@article{PelisoliVosGeieretal.2020, author = {Pelisoli, Ingrid and Vos, Joris and Geier, Stephan and Schaffenroth, Veronika and Baran, Andrzej S.}, title = {Alone but not lonely}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {642}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {0004-6361}, doi = {10.1051/0004-6361/202038473}, pages = {14}, year = {2020}, abstract = {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.}, language = {en} } @misc{MaierWolfKeiligetal.2018, author = {Maier, Philipp and Wolf, J{\"u}rgen and Keilig, Thomas and Krabbe, Alfred and Duffard, Rene and Ortiz, Jose-Luis and Klinkner, Sabine and Lengowski, Michael and M{\"u}ller, Thomas and Lockowandt, Christian and Krockstedt, Christian and Kappelmann, Norbert and Stelzer, Beate and Werner, Klaus and Geier, Stephan and Kalkuhl, Christoph and Rauch, Thomas and Schanz, Thomas and Barnstedt, J{\"u}rgen and Conti, Lauro and Hanke, Lars}, title = {Towards a European Stratospheric Balloon Observatory}, series = {Ground-based and Airborne Telescopes VII}, volume = {10700}, journal = {Ground-based and Airborne Telescopes VII}, publisher = {SPIE-INT Soc Optical Engineering}, address = {Bellingham}, isbn = {978-1-5106-1954-8}, issn = {0277-786X}, doi = {10.1117/12.2319248}, pages = {12}, year = {2018}, abstract = {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.}, language = {en} } @article{IrrgangGeierHeberetal.2019, author = {Irrgang, Andreas and Geier, Stephan and Heber, Ulrich and Kupfer, Thomas and F{\"u}rst, F.}, title = {PG 1610+062: a runaway B star challenging classical ejection mechanisms}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {628}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {1432-0746}, doi = {10.1051/0004-6361/201935429}, pages = {17}, year = {2019}, abstract = {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.}, language = {en} } @article{RatzloffBarlowKupferetal.2019, author = {Ratzloff, Jeffrey K. and Barlow, Brad N. and Kupfer, Thomas and Corcoran, Kyle A. and Geier, Stephan and Bauer, Evan and Corbett, Henry T. and Howard, Ward S. and Glazier, Amy and Law, Nicholas M.}, title = {EVR-CB-001: An Evolving, Progenitor, White Dwarf Compact Binary Discovered with the Evryscope}, series = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, volume = {883}, journal = {The astrophysical journal : an international review of spectroscopy and astronomical physics}, number = {1}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {0004-637X}, doi = {10.3847/1538-4357/ab3727}, pages = {12}, year = {2019}, abstract = {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.}, language = {en} } @article{RomanowskyHandorfJaiseretal.2019, author = {Romanowsky, Erik and Handorf, D{\"o}rthe and Jaiser, Ralf and Wohltmann, Ingo and Dorn, Wolfgang and Ukita, Jinro and Cohen, Judah and Dethloff, Klaus and Rex, Markus}, title = {The role of stratospheric ozone for Arctic-midlatitude linkages}, series = {Scientific reports}, volume = {9}, journal = {Scientific reports}, publisher = {Nature Publ. Group}, address = {London}, issn = {2045-2322}, doi = {10.1038/s41598-019-43823-1}, pages = {7}, year = {2019}, abstract = {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.}, language = {en} } @article{GeierRaddiFusilloetal.2019, author = {Geier, Stephan and Raddi, Roberto and Fusillo, Nicola Pietro Gentile and Marsh, T. R.}, title = {The population of hot subdwarf stars studied with Gaia}, series = {Astronomy and astrophysics : an international weekly journal}, volume = {621}, journal = {Astronomy and astrophysics : an international weekly journal}, publisher = {EDP Sciences}, address = {Les Ulis}, issn = {0004-6361}, doi = {10.1051/0004-6361/201834236}, pages = {13}, year = {2019}, abstract = {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.}, language = {en} } @misc{FinchBrakerReindletal.2019, author = {Finch, Nicolle L. and Braker, I. P. and Reindl, Nicole and Barstow, Martin A. and Casewell, Sarah L. and Burleigh, M. and Kupfer, Thomas and Kilkenny, D. and Geier, Stephan and Schaffenroth, Veronika and Bertolami Miller, Marcelo Miguel and Taubenberger, Stefan and Freudenthal, Joseph}, title = {Spectral Analysis of Binary Pre-white Dwarf Systems}, series = {Radiative signatures from the cosmos}, volume = {519}, journal = {Radiative signatures from the cosmos}, publisher = {Astronomical soc pacific}, address = {San Fransisco}, isbn = {978-1-58381-925-8}, issn = {1050-3390}, pages = {231 -- 238}, year = {2019}, abstract = {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.}, language = {en} } @article{Omel'chenkoOcampoEspindolaKiss2021, author = {Omel'chenko, Oleh and Ocampo-Espindola, Jorge Luis and Kiss, Istv{\´a}n Z.}, title = {Asymmetry-induced isolated fully synchronized state in coupled oscillator populations}, series = {Physical review : E, Statistical, nonlinear and soft matter physics}, volume = {104}, journal = {Physical review : E, Statistical, nonlinear and soft matter physics}, number = {2}, publisher = {American Physical Society}, address = {Melville, NY}, issn = {2470-0045}, doi = {10.1103/PhysRevE.104.L022202}, pages = {6}, year = {2021}, abstract = {A symmetry-breaking mechanism is investigated that creates bistability between fully and partially synchronized states in oscillator networks. Two populations of oscillators with unimodal frequency distribution and different amplitudes, in the presence of weak global coupling, are shown to simplify to a modular network with asymmetrical coupling. With increasing the coupling strength, a synchronization transition is observed with an isolated fully synchronized state. The results are interpreted theoretically in the thermodynamic limit and confirmed in experiments with chemical oscillators.}, language = {en} } @article{MeyerPohlPetrovetal.2023, author = {Meyer, Dominique M.-A. and Pohl, Martin and Petrov, Miroslav and Egberts, Kathrin}, title = {Mixing of materials in magnetized core-collapse supernova remnants}, series = {Monthly notices of the Royal Astronomical Society}, volume = {521}, journal = {Monthly notices of the Royal Astronomical Society}, number = {4}, publisher = {Oxford Univ. Press}, address = {Oxford}, issn = {0035-8711}, doi = {10.1093/mnras/stad906}, pages = {5354 -- 5371}, year = {2023}, abstract = {Core-collapse supernova remnants are structures of the interstellar medium (ISM) left behind the explosive death of most massive stars ( ?40 M-?). Since they result in the expansion of the supernova shock wave into the gaseous environment shaped by the star's wind history, their morphology constitutes an insight into the past evolution of their progenitor star. Particularly, fast-mo ving massiv e stars can produce asymmetric core-collapse superno va remnants. We inv estigate the mixing of materials in core-collapse supernova remnants generated by a moving massive 35 M-? star, in a magnetized ISM. Stellar rotation and the wind magnetic field are time-dependently included into the models which follow the entire evolution of the stellar surroundings from the zero-age main-sequence to 80 kyr after the supernova explosion. It is found that very little main-sequence material is present in remnants from moving stars, that the Wolf-Rayet wind mixes very efficiently within the 10 kyr after the explosion, while the red supergiant material is still unmixed by 30 per cent within 50 kyr after the supernova. Our results indicate that the faster the stellar motion, the more complex the internal organization of the supernova remnant and the more ef fecti ve the mixing of ejecta therein. In contrast, the mixing of stellar wind material is only weakly affected by progenitor motion, if at all.}, language = {en} } @article{DahlenburgChechkinSchumeretal.2021, author = {Dahlenburg, Marcus and Chechkin, Aleksei and Schumer, Rina and Metzler, Ralf}, title = {Stochastic resetting by a random amplitude}, series = {Physical review : E, Statistical, nonlinear and soft matter physics}, volume = {103}, journal = {Physical review : E, Statistical, nonlinear and soft matter physics}, number = {5}, publisher = {American Physical Society}, address = {Woodbury, NY}, issn = {2470-0045}, doi = {10.1103/PhysRevE.103.052123}, pages = {22}, year = {2021}, abstract = {Stochastic resetting, a diffusive process whose amplitude is reset to the origin at random times, is a vividly studied strategy to optimize encounter dynamics, e.g., in chemical reactions. Here we generalize the resetting step by introducing a random resetting amplitude such that the diffusing particle may be only partially reset towards the trajectory origin or even overshoot the origin in a resetting step. We introduce different scenarios for the random-amplitude stochastic resetting process and discuss the resulting dynamics. Direct applications are geophysical layering (stratigraphy) and population dynamics or financial markets, as well as generic search processes.}, language = {en} }