TY - GEN
A1 - Reindl, Nicole
A1 - Finch, Nicolle L.
A1 - Schaffenroth, Veronika
A1 - Barstow, Martin A.
A1 - Casewell, Sarah L.
A1 - Geier, Stephan
A1 - Bertolami Miller, Marcelo Miguel
A1 - Taubenberger, Stefan
T1 - Revealing the true nature of Hen2-428
T2 - Postprints der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe
N2 - The nucleus of Hen 2-428 is a short orbital period (4.2 h) spectroscopic binary, whose status as potential supernovae type Ia progenitor has raised some controversy in the literature. We present preliminary results of a thorough analysis of this interesting system, which combines quantitative non-local thermodynamic (non-LTE) equilibrium spectral modelling, radial velocity analysis, multi-band light curve fitting, and state-of-the art stellar evolutionary calculations. Importantly, we find that the dynamical system mass that is derived by using all available He II lines does not exceed the Chandrasekhar mass limit. Furthermore, the individual masses of the two central stars are too small to lead to an SN Ia in case of a dynamical explosion during the merger process.
T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 1129
KW - binaries: spectroscopic
KW - stars: atmospheres
KW - stars: abundances
KW - Supernovae
Y1 - 2021
U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-459703
SN - 1866-8372
IS - 1129
ER -
TY - JOUR
A1 - Orgis, Thomas
A1 - Brand, Sascha
A1 - Schwarz, Udo
A1 - Handorf, Dörthe
A1 - Dethloff, Klaus
A1 - Kurths, Jürgen
T1 - Influence of interactive stratospheric chemistry on large-scale air mass exchange in a global circulation model
N2 - A new globally uniform Lagrangian transport scheme for large ensembles of passive tracer particles is presented and applied to wind data from a coupled atmosphere-ocean climate model that includes interactive dynamical feedback with stratospheric chemistry. This feedback from the chemistry is found to enhance large-scale meridional air mass exchange in the northern winter stratosphere as well as intrusion of stratospheric air into the troposphere, where both effects are due to a weakened polar vortex.
Y1 - 2009
UR - http://www.springerlink.com/content/1951-6355
U6 - https://doi.org/10.1140/epjst/e2009-01105-8
SN - 1951-6355
ER -
TY - JOUR
A1 - Culpan, Richard
A1 - Geier, Stephan
A1 - Reindl, Nicole
A1 - Pelisoli, Ingrid
A1 - Gentile Fusillo, Nicola Pietro
A1 - Vorontseva, Alina
T1 - The population of hot subdwarf stars studied with Gaia
BT - IV. catalogues of hot subluminous stars based on Gaia EDR3
JF - Astronomy and astrophysics : an international weekly journal
N2 - In light of substantial new discoveries of hot subdwarfs by ongoing spectroscopic surveys and the availability of the Gaia mission Early Data Release 3 (EDR3), we compiled new releases of two catalogues of hot subluminous stars: the data release 3 (DR3) catalogue of the known hot subdwarf stars contains 6616 unique sources and provides multi-band photometry, and astrometry from Gaia EDR3 as well as classifications based on spectroscopy and colours.
This is an increase of 742 objects over the DR2 catalogue.
This new catalogue provides atmospheric parameters for 3087 stars and radial velocities for 2791 stars from the literature. In addition, we have updated the Gaia Data Release 2 (DR2) catalogue of hot subluminous stars using the improved accuracy of the Gaia EDR3 data set together with updated quality and selection criteria to produce the Gaia EDR3 catalogue of 61 585 hot subluminous stars, representing an increase of 21 785 objects.
The improvements in Gaia EDR3 astrometry and photometry compared to Gaia DR2 have enabled us to define more sophisticated selection functions.
In particular, we improved hot subluminous star detection in the crowded regions of the Galactic plane as well as in the direction of the Magellanic Clouds by including sources with close apparent neighbours but with flux levels that dominate the neighbourhood.
KW - subdwarfs
KW - Hertzsprung-Russell and C-M diagrams
KW - binaries: general
KW - stars: horizontal-branch
KW - catalogs
Y1 - 2022
U6 - https://doi.org/10.1051/0004-6361/202243337
SN - 1432-0746
VL - 662
PB - EDP Sciences
CY - Les Ulis
ER -
TY - JOUR
A1 - Reindl, Nicole
A1 - Schaffenroth, Veronika
A1 - Filiz, Semih
A1 - Geier, Stephan
A1 - Pelisoli, Ingrid
A1 - Kepler, Souza Oliveira
T1 - Mysterious, variable, and extremely hot
BT - White dwarfs showing ultra-high excitation lines: I. Photometric variability
JF - Astronomy and astrophysics : an international weekly journal / European Southern Observatory (ESO)
N2 - Context.
About 10% of all stars exhibit absorption lines of ultra-highly excited (UHE) metals (e.g., O VIII) in their optical spectra when entering the white dwarf cooling sequence. This is something that has never been observed in any other astrophysical object, and poses a decades-long mystery in our understanding of the late stages of stellar evolution. The recent discovery of a UHE white dwarf that is both spectroscopically and photometrically variable led to the speculation that the UHE lines might be created in a shock-heated circumstellar magnetosphere.
Aims.
We aim to gain a better understanding of these mysterious objects by studying the photometric variability of the whole population of UHE white dwarfs, and white dwarfs showing only the He II line problem, as both phenomena are believed to be connected.
Methods.
We investigate (multi-band) light curves from several ground- and space-based surveys of all 16 currently known UHE white dwarfs (including one newly discovered) and eight white dwarfs that show only the He II line problem.
Results.
We find that 75(-13)(+8) % of the UHE white dwarfs, and 75(-19)(+9)% of the He II line problem white dwarfs are significantly photometrically variable, with periods ranging from 0.22 d to 2.93 d and amplitudes from a few tenths to a few hundredths of a magnitude. The high variability rate is in stark contrast to the variability rate amongst normal hot white dwarfs (we find 9(2)(+4)%), marking UHE and He II line problem white dwarfs as a new class of variable stars. The period distribution of our sample agrees with both the orbital period distribution of post-common-envelope binaries and the rotational period distribution of magnetic white dwarfs if we assume that the objects in our sample will spin-up as a consequence of further contraction.
Conclusions.
We find further evidence that UHE and He II line problem white dwarfs are indeed related, as concluded from their overlap in the Gaia HRD, similar photometric variability rates, light-curve shapes and amplitudes, and period distributions. The lack of increasing photometric amplitudes towards longer wavelengths, as well as the nondetection of optical emission lines arising from the highly irradiated face of a hypothetical secondary in the optical spectra of our stars, makes it seem unlikely that an irradiated late-type companion is the origin of the photometric variability. Instead, we believe that spots on the surfaces of these stars and/or geometrical effects of circumstellar material might be responsible.
KW - white dwarfs
KW - stars: variables: general
KW - starspots
KW - binaries: close
Y1 - 2021
U6 - https://doi.org/10.1051/0004-6361/202140289
SN - 1432-0746
VL - 647
PB - EDP Sciences
CY - Les Ulis
ER -
TY - JOUR
A1 - Geier, Stephan
A1 - Dorsch, Matti
A1 - Pelisoli, Ingrid
A1 - Reindl, Nicole
A1 - Heber, Ulrich
A1 - Irrgang, Andreas
T1 - Radial velocity variability and the evolution of hot subdwarf stars
JF - Astronomy and astrophysics : an international weekly journal
N2 - Hot subdwarf stars represent a late and peculiar stage in the evolution of low-mass stars, since they are likely formed by close binary interactions. In this work, we perform a radial velocity (RV) variability study of a sample of 646 hot subdwarfs with multi-epoch radial velocities based on spectra from Sloan Digital Sky Survey (SDSS) and Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST). The atmospheric parameters and RVs were taken from the literature. For stars with archival spectra but without literature values, we determined the parameters by fitting model atmospheres. In addition, we redetermined the atmospheric parameters and RVs for all the He-enriched sdO/Bs. This broad sample allowed us to study RV-variability as a function of the location in the T-eff - log g- and T-eff - log n(He)/n(H) diagrams in a statistically significant way. We used the fraction of RV-variable stars and the distribution of the maximum RV variations Delta RVmax as diagnostics. Both indicators turned out to be quite inhomogeneous across the studied parameter ranges. A striking feature is the completely dissimilar behaviour of He-poor and He-rich hot subdwarfs. While the former have a high fraction of close binaries, almost no significant RV variations could be detected for the latter. This has led us to the conclusion that there is likely no evolutionary connection between these subtypes. On the other hand, intermediate He-rich- and extreme He-rich sdOB/Os are more likely to be related. Furthermore, we conclude that the vast majority of this population is formed via one or several binary merger channels. Hot subdwarfs with temperatures cooler than similar to 24 000 K tend to show fewer and smaller RV-variations. These objects might constitute a new subpopulation of binaries with longer periods and late-type or compact companions. The RV-variability properties of the extreme horizontal branch (EHB) and corresponding post-EHB populations of the He-poor hot subdwarfs match and confirm the predicted evolutionary connection between them. Stars found below the canonical EHB at somewhat higher surface gravities show large RV variations and a high RV variability fraction. These properties are consistent with most of them being low-mass EHB stars or progenitors of low-mass helium white dwarfs in close binaries.
KW - subdwarfs
KW - binaries: spectroscopic
KW - stars: horizontal-branch
Y1 - 2022
U6 - https://doi.org/10.1051/0004-6361/202143022
SN - 0004-6361
SN - 1432-0746
VL - 661
PB - EDP Sciences
CY - Les Ulis
ER -
TY - JOUR
A1 - Schaffenroth, Veronika
A1 - Casewell, Sarah L.
A1 - Schneider, D.
A1 - Kilkenny, David
A1 - Geier, Stephan
A1 - Heber, Ulrich
A1 - Irrgang, Andreas
A1 - Przybilla, Norbert
A1 - Marsh, Thomas R.
A1 - Littlefair, Stuart P.
A1 - Dhillon, Vik S.
T1 - A quantitative in-depth analysis of the prototype sdB plus BD system SDSS J08205+0008 revisited in the Gaia era
JF - Monthly notices of the Royal Astronomical Society
N2 - Subdwarf B stars are core-helium-burning stars located on the extreme horizontal branch (EHB). Extensive mass loss on the red giant branch is necessary to form them. It has been proposed that substellar companions could lead to the required mass loss when they are engulfed in the envelope of the red giant star. J08205+0008 was the first example of a hot subdwarf star with a close, substellar companion candidate to be found. Here, we perform an in-depth re-analysis of this important system with much higher quality data allowing additional analysis methods. From the higher resolution spectra obtained with ESO-VLT/XSHOOTER, we derive the chemical abundances of the hot subdwarf as well as its rotational velocity. Using the Gaia parallax and a fit to the spectral energy distribution in the secondary eclipse, tight constraints to the radius of the hot subdwarf are derived. From a long-term photometric campaign, we detected a significant period decrease of -3.2(8) x 10(-12) dd(-1). This can be explained by the non-synchronized hot subdwarf star being spun up by tidal interactions forcing it to become synchronized. From the rate of period decrease we could derive the synchronization time-scale to be 4 Myr, much smaller than the lifetime on EHB. By combining all different methods, we could constrain the hot subdwarf to a mass of 0.39-0.50 M-circle dot and a radius of R-sdB = 0.194 +/- 0.008 R-circle dot, and the companion to 0.061-0.071 M-circle dot with a radius of R-comp = 0.092 +/- 0.005 R-circle dot, below the hydrogen-burning limit. We therefore confirm that the companion is most likely a massive brown dwarf.
KW - stars: abundances
KW - stars: atmospheres
KW - stars: fundamental parameters
KW - stars: horizontal branch
KW - stars: low-mass
KW - subdwarfs
Y1 - 2020
U6 - https://doi.org/10.1093/mnras/staa3661
SN - 0035-8711
SN - 1365-2966
VL - 501
IS - 3
SP - 3847
EP - 3870
PB - Oxford Univ. Press
CY - Oxford
ER -
TY - JOUR
A1 - Neunteufel, Patrick
A1 - Kruckow U., Matthias
A1 - Geier, Stephan
A1 - Hamers, Adrian S.
T1 - Predicted spatial and velocity distributions of ejected companion stars of helium accretion-induced thermonuclear supernovae
JF - Astronomy and astrophysics : an international weekly journal
N2 - Context
Thermonuclear supernovae (SNe), a subset of which are the highly important SNe Type Ia, remain one of the more poorly understood phenomena known to modern astrophysics.
In recent years, the single degenerate helium (He) donor channel, where a white dwarf star accretes He-rich matter from a hydrogen-depleted companion, has emerged as a promising candidate progenitor scenario for these events.
An unresolved question in this scenario is the fate of the companion star, which would be evident as a runaway hot subdwarf O/B stars (He sdO/B) in the aftermath of the SN event.
Aims
Previous studies have shown that the kinematic properties of an ejected companion provide an opportunity to closer examine the properties of an SN progenitor system. However, with the number of observed objects not matching predictions by theory, the viability of this mechanism is called into question.
In this study, we first synthesize a population of companion stars ejected by the aforementioned mechanism, taking into account predicted ejection velocities, the inferred population density in the Galactic mass distribution, and subsequent kinematics in the Galactic potential. We then discuss the astrometric properties of this population.
Methods
We present 10(6) individual ejection trajectories, which were numerically computed with a newly developed, lightweight simulation framework. Initial conditions were randomly generated, but weighted according to the Galactic mass density and ejection velocity data. We then discuss the bulk properties (Galactic distribution and observational parameters) of our sample.
Results
Our synthetic population reflects the Galactic mass distribution.
A peak in the density distribution for close objects is expected in the direction of the Galactic centre. Higher mass runaways should outnumber lower mass ones. If the entire considered mass range is realised, the radial velocity distribution should show a peak at 500 km s(-1).
If only close US 708 analogues are considered, there should be a peak at (similar to 750-850) km s(-1). In either case, US 708 should be a member of the high-velocity tail of the distribution.
Conclusions
We show that the puzzling lack of confirmed surviving companion stars of thermonuclear SNe, though possibly an observation-related selection effect, may indicate a selection against high mass donors in the SD He donor channel.
KW - stars: kinematics and dynamics
KW - binaries: close
KW - supernovae: general
KW - subdwarfs
Y1 - 2021
U6 - https://doi.org/10.1051/0004-6361/202040022
SN - 0004-6361
SN - 1432-0746
VL - 646
PB - EDP Sciences
CY - Les Ulis
ER -
TY - JOUR
A1 - Werner, Klaus
A1 - Reindl, Nicole
A1 - Dorsch, Matti
A1 - Geier, Stephan
A1 - Munari, Ulisse
A1 - Raddi, Roberto
T1 - Non-local thermodynamic equilibrium spectral analysis of five hot, hydrogen-deficient pre-white dwarfs
JF - Astronomy and Astrophysics
N2 - Hot, compact, hydrogen-deficient pre-white dwarfs (pre-WDs) with effective temperatures of Teff > 70 000 K and a surface gravity of 5.0 < logg < 7.0 are rather rare objects despite recent and ongoing surveys. It is believed that they are the outcome of either single star evolution (late helium-shell flash or late helium-core flash) or binary star evolution (double WD merger). Their study is interesting because the surface elemental abundances reflect the physics of thermonuclear flashes and merger events. Spectroscopically they are divided in three different classes, namely PG1159, O(He), or He-sdO. We present a spectroscopic analysis of five such stars that turned out to have atmospheric parameters in the range Teff = 70 000-80 000 K and logg = 5.2-6.3. The three investigated He-sdOs have a relatively high hydrogen mass fraction (10%) that is unexplained by both single (He core flash) and binary evolution (He-WD merger) scenarios. The O(He) star JL 9 is probably a binary helium-WD merger, but its hydrogen content (6%) is also at odds with merger models. We found that RL 104 is the 'coolest' (Teff = 80 000 K) member of the PG1159 class in a pre-WD stage. Its optical spectrum is remarkable because it exhibits C※ IV lines involving Rydberg states with principal quantum numbers up to n = 22. Its rather low mass (0.48-0.02+0.03 M·) is difficult to reconcile with the common evolutionary scenario for PG1159 stars due to it being the outcome of a (very) late He-shell flash. The same mass-problem faces a merger model of a close He-sdO plus CO WD binary that predicts PG1159-like abundances. Perhaps RL 104 originates from a very late He-shell flash in a CO/He WD formed by a merger of two low-mass He-WDs.
KW - stars: atmospheres
KW - stars: abundances
KW - stars: evolution
KW - subdwarfs
KW - white dwarfs
Y1 - 2022
U6 - https://doi.org/10.1051/0004-6361/202142397
SN - 0004-6361
SN - 1432-0746
VL - 658
PB - EDP Sciences
CY - Les Ulis
ER -
TY - JOUR
A1 - Geier, Stephan
T1 - The population of hot subdwarf stars studied with Gaia
BT - III. catalogue of known hot subdwarf stars : data release
JF - Astronomy and astrophysics : an international weekly journal
N2 - In light of substantial new discoveries of hot subdwarfs by ongoing spectroscopic surveys and the availability of new all-sky data from ground-based photometric surveys and the Gaia mission Data Release 2, we compiled an updated catalogue of the known hot subdwarf stars. The catalogue contains 5874 unique sources including 528 previously unknown hot subdwarfs and provides multi-band photometry, astrometry from Gaia, and classifications based on spectroscopy and colours. This new catalogue provides atmospheric parameters of 2187 stars and radial velocities of 2790 stars from the literature. Using colour, absolute magnitude, and reduced proper motion criteria, we identified 268 previously misclassified objects, most of which are less luminous white dwarfs or more luminous blue horizontal branch and main-sequence stars.
KW - subdwarfs
KW - stars: horizontal-branch
KW - catalogs
Y1 - 2020
U6 - https://doi.org/10.1051/0004-6361/202037526
SN - 0004-6361
SN - 1432-0746
VL - 635
PB - EDP Sciences
CY - Les Ulis
ER -
TY - JOUR
A1 - Köhler, Raphael H.
A1 - Handorf, Dörthe
A1 - Jaiser, Ralf
A1 - Dethloff, Klaus
A1 - Zängl, Günther
A1 - Majewski, Detlev
A1 - Rex, Markus
T1 - Improved circulation in the Northern hemisphere by adjusting gravity wave drag parameterizations in seasonal experiments with ICON-NWP
JF - Earth and Space Science : ESS
N2 - The stratosphere is one of the main potential sources for subseasonal to seasonal predictability in midlatitudes in winter. The ability of an atmospheric model to realistically simulate the stratospheric dynamics is essential in order to move forward in the field of seasonal predictions in midlatitudes. Earlier studies with the ICOsahedral Nonhydrostatic atmospheric model (ICON) point out that stratospheric westerlies in ICON are underestimated. This is the first extensive study on the evaluation of Northern Hemisphere stratospheric winter circulation with ICON in numerical weather prediction (NWP) mode. Seasonal experiments with the default setup are able to reproduce the basic climatology of the stratospheric polar vortex. However, westerlies are too weak and major stratospheric warmings too frequent in ICON. Both a reduction of the nonorographic, and a reduction of the orographic gravity wave and wake drag lead to a strengthening of the stratospheric vortex and a bias reduction, in particular in January. However, the effect of the nonorographic gravity wave drag scheme on the stratosphere is stronger. Stratosphere-troposphere coupling is intensified and more realistic due to a reduced gravity wave drag. Furthermore, an adjustment of the subgrid-scale orographic drag parameterization leads to a significant error reduction in the mean sea level pressure. As a result of these findings, we present our current suggested improved setup for seasonal experiments with ICON-NWP.
Plain Language Summary Although seasonal forecasts for midlatitudes have the potential to be highly beneficial to the public sector, they are still characterized by a large amount of uncertainty. Exact simulations of the circulation in the stratosphere can help to improve tropospheric predictability on seasonal time scales. For this reason, we investigate how well the new German atmospheric model is able to simulate the stratospheric circulation. The model reproduces the basic behavior of the Northern Hemisphere stratospheric polar vortex, but the westerly circulation in winter is underestimated. The stratospheric circulation is influenced by gravity waves that exert drag on the flow. These processes are only partly physically represented in the model, but are very important and are hence parameterized. By adjusting the parameterizations for the gravity wave drag, the stratospheric polar vortex is strengthened, thereby yielding a more realistic stratospheric circulation. In addition, the altered parameterizations improve the simulated surface pressure pattern. Based upon this, we present our current suggested improved model setup for seasonal experiments.
Y1 - 2021
U6 - https://doi.org/10.1029/2021EA001676
SN - 2333-5084
VL - 8
IS - 3
PB - American Geophysical Union
CY - Malden, Mass.
ER -
TY - JOUR
A1 - Schaffenroth, Veronika
A1 - Pelisoli, Ingrid
A1 - Barlow, Brad N.
A1 - Geier, Stephan
A1 - Kupfer, Thomas
T1 - Hot subdwarfs in close binaries observed from space I.
BT - orbital, atmospheric, and absolute parameters and the nature of
their companions
JF - Astronomy and astrophysics : an international weekly journal
N2 - Context:
About a third of the hot subdwarfs of spectral type B (sdBs), which are mostly core-helium-burning objects on the extreme horizontal branch, are found in close binaries with cool, low-mass stellar, substellar, or white dwarf companions. They can show light variations due to di fferent phenomena.
Aims:
Many hot subdwarfs now have space-based light curves with a high signal-to-noise ratio available. We used light curves from the Transiting Exoplanet Survey Satellite and the K2 space mission to look for more sdB binaries. Their light curves can be used to study the hot subdwarf primaries and their companions, and obtained orbital, atmospheric, and absolute parameters for those systems, when combined with other analysis methods.
Methods:
By classifying the light variations and combining these with the fit of the spectral energy distribution, the distance derived by the parallaxes obtained by Gaia, and the atmospheric parameters, mainly from the literature, we could derive the nature of the primaries and secondaries in 122 (75%) of the known sdB binaries and 82 newly found reflection e ffect systems. We derived absolute masses, radii, and luminosities for a total of 39 hot subdwarfs with cool, low-mass companions, as well 29 known and newly found sdBs with white dwarf companions.
Results:
The mass distribution of hot subdwarfs with cool, low-mass stellar and substellar companions, di ffers from those with white dwarf companions, implying they come from di fferent populations. By comparing the period and minimum companion mass distributions, we find that the reflection e ffect systems all have M dwarf or brown dwarf companions, and that there seem to be several di fferent populations of hot subdwarfs with white dwarf binaries - one with white dwarf minimum masses around 0.4 M-circle dot, one with longer periods and minimum companion masses up to 0.6 M-circle dot, and at the shortest period, another with white dwarf minimum masses around 0.8 M-circle dot. We also derive the first orbital period distribution for hot subdwarfs with cool, low-mass stellar or substellar systems selected from light variations instead of radial velocity variations. It shows a narrower period distribution, from 1.5 h to 35 h, compared to the distribution of hot subdwarfs with white dwarfs, which ranges from 1 h to 30 days. These period distributions can be used to constrain the previous common-envelope phase.
KW - binaries: close
KW - subdwarfs
KW - white dwarfs
KW - stars: late-type
KW - stars:
KW - horizontal-branch
KW - stars: fundamental parameters
Y1 - 2022
U6 - https://doi.org/10.1051/0004-6361/202244214
SN - 0004-6361
SN - 1432-0746
VL - 666
PB - EDP Sciences
CY - Les Ulis
ER -
TY - JOUR
A1 - Pelisoli, Ingrid
A1 - Vos, Joris
A1 - Geier, Stephan
A1 - Schaffenroth, Veronika
A1 - Baran, Andrzej S.
T1 - Alone but not lonely
BT - observational evidence that binary interaction is always required to form hot subdwarf stars
JF - Astronomy and astrophysics : an international weekly journal
N2 - Context. Hot subdwarfs are core-helium burning stars that show lower masses and higher temperatures than canonical horizontal branch stars. They are believed to be formed when a red giant suffers an extreme mass-loss episode. Binary interaction is suggested to be the main formation channel, but the high fraction of apparently single hot subdwarfs (up to 30%) has prompted single star formation scenarios to be proposed.Aims. We investigate the possibility that hot subdwarfs could form without interaction by studying wide binary systems. If single formation scenarios were possible, there should be hot subdwarfs in wide binaries that have undergone no interaction.Methods. Angular momentum accretion during interaction is predicted to cause the hot subdwarf companion to spin up to the critical velocity. The effect of this should still be observable given the timescales of the hot subdwarf phase. To study the rotation rates of companions, we have analysed light curves from the Transiting Exoplanet Survey Satellite for all known hot subdwarfs showing composite spectral energy distributions indicating the presence of a main sequence wide binary companion. If formation without interaction were possible, that would also imply the existence of hot subdwarfs in very wide binaries that are not predicted to interact. To identify such systems, we have searched for common proper motion companions with projected orbital distances of up to 0.1 pc to all known spectroscopically confirmed hot subdwarfs using Gaia DR2 astrometry.Results. We find that the companions in composite hot subdwarfs show short rotation periods when compared to field main sequence stars. They display a triangular-shaped distribution with a peak around 2.5 days, similar to what is observed for young open clusters. We also report a shortage of hot subdwarfs with candidate common proper motion companions. We identify only 16 candidates after probing 2938 hot subdwarfs with good astrometry. Out of those, at least six seem to be hierarchical triple systems, in which the hot subdwarf is part of an inner binary.Conclusions. The observed distribution of rotation rates for the companions in known wide hot subdwarf binaries provides evidence of previous interaction causing spin-up. Additionally, there is a shortage of hot subdwarfs in common proper motion pairs, considering the frequency of such systems among progenitors. These results suggest that binary interaction is always required for the formation of hot subdwarfs.
KW - subdwarfs
KW - binaries: general
KW - stars: variables: general
Y1 - 2020
U6 - https://doi.org/10.1051/0004-6361/202038473
SN - 0004-6361
SN - 1432-0746
VL - 642
PB - EDP Sciences
CY - Les Ulis
ER -
TY - JOUR
A1 - Irrgang, Andreas
A1 - Geier, Stephan
A1 - Kreuzer, Simon
A1 - Pelisoli, Ingrid Domingos
A1 - Heber, Ulrich
T1 - A stripped helium star in the potential black hole binary LB-1
JF - Astronomy and astrophysics : an international weekly journal
N2 - Context
The recently claimed discovery of a massive (M-BH = 68(-13)(+11) M-circle dot) black hole in the Galactic solar neighborhood has led to controversial discussions because it severely challenges our current view of stellar evolution.
Aims
A crucial aspect for the determination of the mass of the unseen black hole is the precise nature of its visible companion, the B-type star LSV +22 25. Because stars of different mass can exhibit B-type spectra during the course of their evolution, it is essential to obtain a comprehensive picture of the star to unravel its nature and, thus, its mass.
Methods
To this end, we study the spectral energy distribution of LSV +22 25 and perform a quantitative spectroscopic analysis that includes the determination of chemical abundances for He, C, N, O, Ne, Mg, Al, Si, S, Ar, and Fe.
Results
Our analysis clearly shows that LSV +22 25 is not an ordinary main sequence B-type star. The derived abundance pattern exhibits heavy imprints of the CNO bi-cycle of hydrogen burning, that is, He and N are strongly enriched at the expense of C and O. Moreover, the elements Mg, Al, Si, S, Ar, and Fe are systematically underabundant when compared to normal main-sequence B-type stars. We suggest that LSV +22 25 is a stripped helium star and discuss two possible formation scenarios. Combining our photometric and spectroscopic results with the Gaia parallax, we infer a stellar mass of 1.1 +/- 0.5 M-circle dot. Based on the binary system's mass function, this yields a minimum mass of 2-3 M-circle dot for the compact companion, which implies that it may not necessarily be a black hole but a massive neutron- or main sequence star.
Conclusions
The star LSV +22 25 has become famous for possibly having a very massive black hole companion. However, a closer look reveals that the star itself is a very intriguing object. Further investigations are necessary for complete characterization of this object.
KW - stars: abundances
KW - stars: chemically peculiar
KW - stars: early-type
KW - pulsars: individual: LS V+22 25
Y1 - 2020
U6 - https://doi.org/10.1051/0004-6361/201937343
SN - 0004-6361
SN - 1432-0746
VL - 633
PB - EDP Sciences
CY - Les Ulis
ER -
TY - JOUR
A1 - Culpan, Rick
A1 - Pelisoli, Ingrid
A1 - Geier, Stephan
T1 - Clean catalogues of blue horizontal-branch stars using Gaia EDR3
JF - Astronomy and astrophysics : an international weekly journal / European Southern Observatory (ESO)
N2 - Context.
Blue horizontal-branch stars evolve from low-mass stars that have completed their main-sequence lifetimes and undergone a helium flash at the end of their red-giant phase. As such, blue horizontal-branch stars are very old objects that can be used as markers in studies of the Galactic structure and formation history. To create a clean sky catalogue of blue horizontal-branch stars, we cross-matched the Gaia data release 2 (DR2) dataset with existing reference catalogues to define selection criteria based on Gaia DR2 parameters. Following the publication of Gaia early data release 3 (EDR3), these methods were verified and subsequently applied to this latest release.
Aims.
Previous catalogues of blue horizontal-branch stars were developed using spectral analyses or were restricted to individual globular clusters. The purpose of this catalogue is to identify a set of blue horizontal-branch star candidates that have been selected using photometric and astrometric observations and exhibits a low contamination rate. This has been deemed important as the success of the Gaia mission has changed the way that targets are selected for large-scale spectroscopic surveys, meaning that far fewer spectra will be acquired for blue horizontal-branch stars in the future unless they are specifically targeted.
Methods. We cross-matched reference blue horizontal-branch datasets with the Gaia DR2 database and defined two sets of selection criteria. Firstly, in Gaia DR2 - colour and absolute G magnitude space, and secondly, in Gaia DR2 - colour and reduced proper motion space. The main-sequence contamination in both subsets of the catalogue was reduced, at the expense of completeness, by concentrating on the Milky Way's Galactic halo, where relatively young main-sequence stars were not expected. The entire catalogue is limited to those stars with no apparent neighbours within 5 arcsec. These methods were verified and subsequently applied to the Gaia EDR3.
Results.
We present a catalogue, based on Gaia EDR3, of 57 377 blue horizontal-branch stars. The Gaia EDR3 parallax was used in selecting 16 794 candidates and the proper motions were used to identify a further 40 583 candidates.
KW - Hertzsprung-Russell and C-M diagrams
KW - stars: horizontal-branch
KW - catalogs
KW - Galaxy: stellar content
Y1 - 2021
U6 - https://doi.org/10.1051/0004-6361/202040074
SN - 1432-0746
VL - 654
PB - EDP Sciences
CY - Les Ulis
ER -
TY - GEN
A1 - Maier, Philipp
A1 - Wolf, Jürgen
A1 - Keilig, Thomas
A1 - Krabbe, Alfred
A1 - Duffard, Rene
A1 - Ortiz, Jose-Luis
A1 - Klinkner, Sabine
A1 - Lengowski, Michael
A1 - Müller, Thomas
A1 - Lockowandt, Christian
A1 - Krockstedt, Christian
A1 - Kappelmann, Norbert
A1 - Stelzer, Beate
A1 - Werner, Klaus
A1 - Geier, Stephan
A1 - Kalkuhl, Christoph
A1 - Rauch, Thomas
A1 - Schanz, Thomas
A1 - Barnstedt, Jürgen
A1 - Conti, Lauro
A1 - Hanke, Lars
T1 - Towards a European Stratospheric Balloon Observatory
BT - the ESBO design study
T2 - Ground-based and Airborne Telescopes VII
N2 - This paper presents the concept of a community-accessible stratospheric balloon-based observatory that is currently under preparation by a consortium of European research institutes and industry. We present the technical motivation, science case, instrumentation, and a two-stage image stabilization approach of the 0.5-m UV/visible platform. In addition, we briefly describe the novel mid-sized stabilized balloon gondola under design to carry telescopes in the 0.5 to 0.6 m range as well as the currently considered flight option for this platform. Secondly, we outline the scientific and technical motivation for a large balloon-based FIR telescope and the ESBO DS approach towards such an infrastructure.
KW - astronomy
KW - balloon telescopes
KW - UV
KW - far infrared
KW - detectors
KW - observatory
Y1 - 2018
SN - 978-1-5106-1954-8
U6 - https://doi.org/10.1117/12.2319248
SN - 0277-786X
SN - 1996-756X
VL - 10700
PB - SPIE-INT Soc Optical Engineering
CY - Bellingham
ER -
TY - JOUR
A1 - Charpinet, Stéphane
A1 - Brassard, P.
A1 - Fontaine, G.
A1 - Van Grootel, Valerie
A1 - Zong, Weika
A1 - Giammichele, N.
A1 - Heber, Ulrich
A1 - Bognár, Zsófia
A1 - Geier, Stephan
A1 - Green, Elizabeth M.
A1 - Hermes, J. J.
A1 - Kilkenny, D.
A1 - Ostensen, R. H.
A1 - Pelisoli, Ingrid Domingos
A1 - Silvotti, R.
A1 - Telting, J. H.
A1 - Vuckovic, Maja
A1 - Worters, H. L.
A1 - Baran, Andrzej S.
A1 - Bell, Keaton J.
A1 - Bradley, Paul A.
A1 - Debes, J. H.
A1 - Kawaler, S. D.
A1 - Kolaczek-Szymanski, P.
A1 - Murphy, S. J.
A1 - Pigulski, A.
A1 - Sodor, A.
A1 - Uzundag, Murat
A1 - Handberg, R.
A1 - Kjeldsen, H.
A1 - Ricker, G. R.
A1 - Vanderspek, R. K.
T1 - TESS first look at evolved compact pulsators Discovery and asteroseismic probing of the g-mode hot B subdwarf pulsator EC 21494-7018
JF - Astronomy and astrophysics : an international weekly journal
N2 - Context. The TESS satellite was launched in 2018 to perform high-precision photometry from space over almost the whole sky in a search for exoplanets orbiting bright stars. This instrument has opened new opportunities to study variable hot subdwarfs, white dwarfs, and related compact objects. Targets of interest include white dwarf and hot subdwarf pulsators, both carrying high potential for asteroseismology. Aims. We present the discovery and detailed asteroseismic analysis of a new g-mode hot B subdwarf (sdB) pulsator, EC 21494-7018 (TIC 278659026), monitored in TESS first sector using 120-s cadence. Methods. The TESS light curve was analyzed with standard prewhitening techniques, followed by forward modeling using our latest generation of sdB models developed for asteroseismic investigations. By simultaneously best-matching all the observed frequencies with those computed from models, we identified the pulsation modes detected and, more importantly, we determined the global parameters and structural configuration of the star. Results. The light curve analysis reveals that EC 21494-7018 is a sdB pulsator counting up to 20 frequencies associated with independent g-modes. The seismic analysis singles out an optimal model solution in full agreement with independent measurements provided by spectroscopy (atmospheric parameters derived from model atmospheres) and astrometry (distance evaluated from Gaia DR2 trigonometric parallax). Several key parameters of the star are derived. Its mass (0.391 +/- 0.009x2006;M-circle dot) is significantly lower than the typical mass of sdB stars and suggests that its progenitor has not undergone the He-core flash; therefore this progenitor could originate from a massive (greater than or similar to 2;M-circle dot) red giant, which is an alternative channel for the formation of sdBs. Other derived parameters include the H-rich envelope mass (0.0037 +/- 0.0010;M-circle dot), radius (0.1694 +/- 0.0081;R-circle dot), and luminosity (8.2 +/- 1.1;L-circle dot). The optimal model fit has a double-layered He+H composition profile, which we interpret as an incomplete but ongoing process of gravitational settling of helium at the bottom of a thick H-rich envelope. Moreover, the derived properties of the core indicate that EC 21494-7018 has burnt similar to 43% (in mass) of its central helium and possesses a relatively large mixed core (M-core;=;0.198 +/- 0.010;M-circle dot), in line with trends already uncovered from other g-mode sdB pulsators analyzed with asteroseismology. Finally, we obtain for the first time an estimate of the amount of oxygen (in mass; X(O)(core) = 0.16(-0.05)(+0.13)X(O)core=0.16-0.05+0.13$ X(mathrm{O})_{mathrm{core}}=0.16_{-0.05}<^>{+0.13} $) produced at this stage of evolution by an helium-burning core. This result, along with the core-size estimate, is an interesting constraint that may help to narrow down the still uncertain C-12(alpha,;gamma)O-16 nuclear reaction rate.
KW - asteroseismology
KW - stars
KW - interiors
KW - oscillations
KW - horizontal-branch
KW - individual
KW - TIC 278659026
KW - subdwarfs
Y1 - 2019
U6 - https://doi.org/10.1051/0004-6361/201935395
SN - 0004-6361
SN - 1432-0746
VL - 632
PB - EDP Sciences
CY - Les Ulis
ER -
TY - JOUR
A1 - Raddi, Roberto
A1 - Hollands, M. A.
A1 - Koester, D.
A1 - Hermes, J. J.
A1 - Gansicke, B. T.
A1 - Heber, Ulrich
A1 - Shen, Ken J.
A1 - Townsley, D. M.
A1 - Pala, Anna Francesca
A1 - Reding, J. S.
A1 - Toloza, O. F.
A1 - Pelisoli, Ingrid Domingos
A1 - Geier, Stephan
A1 - Fusillo, Nicola Pietro Gentile
A1 - Munari, Ullisse
A1 - Strader, J.
T1 - Partly burnt runaway stellar remnants from peculiar thermonuclear supernovae
JF - Monthly notices of the Royal Astronomical Society
N2 - We report the discovery of three stars that, along with the prototype LP 40-365, form a distinct class of chemically peculiar runaway stars that are the survivors of thermonuclear explosions. Spectroscopy of the four confirmed LP 40-365 stars finds ONe-dominated atmospheres enriched with remarkably similar amounts of nuclear ashes of partial O- and Si-burning. Kinematic evidence is consistent with ejection from a binary supernova progenitor; at least two stars have rest-frame velocities indicating they are unbound to the Galaxy. With masses and radii ranging between 0.20 and 0.28M(circle dot) and between 0.16 and 0.60 R-circle dot, respectively, we speculate these inflated white dwarfs are the partly burnt remnants of either peculiar Type Iax or electron-capture supernovae. Adopting supernova rates from the literature, we estimate that similar to 20 LP 40-365 stars brighter than 19 mag should be detectable within 2 kpc from the Sun at the end of the Gaia mission. We suggest that as they cool, these stars will evolve in their spectroscopic appearance, and eventually become peculiar O-rich white dwarfs. Finally, we stress that the discovery of new LP 40-365 stars will be useful to further constrain their evolution, supplying key boundary conditions to the modelling of explosion mechanisms, supernova rates, and nucleosynthetic yields of peculiar thermonuclear explosions.
KW - stars: individual: LP 40-365
KW - subdwarfs
KW - supernovae: general
KW - white dwarfs
KW - Galaxy: kinematics and dynamics
Y1 - 2019
U6 - https://doi.org/10.1093/mnras/stz1618
SN - 0035-8711
SN - 1365-2966
VL - 489
IS - 2
SP - 1489
EP - 1508
PB - Oxford Univ. Press
CY - Oxford
ER -
TY - JOUR
A1 - Irrgang, Andreas
A1 - Geier, Stephan
A1 - Heber, Ulrich
A1 - Kupfer, Thomas
A1 - Fürst, F.
T1 - PG 1610+062: a runaway B star challenging classical ejection mechanisms
JF - Astronomy and astrophysics : an international weekly journal
N2 - Hypervelocity stars are rare objects, mostly main-sequence (MS) B stars, traveling so fast that they will eventually escape from the Milky Way. Recently, it has been shown that the popular Hills mechanism, in which a binary system is disrupted via a close encounter with the supermassive black hole at the Galactic center, may not be their only ejection mechanism. The analyses of Gaia data ruled out a Galactic center origin for some of them, and instead indicated that they are extreme disk runaway stars ejected at velocities exceeding the predicted limits of classical scenarios (dynamical ejection from star clusters or binary supernova ejection). We present the discovery of a new extreme disk runaway star, PG 1610+062, which is a slowly pulsating B star bright enough to be studied in detail. A quantitative analysis of spectra taken with ESI at the Keck Observatory revealed that PG 1610+062 is a late B-type MS star of 4–5 M⊙ with low projected rotational velocity. Abundances (C, N, O, Ne, Mg, Al, Si, S, Ar, and Fe) were derived differentially with respect to the normal B star HD 137366 and indicate that PG 1610+062 is somewhat metal rich. A kinematic analysis, based on our spectrophotometric distance (17.3 kpc) and on proper motions from Gaia’s second data release, shows that PG 1610+062 was probably ejected from the Carina-Sagittarius spiral arm at a velocity of 550 ± 40 km s−1, which is beyond the classical limits. Accordingly, the star is in the top five of the most extreme MS disk runaway stars and is only the second among the five for which the chemical composition is known.
KW - stars: abundances
KW - stars: individual: HD 137366
KW - stars: kinematics and dynamics
KW - stars: individual: PG 1610+062
KW - stars: early-type
Y1 - 2019
U6 - https://doi.org/10.1051/0004-6361/201935429
SN - 1432-0746
VL - 628
PB - EDP Sciences
CY - Les Ulis
ER -
TY - JOUR
A1 - Schaffenroth, Veronika
A1 - Barlow, Brad N.
A1 - Geier, Stephan
A1 - Vuckovic, Maja
A1 - Kilkenny, D.
A1 - Wolz, M.
A1 - Kupfer, Thomas
A1 - Heber, Ulrich
A1 - Drechsel, H.
A1 - Kimeswenger, S.
A1 - Marsh, T.
A1 - Wolf, M.
A1 - Pelisoli, Ingrid Domingos
A1 - Freudenthal, Joseph
A1 - Dreizler, S.
A1 - Kreuzer, S.
A1 - Ziegerer, E.
T1 - The EREBOS project: Investigating the effect of substellar and low-mass stellar companions on late stellar evolution Survey, target selection, and atmospheric parameters
JF - Astronomy and astrophysics : an international weekly journal
N2 - Eclipsing post-common-envelope binaries are highly important for resolving the poorly understood, very short-lived common-envelope phase of stellar evolution. Most hot subdwarfs (sdO/Bs) are the bare helium-burning cores of red giants that have lost almost all of their hydrogen envelope. This mass loss is often triggered by common-envelope interactions with close stellar or even substellar companions. Cool companions to hot subdwarf stars such as late-type stars and brown dwarfs are detectable from characteristic light-curve variations - reflection effects and often eclipses. In the recently published catalog of eclipsing binaries in the Galactic Bulge and in the Asteroid Terrestrial-impact Last Alert System (ATLAS) survey, we discovered 125 new eclipsing systems showing a reflection effect seen by visual inspection of the light curves and using a machine-learning algorithm, in addition to the 36 systems previously discovered by the Optical Gravitational Lesing Experiment (OGLE) team. The Eclipsing Reflection Effect Binaries from Optical Surveys (EREBOS) project aims at analyzing all newly discovered eclipsing binaries of the HW Vir type (hot subdwarf + close, cool companion) based on a spectroscopic and photometric follow up to derive the mass distribution of the companions, constrain the fraction of substellar companions, and determine the minimum mass needed to strip off the red-giant envelope. To constrain the nature of the primary we derived the absolute magnitude and the reduced proper motion of all our targets with the help of the parallaxes and proper motions measured by the Gaia mission and compared those to the Gaia white-dwarf candidate catalog. It was possible to derive the nature of a subset of our targets, for which observed spectra are available, by measuring the atmospheric parameter of the primary, confirming that less than 10% of our systems are not sdO/Bs with cool companions but are white dwarfs or central stars of planetary nebula. This large sample of eclipsing hot subdwarfs with cool companions allowed us to derive a significant period distribution for hot subdwarfs with cool companions for the first time showing that the period distribution is much broader than previously thought and is ideally suited to finding the lowest-mass companions to hot subdwarf stars. The comparison with related binary populations shows that the period distribution of HW Vir systems is very similar to WD+dM systems and central stars of planetary nebula with cool companions. In the future, several new photometric surveys will be carried out, which will further increase the sample of this project, providing the potential to test many aspects of common-envelope theory and binary evolution.
KW - binaries: eclipsing
KW - brown dwarfs
KW - binaries: spectroscopic
KW - binaries: close
KW - subdwarfs
KW - surveys
Y1 - 2019
U6 - https://doi.org/10.1051/0004-6361/201936019
SN - 1432-0746
VL - 630
PB - EDP Sciences
CY - Les Ulis
ER -
TY - JOUR
A1 - Ratzloff, Jeffrey K.
A1 - Barlow, Brad N.
A1 - Kupfer, Thomas
A1 - Corcoran, Kyle A.
A1 - Geier, Stephan
A1 - Bauer, Evan
A1 - Corbett, Henry T.
A1 - Howard, Ward S.
A1 - Glazier, Amy
A1 - Law, Nicholas M.
T1 - EVR-CB-001: An Evolving, Progenitor, White Dwarf Compact Binary Discovered with the Evryscope
JF - The astrophysical journal : an international review of spectroscopy and astronomical physics
N2 - We present EVR-CB-001, the discovery of a compact binary with an extremely low-mass (0.21 +/- 0.05M(circle dot)) helium core white dwarf progenitor (pre-He WD) and an unseen low-mass (0.32 +/- 0.06M(circle dot)) helium white dwarf (He WD) companion. He WDs are thought to evolve from the remnant helium-rich core of a main-sequence star stripped during the giant phase by a close companion. Low-mass He WDs are exotic objects (only about 0.2% of WDs are thought to be less than 0.3 M-circle dot), and are expected to be found in compact binaries. Pre-He WDs are even rarer, and occupy the intermediate phase after the core is stripped, but before the star becomes a fully degenerate WD and with a larger radius (approximate to 0.2R(circle dot)) than a typical WD. The primary component of EVR-CB-001 (the pre-He WD) was originally thought to be a hot subdwarf (sdB) star from its blue color and under-luminous magnitude, characteristic of sdBs. The mass, temperature (T-eff = 18,500 +/- 500 K), and surface gravity (log(g) = 4.96 +/- 0.04) solutions from this work are lower than values for typical hot subdwarfs. The primary is likely to be a post-red-giant branch, pre-He WD contracting into a He WD, and at a stage that places it nearest to sdBs on color-magnitude and T-eff-log(g) diagrams. EVR-CB-001 is expected to evolve into a fully double degenerate, compact system that should spin down and potentially evolve into a single hot subdwarf star. Single hot subdwarfs are observed, but progenitor systems have been elusive.
Y1 - 2019
U6 - https://doi.org/10.3847/1538-4357/ab3727
SN - 0004-637X
SN - 1538-4357
VL - 883
IS - 1
PB - IOP Publ. Ltd.
CY - Bristol
ER -
TY - JOUR
A1 - Fusillo, Nicola Pietro Gentile
A1 - Tremblay, Pier-Emmanuel
A1 - Gänsicke, Boris T.
A1 - Manser, Christopher J.
A1 - Cunningham, Tim
A1 - Cukanovaite, Elena
A1 - Hollands, Mark
A1 - Marsh, Thomas
A1 - Raddi, Roberto
A1 - Jordan, Stefan
A1 - Toonen, Silvia
A1 - Geier, Stephan
A1 - Barstow, Martin
A1 - Cummings, Jeffrey D.
T1 - A Gaia Data Release 2 catalogue of white dwarfs and a comparison with SDSS
JF - Monthly notices of the Royal Astronomical Society
N2 - We present a catalogue of white dwarf candidates selected from the second data release of Gaia (DR2). We used a sample of spectroscopically confirmed white dwarfs from the Sloan Digital Sky Survey (SDSS) to map the entire space spanned by these objects in the Gaia Hertzsprung–Russell diagram. We then defined a set of cuts in absolute magnitude, colour, and a number of Gaia quality flags to remove the majority of contaminating objects. Finally, we adopt a method analogous to the one presented in our earlier SDSS photometric catalogues to calculate a probability of being a white dwarf (PWD) for all Gaia sources that passed the initial selection. The final catalogue is composed of 486641 stars with calculated PWD from which it is possible to select a sample of ≃260000 high-confidence white dwarf candidates in the magnitude range 8 < G < 21. By comparing this catalogue with a sample of SDSS white dwarf candidates, we estimate an upper limit in completeness of 85 per cent for white dwarfs with G ≤ 20 mag and Teff >7000 K, at high Galactic latitudes (|b| > 20°). However, the completeness drops at low Galactic latitudes, and the magnitude limit of the catalogue varies significantly across the sky as a function of Gaia’s scanning law. We also provide the list of objects within our sample with available SDSS spectroscopy. We use this spectroscopic sample to characterize the observed structure of the white dwarf distribution in the H–R diagram.
KW - catalogues
KW - surveys
KW - white dwarfs
Y1 - 2018
U6 - https://doi.org/10.1093/mnras/sty3016
SN - 0035-8711
SN - 1365-2966
VL - 482
IS - 4
SP - 4570
EP - 4591
PB - Oxford Univ. Press
CY - Oxford
ER -
TY - JOUR
A1 - Romanowsky, Erik
A1 - Handorf, Dörthe
A1 - Jaiser, Ralf
A1 - Wohltmann, Ingo
A1 - Dorn, Wolfgang
A1 - Ukita, Jinro
A1 - Cohen, Judah
A1 - Dethloff, Klaus
A1 - Rex, Markus
T1 - The role of stratospheric ozone for Arctic-midlatitude linkages
JF - Scientific reports
N2 - Arctic warming was more pronounced than warming in midlatitudes in the last decades making this region a hotspot of climate change. Associated with this, a rapid decline of sea-ice extent and a decrease of its thickness has been observed. Sea-ice retreat allows for an increased transport of heat and momentum from the ocean up to the tropo- and stratosphere by enhanced upward propagation of planetary-scale atmospheric waves. In the upper atmosphere, these waves deposit the momentum transported, disturbing the stratospheric polar vortex, which can lead to a breakdown of this circulation with the potential to also significantly impact the troposphere in mid- to late-winter and early spring. Therefore, an accurate representation of stratospheric processes in climate models is necessary to improve the understanding of the impact of retreating sea ice on the atmospheric circulation. By modeling the atmospheric response to a prescribed decline in Arctic sea ice, we show that including interactive stratospheric ozone chemistry in atmospheric model calculations leads to an improvement in tropo-stratospheric interactions compared to simulations without interactive chemistry. This suggests that stratospheric ozone chemistry is important for the understanding of sea ice related impacts on atmospheric dynamics.
Y1 - 2019
U6 - https://doi.org/10.1038/s41598-019-43823-1
SN - 2045-2322
VL - 9
PB - Nature Publ. Group
CY - London
ER -
TY - JOUR
A1 - Latour, Marilyn
A1 - Randall, Suzanna K.
A1 - Calamida, Annalisa
A1 - Geier, Stephan
A1 - Moehler, Sabine
T1 - The ultimate spectroscopic census of extreme horizontal branch stars in omega Centauri
JF - Astronomy and astrophysics : an international weekly journal
N2 - The presence of extreme horizontal branch (EHB) and blue hook stars in some Galactic globular clusters (GGCs) constitutes one of the remaining mysteries of stellar evolution. While several evolutionary scenarios have been proposed to explain the characteristics of this peculiar population of evolved stars, their observational verification has been limited by the availability of spectroscopic data for a statistically significant sample of such objects in any single GGC. We recently launched the SHOTGLAS project with the aim of providing a comprehensive picture of this intriguing stellar population in terms of spectroscopic properties for all readily accessible GGCs hosting an EHB. In this first paper, we focus on omega Cen, a peculiar, massive GGC that hosts multiple stellar populations. We use non-LTE model atmospheres to derive atmospheric parameters (Te ff, log g and N(He) / N(H)) and spectroscopic masses for 152 EHB stars in the cluster. This constitutes the largest spectroscopic sample of EHB stars ever analyzed in a GGC and represents similar to 20% of the EHB population of omega Cen. We also search for close binaries among these stars based on radial velocity variations. Our results show that the EHB population of omega Cen is divided into three spectroscopic groups that are very distinct in the Te ff helium abundance plane. The coolest sdB-type stars (Te ff. 30 000 K) have a hydrogen-rich atmosphere, populate the theoretical EHB region in the Te ff log g plane, and form 26% of our sample. The hottest sdO-type stars (Te ff & 42 000 K) make up 10% of the sample, have a hydrogen-rich atmosphere and are thought to be in a post-EHB evolutionary phase. The majority of our sample is found at intermediate temperatures and consists of sdOB stars that have roughly solar or super-solar atmospheric helium abundances. It is these objects that constitute the blue hook at V > 18 : 5 mag in the omega Cen color-magnitude diagram. Interestingly, the helium-enriched sdOBs do not have a significant counterpart population in the Galactic field, indicating that their formation is dependent on the particular environment found in omega Cen and other select GGCs. Another major di ff erence between the EHB stars in omega Cen and the field is the fraction of close binaries. From our radial velocity survey we identify two binary candidates, however no orbital solutions could be determined. We estimate an EHB close binary fraction of similar to 5% in omega Cen. This low fraction is in line with findings for other GGCs, but in sharp contrast to the situation in the field, where around 50% of the sdB stars reside in close binaries. Finally, the mass distribution derived is very similar for all three spectroscopic groups, however the average mass (0.38 M fi) is lower than that expected from stellar evolution theory. While this mass conundrum has previously been noted for EHB stars in omega Cen, it so far appears to be unique to that cluster.
KW - stars: atmospheres
KW - stars: horizontal-branch
KW - subdwarfs
KW - stars: fundamental parameters
KW - binaries: close
KW - globular clusters: individual: NGC5139
Y1 - 2018
U6 - https://doi.org/10.1051/0004-6361/201833129
SN - 1432-0746
VL - 618
PB - EDP Sciences
CY - Les Ulis
ER -
TY - JOUR
A1 - Geier, Stephan
A1 - Raddi, Roberto
A1 - Fusillo, Nicola Pietro Gentile
A1 - Marsh, T. R.
T1 - The population of hot subdwarf stars studied with Gaia
BT - II. The Gaia DR2 catalogue of hot subluminous stars
JF - Astronomy and astrophysics : an international weekly journal
N2 - Based on data from the ESA Gaia Data Release 2 (DR2) and several ground-based, multi-band photometry surveys we have compiled an all-sky catalogue of 39 800 hot subluminous star candidates selected in Gaia DR2 by means of colour, absolute magnitude, and reduced proper motion cuts. We expect the majority of the candidates to be hot subdwarf stars of spectral type B and O, followed by blue horizontal branch stars of late B-type (HBB), hot post-AGB stars, and central stars of planetary nebulae. The contamination by cooler stars should be about 10%. The catalogue is magnitude limited to Gaia G < 19 mag and covers the whole sky. Except within the Galactic plane and LMC/SMC regions, we expect the catalogue to be almost complete up to about 1.5 kpc. The main purpose of this catalogue is to serve as input target list for the large-scale photometric and spectroscopic surveys which are ongoing or scheduled to start in the coming years. In the long run, securing a statistically significant sample of spectroscopically confirmed hot subluminous stars is key to advance towards a more detailed understanding of the latest stages of stellar evolution for single and binary stars.
KW - subdwarfs
KW - stars: horizontal-branch
KW - catalogs
Y1 - 2019
U6 - https://doi.org/10.1051/0004-6361/201834236
SN - 0004-6361
SN - 1432-0746
VL - 621
PB - EDP Sciences
CY - Les Ulis
ER -
TY - JOUR
A1 - Reindl, Nicole
A1 - Bainbridge, M.
A1 - Przybilla, Norbert
A1 - Geier, Stephan
A1 - Prvak, M.
A1 - Krticka, Jiri
A1 - Ostensen, R. H.
A1 - Telting, J.
A1 - Werner, K.
T1 - Unravelling the baffling mystery of the ultrahot wind phenomenon in white dwarfs
JF - Monthly notices of the Royal Astronomical Society
N2 - The presence of ultrahigh excitation (UHE) absorption lines (e.g. OVIII) in the optical spectra of several of the hottest white dwarfs poses a decades-long mystery and is something that has never been observed in any other astrophysical object. The occurrence of such features requires a dense environment with temperatures near 10(6) K, by far exceeding the stellar effective temperature. Here we report the discovery of a new hot wind white dwarf, GALEXJ014636.8+323615. Astonishingly, we found for the first time rapid changes of the equivalent widths of the UHE features, which are correlated to the rotational period of the star (P=0.242035 d). We explain this with the presence of a wind-fed circumstellar magnetosphere in which magnetically confined wind shocks heat up the material to the high temperatures required for the creation of the UHE lines. The photometric and spectroscopic variability of GALEXJ014636.8+323615 can then be understood as consequence of the obliquity of the magnetic axis with respect to the rotation axis of the white dwarf. This is the first time a wind-fed circumstellar magnetosphere around an apparently isolated white dwarf has been discovered and finally offers a plausible explanation of the ultrahot wind phenomenon.
KW - stars: AGB and post-AGB
KW - stars: evolution
KW - stars: magnetic field
Y1 - 2018
U6 - https://doi.org/10.1093/mnrasl/sly191
SN - 0035-8711
SN - 1365-2966
VL - 482
IS - 1
SP - L93
EP - L98
PB - Oxford Univ. Press
CY - Oxford
ER -
TY - GEN
A1 - Finch, Nicolle L.
A1 - Braker, I. P.
A1 - Reindl, Nicole
A1 - Barstow, Martin A.
A1 - Casewell, Sarah L.
A1 - Burleigh, M.
A1 - Kupfer, Thomas
A1 - Kilkenny, D.
A1 - Geier, Stephan
A1 - Schaffenroth, Veronika
A1 - Bertolami Miller, Marcelo Miguel
A1 - Taubenberger, Stefan
A1 - Freudenthal, Joseph
T1 - Spectral Analysis of Binary Pre-white Dwarf Systems
T2 - Radiative signatures from the cosmos
N2 - Short period double degenerate white dwarf (WD) binaries with periods of less than similar to 1 day are considered to be one of the likely progenitors of type Ia supernovae. These binaries have undergone a period of common envelope evolution. If the core ignites helium before the envelope is ejected, then a hot subdwarf remains prior to contracting into a WD. Here we present a comparison of two very rare systems that contain two hot subdwarfs in short period orbits. We provide a quantitative spectroscopic analysis of the systems using synthetic spectra from state-of-the-art non-LTE models to constrain the atmospheric parameters of the stars. We also use these models to determine the radial velocities, and thus calculate dynamical masses for the stars in each system.
Y1 - 2019
SN - 978-1-58381-925-8
SN - 1050-3390
VL - 519
SP - 231
EP - 238
PB - Astronomical soc pacific
CY - San Fransisco
ER -
TY - JOUR
A1 - Reindl, Nicole
A1 - Geier, Stephan
A1 - Ostensen, R. H.
T1 - Discovery of two bright DO-type white dwarfs
JF - Monthly notices of the Royal Astronomical Society
N2 - We discovered two bright DO-type white dwarfs, GALEXJ053628.3+544854 (J0536+5448) and GALEXJ231128.0+292935(J2311+2929), which rank among the eight brightest DO-type white dwarfs known. Our non-LTE model atmosphere analysis reveals effective temperatures and surface gravities of T-eff = 80000 +/- 4600K and log g = 8.25 +/- 0.15 for J0536+5448 and T-eff = 69400 +/- 900K and log g = 7.80 +/- 0.06 for J2311+2929. The latter shows a significant amount of carbon in its atmosphere (C = 0.003(-0.002)(+0.005), by mass), while for J0536+5448 we could derive only an upper limit of C < 0.003. Furthermore, we calculated spectroscopic distances for the two stars and found a good agreement with the distances derived from the Gaia parallaxes.
KW - stars: abundances
KW - stars: atmospheres
KW - white dwarfs
Y1 - 2018
U6 - https://doi.org/10.1093/mnras/sty1875
SN - 0035-8711
SN - 1365-2966
VL - 480
IS - 1
SP - 1211
EP - 1217
PB - Oxford Univ. Press
CY - Oxford
ER -
TY - THES
A1 - Bojahr, Andre
T1 - Hypersound interaction studied by time-resolved inelastic light and x-ray scattering
T1 - Wechselwirkende Hyperschallwellen untersucht mittels zeitaufgelöster inelastischer Licht- und Röntgenstreuung
N2 - This publications-based thesis summarizes my contribution to the scientific field of ultrafast structural dynamics. It consists of 16 publications, about the generation, detection and coupling of coherent gigahertz longitudinal acoustic phonons, also called hypersonic waves. To generate such high frequency phonons, femtosecond near infrared laser pulses were used to heat nanostructures composed of perovskite oxides on an ultrashort timescale. As a consequence the heated regions of such a nanostructure expand and a high frequency acoustic phonon pulse is generated. To detect such coherent acoustic sound pulses I use ultrafast variants of optical Brillouin and x-ray scattering. Here an incident optical or x-ray photon is scattered by the excited sound wave in the sample. The scattered light intensity measures the occupation of the phonon modes.
The central part of this work is the investigation of coherent high amplitude phonon wave packets which can behave nonlinearly, quite similar to shallow water waves which show a steepening of wave fronts or solitons well known as tsunamis. Due to the high amplitude of the acoustic wave packets in the solid, the acoustic properties can change significantly in the vicinity of the sound pulse. This may lead to a shape change of the pulse. I have observed by time-resolved Brillouin scattering, that a single cycle hypersound pulse shows a wavefront steepening. I excited hypersound pulses with strain amplitudes until 1% which I have calibrated by ultrafast x-ray diffraction (UXRD).
On the basis of this first experiment we developed the idea of the nonlinear mixing of narrowband phonon wave packets which we call "nonlinear phononics" in analogy with the nonlinear optics, which summarizes a kaleidoscope of surprising optical phenomena showing up at very high electric fields. Such phenomena are for instance Second Harmonic Generation, four-wave-mixing or solitons. But in case of excited coherent phonons the wave packets have usually very broad spectra which make it nearly impossible to look at elementary scattering processes between phonons with certain momentum and energy.
For that purpose I tested different techniques to excite narrowband phonon wave packets which mainly consist of phonons with a certain momentum and frequency. To this end epitaxially grown metal films on a dielectric substrate were excited with a train of laser pulses. These excitation pulses drive the metal film to oscillate with the frequency given by their inverse temporal displacement and send a hypersonic wave of this frequency into the substrate. The monochromaticity of these wave packets was proven by ultrafast optical Brillouin and x-ray scattering.
Using the excitation of such narrowband phonon wave packets I was able to observe the Second Harmonic Generation (SHG) of coherent phonons as a first example of nonlinear wave mixing of nanometric phonon wave packets.
N2 - Diese publikationsbasierte Dissertation fasst meinen Beitrag zum Forschungsgebiet der ultraschnellen Strukturdynamik zusammen.
Diese Arbeit besteht aus 16 Publikationen aus den Bereichen der Erzeugung, Detektion und Kopplung von kohärenten Gigahertz longitudinal-akustischen Phononen, auch Hyperschallwellen genannt. Um solch hochfrequente Phononen zu erzeugen, werden Femtosekunden nahinfrarot Laserpulse benutzt, um Nanostrukturen auf einer ultraschnellen Zeitskala zu erhitzen. Die aufgeheizten Regionen der Nanostruktur dehnen sich aufgrund der hohen Temperatur aus und ein hochfrequenter Schallpuls wird generiert. Um solche akustischen Pulse zu detektieren benutze ich ultraschnelle Varianten der Brillouin- und Röntgenstreuung. Dabei wird ein einfallendes optisches oder Röntgenphoton an der erzeugten Schallwelle gestreut. Die gemessene Streuintensität ist hierbei ein Maß für die Besetzung einzelner Phononenzustände.
Der zentrale Teil dieser Arbeit ist die Untersuchung von kohärenten Phonon-Wellenpaketen mit sehr hoher Amplitude. Diese Wellenpakete können sich nichtlinear verhalten, sehr ähnlich zu Flachwasserwellen bei denen nichtlineare Effekte in Form eines Aufsteilens der Wellenfronten oder der Existenz von Solitonen, bekannt als Tsunamis, äußern. Durch die hohe Amplitude der akustischen Wellenpakete können sich die akustischen Eigenschaften des Festkörpers in der Umgebung des Schallpulses signifikant ändern, welches sich dann in einer Formänderung des Schallpulses widerspiegelt. Ich konnte mittels zeitaufgelöster Brillouinstreuung das Aufsteilen der Wellenfronten eines Hyperschallpulses bestehend aus einem einzigen Oszillationszyklus beobachten. Hierbei wurden Hyperschallwellen mit einer Dehnungsamplitude von bis zu 1% angeregt, wobei ich diesen Wert mittels ultraschneller Röntgenbeugung kalibrieren konnte.
Mit diesem ersten Experiment als Basis entwickelten wir die Idee der nichtlinearen Wellenmischung von schmalbandigen Phonon-Wellenpaketen unter dem Titel "nichtlineare Phononik" in Analogie zur nichtlinearen Optik, welche sich aus einer Reihe von verblüffenden optischen Phänomenen bei sehr hohen elektrischen Feldstärken zusammensetzt. Solche Phänomene sind z. B. die optische Frequenzverdopplung, das Vier-Wellen-Mischen oder Solitone. Nur sind im Falle von kohärenten Phononen die erzeugten Spektren sehr breitbandig, was die Untersuchung von spezifischen Phononen mit festem Impuls und definierter Frequenz fast unmöglich macht.
Aus diesem Grund testete ich verschiedene Methoden um schmalbandige Phonon-Wellenpakete anzuregen, welche im Wesentlichen aus Phononen bestimmten Impulses und definierter Frequenz bestehen. Dafür wurden schließ lich epitaktisch auf ein dielektrisches Substrat aufgewachsene Metallfilme mit einen Laserpulszug angeregt. Hier sorgen die Lichtpulse für eine periodische Oszillation des Metalfilms, wobei die Anregefrequenz durch den inversen zeitlichen Abstand der Lichtpulse gegeben ist. Diese periodische Oszillation sendet dann ein Hyperschallwellenpaket eben dieser Frequenz ins Substrat. Die Monochromie dieser Wellenpakete konnte dabei mittels ultraschneller Brillouin- und Röntgenstreuung bestätigt werden.
Durch die Benutzung dieser schmalbandigen Phonon-Wellenpakete war es mir möglich, die Frequenzverdopplung (SHG) von kohärenten Phononen zu beobachten, was ein erstes Beispiel für die nichtlineare Wellenmischung von nanometrischen Phonon-Wellenpaketen ist.
KW - hypersound
KW - nonlinear acoustics
KW - ultrafast
KW - Brillouin scattering
KW - x-ray diffraction
KW - self-steepening
KW - second-harmonic generation
KW - Phononen
KW - Wechselwirkung
KW - Anharmonizität
KW - nichtlineare Wellenmischung
KW - zweite Harmonische
KW - Phononenstreuung
KW - nichlineare Phononik
Y1 - 2016
U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-93860
ER -
TY - THES
A1 - Maerten, Lena
T1 - Spectroscopic perspectives on ultrafast coupling phenomena in perovskite oxides
T1 - Spektroskopische Untersuchung ultraschneller Kopplungsphänomene in Perowskit-Oxiden
N2 - In this thesis, I study ultrafast dynamics in perovskite oxides using time resolved broadband spectroscopy. I focus on the observation of coherent phonon propagation by time resolved Brillouin scattering: following the excition of metal transducer films with a femtosecond infrared pump pulse, coherent phonon dynamics in the GHz frequency range are triggered. Their propagation is monitored using a delayed white light probe pulse. The technique is illustrated on various thin films and multilayered samples. I apply the technique to investigate the linear and nonlinear acoustic response in bulk SrTiO_3, which displays a ferroelastic phase transition from a cubic to a tetragonal structural phase at T_a=105 K. In the linear regime, I observe a coupling of the observed acoustic phonon mode to the softening optic modes describing the phase transition. In the nonlinear regime, I find a giant slowing down of the sound velocity in the low temperature phase that is only observable for a strain amplitude exceeding the tetragonality of the material. It is attributed to a coupling of the high frequency phonons to ferroelastic domain walls in the material. I propose a new mechanism for the coupling of strain waves to the domain walls that is only effective for high amplitude strain. A detailed study of the phonon attenuation across a wide temperature range shows that the phonon attenuation at low temperatures is influenced by the domain configuration, which is determined by interface strain. Preliminary measurements on magnetic-ferroelectric multilayers reveal that the excitation fluence needs to be carefully controlled when dynamics at phase transitions are studied.
N2 - In dieser Doktorarbeit untersuche ich ultraschnelle Dynamik in perovskitischen Oxiden mittels zeitaufgelöster optischer Spektroskopie. Der Schwerpunkt liegt dabei auf Phononendynamik, die mithilfe von zeitaufgelöster Brillouin-Streuung sichtbar gemacht wird: durch die Anregung einer metallischen Transducer-Schicht mit einem ultrakurzen Anregepuls wird eine kohärente Phononendynamik im GHz Frequenzbereich erzeugt. Die Ausbreitung der Schallpulse wird mit einem Weißlicht-Abfragepuls aufgezeichnet. Diese Methode wird am Beispiel verschiedener Dünnschicht- und Übergitterproben illustriert. Die Methode und das gewonnene Verständnis wende ich an, um lineare und nichtlineare akustische Eigenschaften an einem SrTiO_3-Kristall zu untersuchen. Dieser weist einen ferroelastischen Phasenübergang von kubischer zu tetragonaler Kristallstruktur bei T_a=105 K auf. Im linearen Regime beobachte ich eine Kopplung der untersuchten akustischen Mode an eine weichwerdende optische Mode, welche den Phasenübergang charakterisiert. Im nichtlinearen Regime tritt eine gigantische Verlangsamung der Schallgeschwindigkeit unterhalb von T_a auf, wenn die induzierte Gitterverzerrung die Tetragonalität des Materials übersteigt. Dies kann auf eine Kopplung der hochfrequenten akustischen Mode an ferroelastische Domänenwände bei tiefen Temperaturen zurückgeführt werden. Ich entwickle einen neuen Mechanismus, der die Kopplung der Verzerrungswelle an die Domänenwände beschreibt. Eine detaillierte Untersuchung der Phononendämpfung in SrTiO_3 über einen weiten Temperaturbereich zeigt, dass diese bei tiefen Temperaturen durch die Domänenkonfiguration beeinflusst ist. Die Domänenkonfiguration ist durch Verzerrungen an der Kristall-Transducer Grenzfläche bestimmt. Erste Untersuchungen an magnetisch-ferroelektrischen Übergittern zeigen, dass die Anregungsfluenz vorsichtig eingestellt werden muss, um die Dynamik an Phasenübergängen zu untersuchen.
KW - coherent phonons
KW - phonon dynamics
KW - time resolved
KW - Brillouin scattering
KW - perovskite oxides
KW - optical spectroscopy
KW - hypersound propagation
KW - phonon damping
KW - domain wall motion
KW - phonon backfolding
KW - superlattice dispersion
KW - kohärente Phononen
KW - Phononen Dynamik
KW - zeitaufgelöst
KW - Brillouin Streuung
KW - Perowskit-Oxide
KW - optische Spektroskopie
KW - Hyperschall Propagation
KW - Phononen Dämpfung
KW - Domänenwandbewegung
KW - Phononen Rückfaltung
KW - Übergitter Dispersion
Y1 - 2015
U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-77623
ER -
TY - GEN
A1 - Lomadze, Nino
A1 - Kopyshev, Alexey
A1 - Bargheer, Matias
A1 - Wollgarten, Markus
A1 - Santer, Svetlana
T1 - Mass production of polymer nanowires filled with metal nanoparticles
N2 - Despite the ongoing progress in nanotechnology and its applications, the development of strategies for connecting nano-scale systems to micro- or macroscale elements is hampered by the lack of structural components that have both, nano- and macroscale dimensions. The production of nano-scale wires with macroscale length is one of the most interesting challenges here. There are a lot of strategies to fabricate long nanoscopic stripes made of metals, polymers or ceramics but none is suitable for mass production of ordered and dense arrangements of wires at large numbers. In this paper, we report on a technique for producing arrays of ordered, flexible and free-standing polymer nano-wires filled with different types of nano-particles. The process utilizes the strong response of photosensitive polymer brushes to irradiation with UV-interference patterns, resulting in a substantial mass redistribution of the polymer material along with local rupturing of polymer chains. The chains can wind up in wires of nano-scale thickness and a length of up to several centimeters. When dispersing nano-particles within the film, the final arrangement is similar to a core-shell geometry with mainly nano-particles found in the core region and the polymer forming a dielectric jacket.
T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 387
Y1 - 2017
U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-402712
ER -
TY - JOUR
A1 - Shayduk, Roman
A1 - Hallmann, Jörg
A1 - Rodriguez-Fernandez, Angel
A1 - Scholz, Markus
A1 - Lu, Wei
A1 - Bösenberg, Ulrike
A1 - Möller, Johannes
A1 - Zozulya, Alexey
A1 - Jiang, Man
A1 - Wegner, Ulrike
A1 - Secareanu, Radu-Costin
A1 - Palmer, Guido
A1 - Emons, Moritz
A1 - Lederer, Max
A1 - Volkov, Sergey
A1 - Lindfors-Vrejoiu, Ionela
A1 - Schick, Daniel
A1 - Herzog, Marc
A1 - Bargheer, Matias
A1 - Madsen, Anders
T1 - Femtosecond x-ray diffraction study of multi-THz coherent phonons in SrTiO3
JF - Applied physics letters
N2 - We report generation of ultra-broadband longitudinal acoustic coherent phonon wavepackets in SrTiO3 (STO) with frequency components extending throughout the first Brillouin zone. The wavepackets are efficiently generated in STO using femtosecond infrared laser excitation of an atomically flat 1.6 nm-thick epitaxial SrRuO3 film. We use femtosecond x-ray diffraction at the European X-Ray Free Electron Laser Facility to study the dispersion and damping of phonon wavepackets. The experimentally determined damping constants for multi-THz frequency phonons compare favorably to the extrapolation of a simple ultrasound damping model over several orders of magnitude.
Y1 - 2022
U6 - https://doi.org/10.1063/5.0083256
SN - 0003-6951
SN - 1077-3118
VL - 120
IS - 20
PB - AIP Publishing
CY - Melville
ER -
TY - JOUR
A1 - Schick, Daniel
A1 - Herzog, Marc
A1 - Wen, Haidan
A1 - Chen, Pice
A1 - Adamo, Carolina
A1 - Gaal, Peter
A1 - Schlom, Darrell G.
A1 - Evans, Paul G.
A1 - Li, Yuelin
A1 - Bargheer, Matias
T1 - Localized excited charge carriers generate ultrafast inhomogeneous strain in the multiferroic BiFeO3
JF - Physical review letters
N2 - We apply ultrafast x-ray diffraction with femtosecond temporal resolution to monitor the lattice dynamics in a thin film of multiferroic BiFeO3 after above-band-gap photoexcitation. The sound-velocity limited evolution of the observed lattice strains indicates a quasi-instantaneous photoinduced stress which decays on a nanosecond time scale. This stress exhibits an inhomogeneous spatial profile evidenced by the broadening of the Bragg peak. These new data require substantial modification of existing models of photogenerated stresses in BiFeO3: the relevant excited charge carriers must remain localized to be consistent with the data.
Y1 - 2014
U6 - https://doi.org/10.1103/PhysRevLett.112.097602
SN - 0031-9007
SN - 1079-7114
VL - 112
IS - 9
PB - American Physical Society
CY - College Park
ER -
TY - JOUR
A1 - Herzog, Marc
A1 - Schick, Daniel
A1 - Leitenberger, Wolfram
A1 - Shayduk, Roman
A1 - van der Veen, Renske M.
A1 - Milne, Christopher J.
A1 - Johnson, Steven Lee
A1 - Vrejoiu, Ionela
A1 - Bargheer, Matias
T1 - Tailoring interference and nonlinear manipulation of femtosecond x-rays
JF - New journal of physics : the open-access journal for physics
N2 - We present ultrafast x-ray diffraction (UXRD) experiments on different photoexcited oxide superlattices. All data are successfully simulated by dynamical x-ray diffraction calculations based on a microscopic model, that accounts for the linear response of phonons to the excitation laser pulse. Some Bragg reflections display a highly nonlinear strain dependence. The origin of linear and two distinct nonlinear response phenomena is discussed in a conceptually simpler model using the interference of envelope functions that describe the diffraction efficiency of the average constituent nanolayers. The combination of both models facilitates rapid and accurate simulations of UXRD experiments.
Y1 - 2012
U6 - https://doi.org/10.1088/1367-2630/14/1/013004
SN - 1367-2630
VL - 14
IS - 1
PB - IOP Publ. Ltd.
CY - Bristol
ER -
TY - JOUR
A1 - Schick, Daniel
A1 - Bojahr, Andre
A1 - Herzog, Marc
A1 - von Korff Schmising, Clemens
A1 - Shayduk, Roman
A1 - Leitenberger, Wolfram
A1 - Gaa, P.
A1 - Bargheer, Matias
T1 - Normalization schemes for ultrafast x-ray diffraction using a table-top laser-driven plasma source
JF - Review of scientific instruments : a monthly journal devoted to scientific instruments, apparatus, and techniques
N2 - We present an experimental setup of a laser-driven x-ray plasma source for femtosecond x-ray diffraction. Different normalization schemes accounting for x-ray source intensity fluctuations are discussed in detail. We apply these schemes to measure the temporal evolution of Bragg peak intensities of perovskite superlattices after ultrafast laser excitation.
Y1 - 2012
U6 - https://doi.org/10.1063/1.3681254
SN - 0034-6748
VL - 83
IS - 2
PB - American Institute of Physics
CY - Melville
ER -
TY - THES
A1 - Pudell, Jan-Etienne
T1 - Lattice dynamics
T1 - Gitterdynamik
BT - observed with x-ray diffraction
BT - beobachtet mit Röntgenbeugung
N2 - In this thesis I summarize my contribution to the research field of ultrafast structural dynamics in condensend matter. It consists of 17 publications that cover the complex interplay between electron, magnon, and phonon subsystems in solid materials and the resulting lattice dynamics after ultrafast photoexcitation. The investigation of such dynamics is necessary for the physical understanding of the processes in materials that might become important in the future as functional materials for technological applications, for example in data storage applications, information processing, sensors, or energy harvesting.
In this work I present ultrafast x-ray diffraction (UXRD) experiments based on the optical pump – x-ray probe technique revealing the time-resolved lattice strain. To study these dynamics the samples (mainly thin film heterostructures) are excited by femtosecond near-infrared or visible light pulses. The induced strain dynamics caused by stresses of the excited subsystems are measured in a pump-probe scheme with x-ray diffraction (XRD) as a probe. The UXRD setups used during my thesis are a laser-driven table-top x-ray source and large-scale synchrotron facilities with dedicated time-resolved diffraction setups. The UXRD experiments provide quantitative access to heat reservoirs in nanometric layers and monitor the transient responses of these layers with coupled electron, magnon, and phonon subsystems. In contrast to optical probes, UXRD allows accessing the material-specific information, which is unavailable for optical light due to the detection of multiple indistinguishable layers in the range of the penetration depth.
In addition, UXRD facilitates a layer-specific probe for layers buried opaque heterostructures to study the energy flow. I extended this UXRD technique to obtain the driving stress profile by measuring the strain dynamics in the unexcited buried layer after excitation of the adjacent absorbing layers with femtosecond laser pulses. This enables the study of negative thermal expansion (NTE) in magnetic materials, which occurs due to the loss of the magnetic order. Part of this work is the investigation of stress profiles which are the source of coherent acoustic phonon wave packets (hypersound waves). The spatiotemporal shape of these stress profiles depends on the energy distribution profile and the ability of the involved subsystems to produce stress. The evaluation of the UXRD data of rare-earth metals yields a stress profile that closely matches the optical penetration profile: In the paramagnetic (PM) phase the photoexcitation results in a quasi-instantaneous expansive stress of the metallic layer whereas in the antiferromagnetic (AFM) phase a quasi-instantaneous contractive stress and a second contractive stress contribution rising on a 10 ps time scale adds to the PM contribution. These two time scales are characteristic for the magnetic contribution and are in agreement with related studies of the magnetization dynamics of rare-earth materials.
Several publications in this thesis demonstrate the scientific progress in the field of active strain control to drive a second excitation or engineer an ultrafast switch. These applications of ultrafast dynamics are necessary to enable control of functional material properties via strain on ultrafast time scales.
For this thesis I implemented upgrades of the existing laser-driven table-top UXRD setup in order to achieve an enhancement of x-ray flux to resolve single digit nanometer thick layers. Furthermore, I developed and built a new in-situ time-resolved magneto-optic Kerr effect (MOKE) and optical reflectivity setup at the laser-driven table-top UXRD setup to measure the dynamics of lattice, electrons and magnons under the same excitation conditions.
N2 - In dieser Doktoarbeit sind meine Beiträge zum Forschungsgebiet der ltraschnellen Strukturdynamik in kondensierter Materie zusammegefasst. Sie besteht aus 17 Publikationen, welche dieWechselwirkung zwischen Elektron-, Magnon- und Phononsystem in Festkörpern, sowie die dadurch verursachte Gitterdynamik nach ultraschneller optischer Anregung diskutieren. Die Untersuchung dieser Dynamik ist erforderlich für das physikalische Verständnis der Prozesse in Materialien, die in Zukunft als Funktionsmaterialien für technologische Anwendungen, z.B. in der Datenspeicherung und Informationsverarbeitung, sowie bei Sensoren und der Energiegewinnung, wichtig werden könnten.
In dieser Arbeit präsentiere ich Experimente, welche ultraschneller Röntgenbeugung (UXRD) als Technik nutzen. Sie basiert auf der Anrege-Abfrage-Technik: Die Dynamik in der Probe (hauptsächlich Dünnfilm-Heterostrukturen) wird durch Femtosekunden-Lichtpulse im nahen Infrarot oder im sichtbaren Bereich angeregt. Die Dehnung des Materials, welche die Spannung (Druck) der angeregten Teilsysteme hervorruft, wird mit Röntgenbeugung als Abfrage gemessen. Während meiner Doktorandentätigkeit habe ich zwei Arten von Aufbauten zur UXRD genutzt: lasergetriebene laborbasierte Röntgenquellen und Synchrotronstrahlungsquellen mit zugehörigen zeitaufgelösten Messinstrumenten. Mit den UXRD-Experimenten kann die gespeicherte Energie unterschiedlicher gekoppelter Teilsysteme, wie Elektronen, Mangonen und Phononen, einer nur wenige Nanometer dicken Schicht gemessen werden. Im Vergleich zu optischenMesstechniken bietet UXRD den Zugriff auf materialspezifische Informationen, die für optisches Licht aufgrund der Detektion mehrerer nicht unterscheidbarer Schichten im Bereich der Eindringtiefe nicht zur Verfügung steht.
Darüber hinaus lässt sich mit UXRD eine für optische Detektion verdeckte Schicht als schichtspezifische Sonde nutzen, um den Energietransport zu untersuchen. Dieses Prinzip wurde dazu genutzt, um das treibende Spannungsprofil mittels der Dehnungsdynamik in einer angrenzenden, optisch nicht angeregten Schicht zu messen. Dies ermöglichte die Untersuchung der Dichteanomalie in magnetischen Materialen, die durch den Verlust der magnetischen Ordnung entsteht. Ebenfalls Teil dieser Arbeit ist die Untersuchung von Spannungsprofilen als Quelle von kohärenten akustischen Phononen (Hyperschallwellen). Das raumzeitliche Profil des Spannungsprofils hängt von der Energieverteilung innerhalb der Teilsysteme und ihrer Fähigkeit ab, Energie in Dehnung umzusetzen. Die Auswertung von UXRD Experimenten an Metallen der Seltenen Erden ergab ein Spannungsprofil, dass dem Absorptionsprofil der optischen Anregung entsprach: In der paramagnetischen Phase erzeugte es einen instantanen expansiven Druck, wohingegen in der antiferromagnetischen Phase ein instantaner und ein auf einer 10 ps Zeitskala ansteigender kontrahierender Druck zusätzlich auftritt. Die beiden charakteristischen Zeitskalen in der antiferromagnetischen Phase sind in Übereinstimmung mit verschiedenen Studien der Demagnetisierungsdynamik in den Metallen der Seltenen Erden.
Einige Publikationen dieser Arbeit beschäftigen sich mit Feld der aktiven Dehnungskontrolle. Dies ermöglich die Kontrolle von Funktionsmaterialen via Dehnung auf ultraschnellen Zeitskalen.
Im Rahmen meiner Doktorandentätigkeit habe ich den lasergetriebenen UXRD Aufbau optimiert, um mit dem hohen Röntgenfluss Experimente mit nur einigen Nanometer dicken Schichten zu ermöglichen. Diese Maschine habe ich um einen zeitaufgelösten Aufbau zur in situ Messung der Reflektivität und Magnetisierungsdynamik mittels magnetooptischem Kerr-Effekt ergänzt. Dies ermöglicht die gleichzeitige Messung von Gitter-, Elektronen- und Magnonendynamik unter derselben Anregebedingung.
KW - ultrafast x-ray diffraction
KW - ultraschnelle Röntgendiffraktion
KW - lattice dynamics
KW - Gitterdynamik
KW - nanoscale heat transfer
KW - nanoskaliger Wärmetransport
KW - ultrafast magnetism
KW - ultraschneller Magnetimus
KW - mechanical and acoustical properties
KW - mechanische und akustische Eigenschaften
Y1 - 2020
U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-484453
ER -
TY - JOUR
A1 - Deb, Marwan
A1 - Popova, Elena
A1 - Jaffrès, Henri-Yves
A1 - Keller, Niels
A1 - Bargheer, Matias
T1 - Polarization-dependent subpicosecond demagnetization in iron garnets
JF - Physical review : B, covering condensed matter and materials physics
N2 - Controlling the magnetization dynamics at the fastest speed is a major issue of fundamental condensed matter physics and its applications for data storage and processing technologies. It requires a deep understanding of the interactions between the degrees of freedom in solids, such as spin, electron, and lattice as well as their responses to external stimuli. In this paper, we systematically investigate the fluence dependence of ultrafast magnetization dynamics induced by below-bandgap ultrashort laser pulses in the ferrimagnetic insulators BixY3-xFe5O12 with 1 xBi 3. We demonstrate subpicosecond demagnetization dynamics in this material followed by a very slow remagnetization process. We prove that this demagnetization results from an ultrafast heating of iron garnets by two-photon absorption (TPA), suggesting a phonon-magnon thermalization time of 0.6 ps. We explain the slow remagnetization timescale by the low phonon heat conductivity in garnets. Additionally, we show that the amplitudes of the demagnetization, optical change, and lattice strain can be manipulated by changing the ellipticity of the pump pulses. We explain this phenomenon considering the TPA circular dichroism. These findings open exciting prospects for ultrafast manipulation of spin, charge, and lattice dynamics in magnetic insulators by ultrafast nonlinear optics.
Y1 - 2022
U6 - https://doi.org/10.1103/PhysRevB.106.184416
SN - 2469-9950
SN - 2469-9969
VL - 106
IS - 18
PB - American Institute of Physics, American Physical Society
CY - Woodbury, NY
ER -
TY - JOUR
A1 - Deb, Marwan
A1 - Popova, Elena
A1 - Jaffrès, Henri-Yves
A1 - Keller, Niels
A1 - Bargheer, Matias
T1 - Controlling high-frequency spin-wave dynamics using double-pulse laser excitation
JF - Physical review applied
N2 - Manipulating spin waves is highly required for the development of innovative data transport and processing technologies. Recently, the possibility of triggering high-frequency standing spin waves in magnetic insulators using femtosecond laser pulses was discovered, raising the question about how one can manipulate their dynamics. Here we explore this question by investigating the ultrafast magnetiza-tion and spin-wave dynamics induced by double-pulse laser excitation. We demonstrate a suppression or enhancement of the amplitudes of the standing spin waves by precisely tuning the time delay between the two pulses. The results can be understood as the constructive or destructive interference of the spin waves induced by the first and second laser pulses. Our findings open exciting perspectives towards generating single-mode standing spin waves that combine high frequency with large amplitude and low magnetic damping.
Y1 - 2022
U6 - https://doi.org/10.1103/PhysRevApplied.18.044001
SN - 2331-7019
VL - 18
IS - 4
PB - American Physical Society
CY - College Park
ER -
TY - GEN
A1 - Stete, Felix
A1 - Koopman, Wouter-Willem Adriaan
A1 - Bargheer, Matias
T1 - Signatures of strong coupling on nanoparticles
BT - revealing absorption anticrossing by tuning the dielectric environment
T2 - Quantum Nano-Photonics
N2 - The electromagnetic coupling of molecular excitations to plasmonic nanoparticles offers a promising method to manipulate the light-matter interaction at the nanoscale. Plasmonic nanoparticles foster exceptionally high coupling strengths, due to their capacity to strongly concentrate the light-field to sub-wavelength mode volumes. A particularly interesting coupling regime occurs, if the coupling increases to a level such that the coupling strength surpasses all damping rates in the system. In this so-called strong-coupling regime hybrid light-matter states emerge, which can no more be divided into separate light and matter components. These hybrids unite the features of the original components and possess new resonances whose positions are separated by the Rabi splitting energy h Omega. Detuning the resonance of one of the components leads to an anticrossing of the two arising branches of the new resonances omega(+) and omega(-) with a minimal separation of Omega = omega(+) - omega(-).
Y1 - 2018
SN - 978-94-024-1546-9
SN - 978-94-024-1544-5
SN - 978-94-024-1543-8
U6 - https://doi.org/10.1007/978-94-024-1544-5_53
SN - 1871-465X
SP - 445
EP - 447
PB - Springer
CY - Dordrecht
ER -
TY - GEN
A1 - Stete, Felix
A1 - Schossau, Phillip Gerald
A1 - Koopman, Wouter-Willem Adriaan
A1 - Bargheer, Matias
T1 - Size Dependence of the Coupling Strength in Plasmon-Exciton Nanoparticles
T2 - Quantum Nano-Photonics
N2 - The coupling between molecular excitations and nanoparticles leads to promising applications. It is for example used to enhance the optical cross-section of molecules in surface enhanced Raman scattering, Purcell enhancement or plasmon enhanced dye lasers. In a coupled system new resonances emerge resulting from the original plasmon (ωpl) and exciton (ωex) resonances as
ω±=12(ωpl+ωex)±14(ωpl−ωex)2+g2−−−−−−−−−−−−−−−√,
(1)
where g is the coupling parameter. Hence, the new resonances show a separation of Δ = ω+ − ω− from which the coupling strength can be deduced from the minimum distance between the two resonances, Ω = Δ(ω+ = ω−).
Y1 - 2018
SN - 978-94-024-1546-9
SN - 978-94-024-1544-5
SN - 978-94-024-1543-8
U6 - https://doi.org/10.1007/978-94-024-1544-5_26
SN - 1871-465X
SP - 381
EP - 383
PB - Springer
CY - Dordrecht
ER -
TY - THES
A1 - Haseeb, Haider
T1 - Charge and heat transport across interfaces in nanostructured porous silicon
T1 - Ladungs- und Wärmetransport über Grenzflächen in nanostrukturiertem porösem Silizium
N2 - This thesis discusses heat and charge transport phenomena in single-crystalline Silicon penetrated by nanometer-sized pores, known as mesoporous Silicon (pSi). Despite the extensive attention given to it as a thermoelectric material of interest, studies on microscopic thermal and electronic transport beyond its macroscopic characterizations are rarely reported. In contrast, this work reports the interplay of both.
PSi samples synthesized by electrochemical anodization display a temperature dependence of specific heat 𝐶𝑝 that deviates from the characteristic 𝑇^3 behaviour (at 𝑇<50𝐾). A thorough analysis reveals that both 3D and 2D Einstein and Debye modes contribute to this specific heat. Additional 2D Einstein modes (~3 𝑚𝑒𝑉) agree reasonably well with the boson peak of SiO2 in pSi pore walls. 2D Debye modes are proposed to account for surface acoustic modes causing a significant deviation from the well-known 𝑇^3 dependence of 𝐶𝑝 at 𝑇<50𝐾.
A novel theoretical model gives insights into the thermal conductivity of pSi in terms of porosity and phonon scattering on the nanoscale. The thermal conductivity analysis utilizes the peculiarities of the pSi phonon dispersion probed by the inelastic neutron scattering experiments. A phonon mean-free path of around 10 𝑛𝑚 extracted from the presented model is proposed to cause the reduced thermal conductivity of pSi by two orders of magnitude compared to p-doped bulk Silicon. Detailed analysis indicates that compound averaging may cause a further 10-50% reduction. The percolation threshold of 65% for thermal conductivity of pSi samples is subsequently determined by employing theoretical effective medium models.
Temperature-dependent electrical conductivity measurements reveal a thermally activated transport process. A detailed analysis of the activation energy 𝐸𝐴𝜎 in the thermally activated transport exhibits a Meyer Neldel compensation rule between different samples that originates in multi-phonon absorption upon carrier transport. Activation energies 𝐸𝐴𝑆 obtained from temperature-dependent thermopower measurements provide further evidence for multi-phonon assisted hopping between localized states as a dominant charge transport mechanism in pSi, as they systematically differ from the determined 𝐸𝐴𝜎 values.
N2 - Diese Dissertation befasst sich mit Wärme- und Ladungstransportphänomenen in mesoporösem Silizium (pSi) oder etwas genauer in einkristallinem Silizium, welches mit nanometergroßen Poren durchsetzt ist. Trotz der großen Aufmerksamkeit, die diesem thermoelektrischen Material zuteil wird, wird nur selten über Studien zum mikroskopischen thermischen und elektronischen Transport jenseits seiner makroskopischen Charakterisierung berichtet. Im Gegensatz dazu wird in dieser Studie das Zusammenspiel von beidem untersucht.
PSi-Proben, die durch elektrochemische Anodisierung synthetisiert wurden, zeigen eine Temperaturabhängigkeit der spezifischen Wärme 𝐶𝑝, die vom charakteristischen 𝑇3 Verhalten (bei 𝑇<50𝐾) abweicht. Eine gründliche Analyse zeigt, dass sowohl 3D- als auch 2D-Einstein- und Debye-Moden zu dieser spezifischen Wärme beitragen. Zusätzliche 2D-Einstein-Moden (~3 𝑚𝑒𝑉) stimmen gut mit dem Bosonen-Peak von SiO2 in teilweise oxidierten pSi-Porenwänden überein. 2D-Debye-Moden werden vorgeschlagen, um akustische Oberflächenmoden zu erklären, die eine signifikante Abweichung von der bekannten 𝑇3Abhängigkeit von 𝐶𝑝 bei 𝑇<50𝐾 verursachen.
Ein neuartiges theoretisches Modell gibt Einblicke in die Wärmeleitfähigkeit von pSi in Bezug auf Porosität und Phononenstreuung auf der Nanoskala. Die Analyse der Wärmeleitfähigkeit nutzt die Besonderheiten der pSi-Phononendispersion, die durch Experimente mit inelastischer Neutronenstreuung untersucht wurden. Ein mittlerer freier Weg der Phononen von etwa 10 𝑛𝑚, der aus dem vorgestellten Modell extrahiert wurde, wird als Ursache für die um zwei Größenordnungen geringere Wärmeleitfähigkeit von pSi im Vergleich zu p-dotiertem Silizium vorgeschlagen. Eine detaillierte Analyse zeigt, dass die Porosität selbst eine weitere Verringerung der Wärmeleitfähigkeit um 10-50% verursachen kann. Die Perkolationsschwelle von 65 % für die Wärmeleitfähigkeit von pSi-Proben wird anschließend mit Hilfe eines theoretischen Ansatzes für effektive Medien bestimmt.
Temperaturabhängige Messungen der elektrischen Leitfähigkeit lassen einen thermisch aktivierten Transportprozess erkennen. Eine detaillierte Analyse der Aktivierungsenergie 𝐸𝐴𝜎 im thermisch aktivierten Transport zeigt eine Meyer-Neldel-Kompensationsregel zwischen verschiedenen Proben, die auf Multiphononenabsorption beim Ladungsträgertransport zurückzuführen ist. Aktivierungsenergien 𝐸𝐴𝑆, die aus temperaturabhängigen Seebeck-Messungen gewonnen wurden, liefern weitere Beweise für Multiphononen-unterstütztes Springen zwischen lokalisierten Zuständen als dominanten Ladungstransportmechanismus in pSi, da sie sich systematisch von den ermittelten 𝐸𝐴𝜎 Werten unterscheiden.
KW - mesoporous
KW - silicon
KW - Meyer-Neldel-rule
KW - nanomaterials
KW - Meyer-Neldel-Regel
KW - mesoporös
KW - Nanomaterialien
KW - Silizium
Y1 - 2023
U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-611224
ER -
TY - JOUR
A1 - Deb, Marwan
A1 - Popova, Elena
A1 - Hehn, Michel
A1 - Keller, Niels
A1 - Petit-Watelot, Sebastien
A1 - Bargheer, Matias
A1 - Mangin, Stephane
A1 - Malinowski, Gregory
T1 - Femtosecond Laser-Excitation-Driven High Frequency Standing Spin Waves in Nanoscale Dielectric Thin Films of Iron Garnets
JF - Physical review letters
N2 - We demonstrate that femtosecond laser pulses allow triggering high-frequency standing spin-wave modes in nanoscale thin films of a bismuth-substituted yttrium iron garnet. By varying the strength of the external magnetic field, we prove that two distinct branches of the dispersion relation are excited for all the modes. This is reflected in particular at a very weak magnetic field (similar to 33 mT) by a spin dynamics with a frequency up to 15 GHz, which is 15 times higher than the one associated with the ferromagnetic resonance mode. We argue that this phenomenon is triggered by ultrafast changes of the magnetic anisotropy via laser excitation of incoherent and coherent phonons. These findings open exciting prospects for ultrafast photo magnonics.
Y1 - 2019
U6 - https://doi.org/10.1103/PhysRevLett.123.027202
SN - 0031-9007
SN - 1079-7114
VL - 123
IS - 2
PB - American Physical Society
CY - College Park
ER -
TY - JOUR
A1 - Sarhan, Radwan Mohamed
A1 - Koopman, Wouter-Willem Adriaan
A1 - Schuetz, Roman
A1 - Schmid, Thomas
A1 - Liebig, Ferenc
A1 - Koetz, Joachim
A1 - Bargheer, Matias
T1 - The importance of plasmonic heating for the plasmondriven photodimerization of 4-nitrothiophenol
JF - Scientific Reports
N2 - Metal nanoparticles form potent nanoreactors, driven by the optical generation of energetic electrons and nanoscale heat. The relative influence of these two factors on nanoscale chemistry is strongly debated. This article discusses the temperature dependence of the dimerization of 4-nitrothiophenol (4-NTP) into 4,4′-dimercaptoazobenzene (DMAB) adsorbed on gold nanoflowers by Surface-Enhanced Raman Scattering (SERS). Raman thermometry shows a significant optical heating of the particles. The ratio of the Stokes and the anti-Stokes Raman signal moreover demonstrates that the molecular temperature during the reaction rises beyond the average crystal lattice temperature of the plasmonic particles. The product bands have an even higher temperature than reactant bands, which suggests that the reaction proceeds preferentially at thermal hot spots. In addition, kinetic measurements of the reaction during external heating of the reaction environment yield a considerable rise of the reaction rate with temperature. Despite this significant heating effects, a comparison of SERS spectra recorded after heating the sample by an external heater to spectra recorded after prolonged illumination shows that the reaction is strictly photo-driven. While in both cases the temperature increase is comparable, the dimerization occurs only in the presence of light. Intensity dependent measurements at fixed temperatures confirm this finding.
KW - enhanced raman-scattering
KW - charge-transfer
KW - metal
KW - nanoparticles
KW - catalysis
KW - AU
KW - 4-nitrobenzenethiol
KW - aminothiophenol
KW - photocatalysis
KW - wavelength
Y1 - 2019
U6 - https://doi.org/10.1038/s41598-019-38627-2
SN - 2045-2322
VL - 9
PB - Macmillan Publishers Limited
CY - London
ER -
TY - JOUR
A1 - Iurchuk, V.
A1 - Schick, D.
A1 - Bran, J.
A1 - Colson, D.
A1 - Forget, A.
A1 - Halley, D.
A1 - Koc, Azize
A1 - Reinhardt, Mathias
A1 - Kwamen, C.
A1 - Morley, N. A.
A1 - Bargheer, Matias
A1 - Viret, M.
A1 - Gumeniuk, R.
A1 - Schmerber, G.
A1 - Doudin, B.
A1 - Kundys, B.
T1 - Optical Writing of Magnetic Properties by Remanent Photostriction
JF - Physical review letters
N2 - We present an optically induced remanent photostriction in BiFeO3, resulting from the photovoltaic effect, which is used to modify the ferromagnetism of Ni film in a hybrid BiFeO3/Ni structure. The 75% change in coercivity in the Ni film is achieved via optical and nonvolatile control. This photoferromagnetic effect can be reversed by static or ac electric depolarization of BiFeO3. Hence, the strain dependent changes in magnetic properties are written optically, and erased electrically. Light-mediated straintronics is therefore a possible approach for low-power multistate control of magnetic elements relevant for memory and spintronic applications.
Y1 - 2016
U6 - https://doi.org/10.1103/PhysRevLett.117.107403
SN - 0031-9007
SN - 1079-7114
VL - 117
PB - American Physical Society
CY - College Park
ER -
TY - JOUR
A1 - Shayduk, Roman
A1 - Herzog, Marc
A1 - Bojahr, Andre
A1 - Schick, Daniel
A1 - Gaal, Peter
A1 - Leitenberger, Wolfram
A1 - Navirian, Hengameh
A1 - Sander, Mathias
A1 - Goldshteyn, Jevgenij
A1 - Vrejoiu, Ionela
A1 - Bargheer, Matias
T1 - Direct time-domain sampling of subterahertz coherent acoustic phonon spectra in SrTiO3 using ultrafast x-ray diffraction
JF - Physical review : B, Condensed matter and materials physics
N2 - We synthesize sub-THz longitudinal quasimonochromatic acoustic phonons in a SrTiO3 single crystal using a SrRuO3/SrTiO3 superlattice as an optical-acoustic transducer. The generated acoustic phonon spectrum is determined using ultrafast x-ray diffraction. The analysis of the generated phonon spectrum in the time domain reveals a k-vector dependent phonon lifetime. It is observed that even at sub-THz frequencies the phonon lifetime agrees with the 1/omega(2) power law known from Akhiezer's model for hyper sound attenuation. The observed shift of the synthesized spectrum to the higher q is discussed in the framework of nonlinear effects appearing due to the high amplitude of the synthesized phonons.
Y1 - 2013
U6 - https://doi.org/10.1103/PhysRevB.87.184301
SN - 1098-0121
VL - 87
IS - 18
PB - American Physical Society
CY - College Park
ER -
TY - JOUR
A1 - Reinhardt, Matthias
A1 - Koc, Azize
A1 - Leitenberger, Wolfram
A1 - Gaal, Peter
A1 - Bargheer, Matias
T1 - Optimized spatial overlap in optical pump-X-ray probe experiments with high repetition rate using laser-induced surface distortions
JF - Journal of synchrotron radiation
N2 - Ultrafast X-ray diffraction experiments require careful adjustment of the spatial overlap between the optical excitation and the X-ray probe pulse. This is especially challenging at high laser repetition rates. Sample distortions caused by the large heat load on the sample and the relatively low optical energy per pulse lead to only tiny signal changes. In consequence, this results in small footprints of the optical excitation on the sample, which turns the adjustment of the overlap difficult. Here a method for reliable overlap adjustment based on reciprocal space mapping of a laser excited thin film is presented.
KW - ultrafast X-ray diffraction
KW - nanostructures
KW - surface deformation
KW - heat diffusion
KW - optical pump
Y1 - 2016
U6 - https://doi.org/10.1107/S1600577515024443
SN - 1600-5775
VL - 23
SP - 474
EP - 479
PB - International Union of Crystallography
CY - Chester
ER -
TY - THES
A1 - Tchoumba Kwamen, Christelle Larodia
T1 - Investigating the dynamics of polarization reversal in ferroelectric thin films by time-resolved X-ray diffraction
T1 - Untersuchung der Dynamik der Polarisationsumkehr in ferroelektrischen Dünnschichten durch zeitaufgelöste Röntgenbeugung
N2 - Ferroic materials have attracted a lot of attention over the years due to their wide range of applications in sensors, actuators, and memory devices. Their technological applications originate from their unique properties such as ferroelectricity and piezoelectricity. In order to optimize these materials, it is necessary to understand the coupling between their nanoscale structure and transient response, which are related to the atomic structure of the unit cell.
In this thesis, synchrotron X-ray diffraction is used to investigate the structure of ferroelectric thin film capacitors during application of a periodic electric field. Combining electrical measurements with time-resolved X-ray diffraction on a working device allows for visualization of the interplay between charge flow and structural motion. This constitutes the core of this work. The first part of this thesis discusses the electrical and structural dynamics of a ferroelectric Pt/Pb(Zr0.2,Ti0.8)O3/SrRuO3 heterostructure during charging, discharging, and polarization reversal. After polarization reversal a non-linear piezoelectric response develops on a much longer time scale than the RC time constant of the device. The reversal process is inhomogeneous and induces a transient disordered domain state. The structural dynamics under sub-coercive field conditions show that this disordered domain state can be remanent and can be erased with an appropriate voltage pulse sequence. The frequency-dependent dynamic characterization of a Pb(Zr0.52,Ti0.48)O3 layer, at the morphotropic phase boundary, shows that at high frequency, the limited domain wall velocity causes a phase lag between the applied field and both the structural and electrical responses. An external modification of the RC time constant of the measurement delays the switching current and widens the electromechanical hysteresis loop while achieving a higher compressive piezoelectric strain within the crystal.
In the second part of this thesis, time-resolved reciprocal space maps of multiferroic BiFeO3 thin films were measured to identify the domain structure and investigate the development of an inhomogeneous piezoelectric response during the polarization reversal. The presence of 109° domains is evidenced by the splitting of the Bragg peak.
The last part of this work investigates the effect of an optically excited ultrafast strain or heat pulse propagating through a ferroelectric BaTiO3 layer, where we observed an additional current response due to the laser pulse excitation of the metallic bottom electrode of the heterostructure.
N2 - Ferroika haben aufgrund vielfältiger Anwendungsmöglichkeiten in Sensoren, Motoren und Speichermedien in den letzten Jahren viel Aufmerksamkeit erhalten. Das Interesse für technologische Anwendungen ist in ihren einzigartigen Eigenschaften wie Ferroelektrizität und Piezoelektrizität begründet. Um die Eigenschaften dieser Materialien zu optimieren ist es notwendig, die Kopplung zwischen ihrer Nanostruktur und der zeitabhängigen Antwort auf die Anregung zu verstehen, welcher von der Atomstruktur der Einheitszelle abhängig ist.
In dieser Arbeit wird Röntgenbeugung an einem Synchrotron verwendet, um die Struktur eines ferroelektrischen Dünnschichtkondensators während eines angelegten elektrischen Feld zu beobachten. Den Kern dieser Arbeit bildet die Kombination aus elektrischen zeitaufgelösten Röntgenbeugungsmessungen an einem betriebsfähigen Kondensator, was die Visualisierung des Zusammenspiels zwischen Ladungsbewegung und Strukturdynamik ermöglicht. Der erste Teil der Arbeit befasst sich mit der elektrischen und strukturellen Dynamik einer ferroelektrischen Pt/Pb(Zr0.2,Ti0.8)O3/SrRuO3 Heterostruktur während des Ladens, Entladens und der Polarisationsumkehr. Nach der Umkehr der Polarisation bildet sich auf einer längeren Zeitskala als die RC-Zeitkonstante der Probe ein nichtlineares piezoelektrisches Signal aus. Der Umkehrungsprozess ist inhomogen und induziert einen vorübergehenden Zustand ungeordneter Domänen. Die strukturelle Dynamik mit einem angelegten elektrischen Feld unterhalb des Koerzitivfelds zeigt, dass dieser ungeordnete Zustand remanent sein kann und mit einer entsprechenden Abfolge von Spannungspulsen wieder entfernt werden kann. Die frequenzabhängige Charakterisierung der Dynamik einer Pb(Zr0.52,Ti0.48)O3 Schicht mit einer Zusammensetzung, die der morphotropen Phasengrenze entspricht, zeigt, dass bei hohen Frequenzen die begrenzte Domänenwandgeschwindigkeit eine Phasenverzögerung zwischen dem angelegten Feld und dem strukturellen sowie dem elektrischen Signal verursacht. Eine externe Änderung der RC-Zeitkonstante verzögert den Schaltstrom und verbreitert die elektromechanische Hysteresekurve, während im Kristall eine höhere kompressive piezoeletrische Spannung erzeugt wird.
In dem zweiten Teil dieser Arbeit wurde der reziproke Raum von multiferroischen dünnen BiFeO3 Filmen vermessen, um die Domänenstruktur zu identifizieren und die Entwicklung eines inhomogenen piezoelektrischen Signals während der Polarisationsumkehr zu untersuchen. Das Aufspalten des Bragg Reflexes ist ein Hinweis auf die Existenz von 109° Domänen.
Der letzte Teil der Arbeit beschäftigt sich mit dem Effekt, den ein durch optische Anregung erzeugter ultraschneller Verspannungs- oder Wärmepuls hervorruft, der durch eine ferroelektrische BaTiO3 Schicht propagiert. Dabei wurde durch die Anregung der unteren metallischen Elektrode der Heterostruktur durch den Laserpuls ein zusätzliches Ladungssignal beobachtet.
KW - ferroelectrics
KW - X-ray diffraction
KW - structural dynamics
KW - Ferroelektrika
KW - Röntgenbeugung
KW - Strukturdynamik
Y1 - 2019
U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-427815
ER -
TY - JOUR
A1 - Deb, Marwan
A1 - Popova, Elena
A1 - Hehn, Michel
A1 - Keller, Niels
A1 - Petit-Watelot, Sebastien
A1 - Bargheer, Matias
A1 - Mangin, Stephane
A1 - Malinowski, Gregory
T1 - Damping of Standing Spin Waves in Bismuth-Substituted Yttrium Iron Garnet as Seen via the Time-Resolved Magneto-Optical Kerr Effect
JF - Physical review applied
N2 - We investigate spin-wave resonance modes and their damping in insulating thin films of bismuth-substituted yttrium iron garnet by performing femtosecond magneto-optical pump-probe experiments. For large magnetic fields in the range below the magnetization saturation, we find that the damping of high-order standing spin-wave (SSW) modes is about 40 times lower than that for the fundamental one. The observed phenomenon can be explained by considering different features of magnetic anisotropy and exchange fields that, respectively, define the precession frequency for fundamental and high-order SSWs. These results provide further insight into SSWs in iron garnets and may be exploited in many new photomagnonic devices.
Y1 - 2019
U6 - https://doi.org/10.1103/PhysRevApplied.12.044006
SN - 2331-7019
VL - 12
IS - 4
PB - American Physical Society
CY - College Park
ER -
TY - THES
A1 - Jay, Raphael Martin
T1 - Principles of charge distribution and separation
BT - the case of iron complexes probed by X-ray spectroscopy
N2 - The electronic charge distributions of transition metal complexes fundamentally determine their chemical reactivity. Experimental access to the local valence electronic structure is therefore crucial in order to determine how frontier orbitals are delocalized between different atomic sites and electronic charge is spread throughout the transition metal complex. To that end, X-ray spectroscopies are employed in this thesis to study a series of solution-phase iron complexes with respect to the response of their local electronic charge distributions to different external influences. Using resonant inelastic X-ray scattering (RIXS) and X-ray absorption spectroscopy (XAS) at the iron L-edge, changes in local charge densities are investigated at the iron center depending on different ligand cages as well as solvent environments. A varying degree of charge delocalization from the metal center onto the ligands is observed, which is governed by the capabilities of the ligands to accept charge density into their unoccupied orbitals. Specific solvents are furthermore shown to amplify this process. Solvent molecules of strong Lewis-acids withdraw charge from the ligand allowing in turn for more metal charge to be delocalized onto the ligand. The resulting local charge deficiencies at the metal center are, however, counteracted by competing electron-donation channels from the ligand towards the iron, which are additionally revealed. This is interpreted as a compensating effect which strives to maintain local charge densities at the iron center. This mechanism of charge density preservation is found to be of general nature. Using time-resolved RIXS and XAS at the iron L-edge, an analogous interplay of electron donation and back-donation channels is also revealed for the case of charge-transfer excited states. In such transient configurations, the electronic occupation of iron-centered frontier orbitals has been altered by an optical excitation. Changes in local charge densities that are expected to follow an increased or decreased population of iron-centered orbitals are, however, again counteracted. By scaling the degree of electron donation from the ligand onto the metal, local charge densities at the iron center can be efficiently maintained. Since charge-transfer excitations, however, often constitute the initial step in many electron transfer processes, these findings challenge common notions of charge-separation in transition metal dyes.
KW - L-edge spectroscopy
KW - charge-transfer excitations
KW - transition metal complexes
KW - electronic structure
KW - photo-chemical pathways
Y1 - 2020
ER -
TY - JOUR
A1 - Bojahr, Andre
A1 - Herzog, Marc
A1 - Schick, Daniel
A1 - Vrejoiu, Ionela
A1 - Bargheer, Matias
T1 - Calibrated real-time detection of nonlinearly propagating strain waves
JF - Physical review : B, Condensed matter and materials physics
N2 - Epitaxially grown metallic oxide transducers support the generation of ultrashort strain pulses in SrTiO3 (STO) with high amplitudes up to 0.5%. The strain amplitudes are calibrated by real-time measurements of the lattice deformation using ultrafast x-ray diffraction. We determine the speed at which the strain fronts propagate by broadband picosecond ultrasonics and conclude that, above a strain level of approx. 0.2%, the compressive and tensile strain components travel at considerably different sound velocities, indicating nonlinear wave behavior. Simulations based on an anharmonic linear-chain model are in excellent accord with the experimental findings and show how the spectrum of coherent phonon modes changes with time.
Y1 - 2012
U6 - https://doi.org/10.1103/PhysRevB.86.144306
SN - 1098-0121
VL - 86
IS - 14
PB - American Physical Society
CY - College Park
ER -
TY - JOUR
A1 - Kiel, Mareike
A1 - Möhwald, Helmuth
A1 - Bargheer, Matias
T1 - Broadband measurements of the transient optical complex dielectric function of a nanoparticle/polymer composite upon ultrafast excitation
JF - Physical review : B, Condensed matter and materials physics
N2 - We determined experimentally the complex transient optical dielectric function of a well-characterized polyelectrolyte/gold-nanoparticle composite system over a broad spectral range upon short pulse laser excitation by simultaneously measuring the time-dependent reflectance and transmittance of white light pulses with femtosecond pump-probe spectroscopy. We extracted directly the ultrafast changes in the real and imaginary parts of the effective dielectric function, epsilon(eff)(r) (omega,t)and epsilon(eff)(i) (omega,t), from the experiment. This complete experimental set of information on the time-dependent complex dielectric function challenges theories modeling the transient dielectric function of gold particles and the effective medium.
Y1 - 2011
U6 - https://doi.org/10.1103/PhysRevB.84.165121
SN - 1098-0121
VL - 84
IS - 16
PB - American Physical Society
CY - College Park
ER -