TY  - JOUR
A1  - Li, Tian-yi
A1  - Benduhn, Johannes
A1  - Qiao, Zhi
A1  - Liu, Yuan
A1  - Li, Yue
A1  - Shivhare, Rishi
A1  - Jaiser, Frank
A1  - Wang, Pei
A1  - Ma, Jie
A1  - Zeika, Olaf
A1  - Neher, Dieter
A1  - Mannsfeld, Stefan C. B.
A1  - Ma, Zaifei
A1  - Vandewal, Koen
A1  - Leo, Karl
T1  - Effect of H- and J-Aggregation on the Photophysical and Voltage Loss of Boron Dipyrromethene Small Molecules in Vacuum-Deposited Organic Solar Cells
JF  - The journal of physical chemistry letters
N2  - An understanding of the factors limiting the open-circuit voltage (V-oc) and related photon energy loss mechanisms is critical to increase the power conversion efficiency (PCE) of small-molecule organic solar cells (OSCs), especially those with near-infrared (NIR) absorbers. In this work, two NIR boron dipyrromethene (BODIPY) molecules are characterized for application in planar (PHJ) and bulk (BHJ) heterojunction OSCs. When two H atoms are substituted by F atoms on the peripheral phenyl rings of the molecules, the molecular aggregation type in the thin film changes from the H-type to J-type. For PHJ devices, the nonradiative voltage loss of 0.35 V in the J-aggregated BODIPY is lower than that of 0.49 V in the H-aggregated device. In BHJ devices with a nonradiative voltage loss of 0.35 V, a PCE of 5.5% is achieved with an external quantum efficiency (EQE) maximum of 68% at 700 nm.
Y1  - 2019
U6  - https://doi.org/10.1021/acs.jpclett.9b01222
SN  - 1948-7185
VL  - 10
IS  - 11
SP  - 2684
EP  - 2691
PB  - American Chemical Society
CY  - Washington
ER  - 
TY  - JOUR
A1  - Diercke, Andrea
A1  - Kuckein, Christoph
A1  - Denker, Carsten
T1  - Dynamics and connectivity of an extended arch filament system
JF  - Astronomy and astrophysics : an international weekly journal
N2  - Aims. In this study, we analyzed a filament system, which expanded between moving magnetic features (MMFs) of a decaying sunspot and opposite flux outside of the active region from the nearby quiet-Sun network. This configuration deviated from a classical arch filament system (AFS), which typically connects two pores in an emerging flux region. Thus, we called this system an extended AFS. We contrasted classical and extended AFSs with an emphasis on the complex magnetic structure of the latter. Furthermore, we examined the physical properties of the extended AFS and described its dynamics and connectivity. Methods. The extended AFS was observed with two instruments at the Dunn Solar Telescope (DST). The Rapid Oscillations in the Solar Atmosphere (ROSA) imager provided images in three different wavelength regions, which covered the dynamics of the extended AFS at different atmospheric heights. The Interferometric Bidimensional Spectropolarimeter (IBIS) provided spectroscopic Ha data and spectropolarimetric data that was obtained in the near-infrared (NIR) Call lambda 8542 angstrom line. We derived the corresponding line-of-sight (LOS) velocities and used He II lambda 304 angstrom extreme ultraviolet (EUV) images of the Atmospheric Imaging Assembly (AIA) and LOS magnetograms of the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) as context data. Results. The NIR Call Stokes-V maps are not suitable to definitively define a clear polarity inversion line and to classify this chromospheric structure. Nevertheless, this unusual AFS connects the MMFs of a decaying sunspot with the network field. At the southern footpoint, we measured that the flux decreases over time. We find strong downflow velocities at the footpoints of the extended AFS, which increase in a time period of 30 min. The velocities are asymmetric at both footpoints with higher velocities at the southern footpoint. An EUV brigthening appears in one of the arch filaments, which migrates from the northern footpoint toward the southern one. This activation likely influences the increasing redshift at the southern footpoint. Conclusions. The extended AFS exhibits a similar morphology as classical AFSs, for example, threaded filaments of comparable length and width. Major differences concern the connection from MMFs around the sunspot with the flux of the neighboring quietSun network, converging footpoint motions, and longer lifetimes of individual arch filaments of about one hour, while the extended AFS is still very dynamic.
KW  - methods: observational
KW  - Sun: filaments, prominences
KW  - Sun: activity
KW  - techniques: image processing
KW  - Sun: chromosphere
Y1  - 2019
U6  - https://doi.org/10.1051/0004-6361/201935583
SN  - 1432-0746
VL  - 629
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Sander, Andreas Alexander Christoph
A1  - Vink, Jorick S.
A1  - Hamann, Wolf-Rainer
T1  - Driving classical Wolf-Rayet winds
BT  - a Gamma- and Z-dependent mass-loss
JF  - Monthly notices of the Royal Astronomical Society
N2  - Classical Wolf-Rayet (cWR) stars are at a crucial evolutionary stage for constraining the fates of massive stars. The feedback of these hot, hydrogen-depleted stars dominates their surrounding by tremendous injections of ionizing radiation and kinetic energy. The strength of a Wolf-Rayet (WR) wind decides the eventual mass of its remnant, likely a massive black hole. However, despite their major influence and importance for gravitational wave detection statistics, WR winds are particularly poorly understood. In this paper, we introduce the first set of hydrodynamically consistent stellar atmosphere models for cWR stars of both the carbon (C) and the nitrogen (N) sequence, i.e. WC and WN stars, as a function of stellar luminosity-to-mass ratio (or Eddington Gamma) and metallicity. We demonstrate the inapplicability of the CAK wind theory for cWR stars and confirm earlier findings that their winds are launched at the (hot) iron (Fe) opacity peak. For log Z/Z(circle dot) > -2, Fe is also the main accelerator throughout the wind. Contrasting previous claims of a sharp lower mass-loss limit forWR stars, we obtain a smooth transition to optically thin winds. Furthermore, we find a strong dependence of the mass-loss rates on Eddington Gamma, both at solar and subsolar metallicity. Increases inWCcarbon and oxygen abundances turn out to slightly reduce the predicted mass-loss rates. Calculations at subsolar metallicities indicate that below the metallicity of the Small Magellanic Cloud, WR mass-loss rates decrease much faster than previously assumed, potentially allowing for high black hole masses even in the local Universe.
KW  - stars: atmospheres
KW  - stars: early-type
KW  - stars: fundamental parameters
KW  - stars: mass-loss
KW  - stars: winds, outflows
KW  - stars: Wolf-Rayet
Y1  - 2019
U6  - https://doi.org/10.1093/mnras/stz3064
SN  - 0035-8711
SN  - 1365-2966
VL  - 491
IS  - 3
SP  - 4406
EP  - 4425
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Tubiana, C.
A1  - Rinaldi, G.
A1  - Guettler, C.
A1  - Snodgrass, C.
A1  - Shi, X.
A1  - Hu, X.
A1  - Marschall, R.
A1  - Fulle, M.
A1  - Bockeele-Morvan, D.
A1  - Naletto, G.
A1  - Capaccioni, F.
A1  - Sierks, H.
A1  - Arnold, G.
A1  - Barucci, M. A.
A1  - Bertaux, J-L
A1  - Bertini, I
A1  - Bodewits, D.
A1  - Capria, M. T.
A1  - Ciarniello, M.
A1  - Cremonese, G.
A1  - Crovisier, J.
A1  - Da Deppo, V
A1  - Debei, S.
A1  - De Cecco, M.
A1  - Deller, J.
A1  - De Sanctis, M. C.
A1  - Davidsson, B.
A1  - Doose, L.
A1  - Erard, S.
A1  - Filacchione, G.
A1  - Fink, U.
A1  - Formisano, M.
A1  - Fornasier, S.
A1  - Gutierrez, P. J.
A1  - Ip, W-H
A1  - Ivanovski, S.
A1  - Kappel, David
A1  - Keller, H. U.
A1  - Kolokolova, L.
A1  - Koschny, D.
A1  - Krueger, H.
A1  - La Forgia, F.
A1  - Lamy, P. L.
A1  - Lara, L. M.
A1  - Lazzarin, M.
A1  - Levasseur-Regourd, A. C.
A1  - Lin, Z-Y
A1  - Longobardo, A.
A1  - Lopez-Moreno, J. J.
A1  - Marzari, F.
A1  - Migliorini, A.
A1  - Mottola, S.
A1  - Rodrigo, R.
A1  - Taylor, F.
A1  - Toth, I
A1  - Zakharov, V
T1  - Diurnal variation of dust and gas production in comet 67P/Churyumov-Gerasimenko at the inbound equinox as seen by OSIRIS and VIRTIS-M on board Rosetta
JF  - Astronomy and astrophysics : an international weekly journal
N2  - Context. On 27 April 2015, when comet 67P/Churyumov-Gerasimenko was at 1.76 au from the Sun and moving toward perihelion, the OSIRIS and VIRTIS-M instruments on board the Rosetta spacecraft simultaneously observed the evolving dust and gas coma during a complete rotation of the comet. Aims. We aim to characterize the spatial distribution of dust, H2O, and CO2 gas in the inner coma. To do this, we performed a quantitative analysis of the release of dust and gas and compared the observed H2O production rate with the rate we calculated using a thermophysical model. Methods. For this study we selected OSIRIS WAC images at 612 nm (dust) and VIRTIS-M image cubes at 612 nm, 2700 nm (H2O emission band), and 4200 nm (CO2 emission band). We measured the average signal in a circular annulus to study the spatial variation around the comet, and in a sector of the annulus to study temporal variation in the sunward direction with comet rotation, both at a fixed distance of 3.1 km from the comet center. Results. The spatial correlation between dust and water, both coming from the sunlit side of the comet, shows that water is the main driver of dust activity in this time period. The spatial distribution of CO2 is not correlated with water and dust. There is no strong temporal correlation between the dust brightness and water production rate as the comet rotates. The dust brightness shows a peak at 0 degrees subsolar longitude, which is not pronounced in the water production. At the same epoch, there is also a maximum in CO2 production. An excess of measured water production with respect to the value calculated using a simple thermophysical model is observed when the head lobe and regions of the southern hemisphere with strong seasonal variations are illuminated (subsolar longitude 270 degrees-50 degrees). A drastic decrease in dust production when the water production (both measured and from the model) displays a maximum occurs when typical northern consolidated regions are illuminated and the southern hemisphere regions with strong seasonal variations are instead in shadow (subsolar longitude 50 degrees-90 degrees). Possible explanations of these observations are presented and discussed.
KW  - comets: general
KW  - comets: individual: 67P/Churyumov-Gerasimenko
KW  - methods: data analysis
Y1  - 2019
U6  - https://doi.org/10.1051/0004-6361/201834869
SN  - 1432-0746
VL  - 630
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Ekhtiari, Nikoo
A1  - Agarwal, Ankit
A1  - Marwan, Norbert
A1  - Donner, Reik Volker
T1  - Disentangling the multi-scale effects of sea-surface temperatures on global precipitation
BT  - a coupled networks approach
JF  - Chaos : an interdisciplinary journal of nonlinear science
N2  - The oceans and atmosphere interact via a multiplicity of feedback mechanisms, shaping to a large extent the global climate and its variability. To deepen our knowledge of the global climate system, characterizing and investigating this interdependence is an important task of contemporary research. However, our present understanding of the underlying large-scale processes is greatly limited due to the manifold interactions between essential climatic variables at different temporal scales. To address this problem, we here propose to extend the application of complex network techniques to capture the interdependence between global fields of sea-surface temperature (SST) and precipitation (P) at multiple temporal scales. For this purpose, we combine time-scale decomposition by means of a discrete wavelet transform with the concept of coupled climate network analysis. Our results demonstrate the potential of the proposed approach to unravel the scale-specific interdependences between atmosphere and ocean and, thus, shed light on the emerging multiscale processes inherent to the climate system, which traditionally remain undiscovered when investigating the system only at the native resolution of existing climate data sets. Moreover, we show how the relevant spatial interdependence structures between SST and P evolve across time-scales. Most notably, the strongest mutual correlations between SST and P at annual scale (8-16 months) concentrate mainly over the Pacific Ocean, while the corresponding spatial patterns progressively disappear when moving toward longer time-scales. Published under license by AIP Publishing.
Y1  - 2019
U6  - https://doi.org/10.1063/1.5095565
SN  - 1054-1500
SN  - 1089-7682
VL  - 29
IS  - 6
PB  - American Institute of Physics
CY  - Melville
ER  - 
TY  - JOUR
A1  - Göttgens, Fabian
A1  - Weilbacher, Peter Michael
A1  - Roth, Martin M.
A1  - Dreizler, Stefan
A1  - Giesers, Benjamin
A1  - Husser, Tim-Oliver
A1  - Kamann, Sebastian
A1  - Brinchmann, Jarle
A1  - Kollatschny, Wolfram
A1  - Monreal-Ibero, Ana
A1  - Schmidt, Kasper Borello
A1  - Wendt, Martin
A1  - Wisotzki, Lutz
A1  - Bacon, Roland
T1  - Discovery of an old nova remnant in the Galactic globular cluster M 22
JF  - Astronomy and astrophysics : an international weekly journal
N2  - A nova is a cataclysmic event on the surface of a white dwarf in a binary system that increases the overall brightness by several orders of magnitude. Although binary systems with a white dwarf are expected to be overabundant in globular clusters compared with in the Galaxy, only two novae from Galactic globular clusters have been observed. We present the discovery of an emission nebula in the Galactic globular cluster M 22 (NGC 6656) in observations made with the integral-field spectrograph MUSE. We extracted the spectrum of the nebula and used the radial velocity determined from the emission lines to confirm that the nebula is part of NGC 6656. Emission-line ratios were used to determine the electron temperature and density. It is estimated to have a mass of 1-17 x 10(-5) M-circle dot. This mass and the emission-line ratios indicate that the nebula is a nova remnant. Its position coincides with the reported location of a "guest star", an ancient Chinese term for transients, observed in May 48 BCE. With this discovery, this nova may be one of the oldest confirmed extra-solar events recorded in human history.
KW  - globular clusters: individual: NGC 6656
KW  - novae, cataclysmic variables
KW  - techniques: imaging spectroscopy
Y1  - 2019
U6  - https://doi.org/10.1051/0004-6361/201935221
SN  - 1432-0746
VL  - 626
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Gvaramadze, V. V.
A1  - Kniazev, Alexei Y.
A1  - Oskinova, Lida
T1  - Discovery of a putative supernova remnant around the long-period X-ray pulsar SXP 1323 in the Small Magellanic Cloud
JF  - Monthly notices of the Royal Astronomical Society
N2  - We report the discovery of a circular shell centred on the Be X-ray binary (BeXB) SXP 1323 in the Small Magellanic Cloud. The shell was detected in an H α image obtained with the Very Large Telescope. Follow-up spectroscopy with the Southern African Large Telescope showed that the shell expands with a velocity of ≈100kms−1 and that its emission is due to shock excitation. We suggest that this shell is a remnant of the supernova explosion that led to the formation of SXP 1323’s neutron star ≈40000 yr ago. SXP 1323 represents the second known case of a BeXB associated with a supernova remnant (the first one is SXP 1062). Interestingly, both of these BeXBs harbour long-period pulsars and are located in a low-metallicity galaxy.
KW  - stars: emission-line, Be
KW  - stars: individual: [MA93] 1393
KW  - stars: massive
KW  - ISM: supernova remnants
KW  - X-rays: binaries
KW  - X-rays: individual: SXP 1323
Y1  - 2019
U6  - https://doi.org/10.1093/mnrasl/slz018
SN  - 0035-8711
SN  - 1365-2966
VL  - 485
IS  - 1
SP  - L6
EP  - L10
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Kühn, Danilo
A1  - Müller, Moritz
A1  - Sorgenfrei, Nomi
A1  - Giangrisostomi, Erika
A1  - Jay, Raphael Martin
A1  - Ovsyannikov, Ruslan
A1  - Martensson, Nils
A1  - Sanchez-Portal, Daniel
A1  - Föhlisch, Alexander
T1  - Directional sub-femtosecond charge transfer dynamics and the dimensionality of 1T-TaS2
JF  - Scientific reports
N2  - For the layered transition metal dichalcogenide 1T-TaS2, we establish through a unique experimental approach and density functional theory, how ultrafast charge transfer in 1T-TaS2 takes on isotropic three-dimensional character or anisotropic two-dimensional character, depending on the commensurability of the charge density wave phases of 1T-TaS2. The X-ray spectroscopic core-hole-clock method prepares selectively in-and out-of-plane polarized sulfur 3p orbital occupation with respect to the 1T-TaS2 planes and monitors sub-femtosecond wave packet delocalization. Despite being a prototypical two-dimensional material, isotropic three-dimensional charge transfer is found in the commensurate charge density wave phase (CCDW), indicating strong coupling between layers. In contrast, anisotropic two-dimensional charge transfer occurs for the nearly commensurate phase (NCDW). In direct comparison, theory shows that interlayer interaction in the CCDW phase - not layer stacking variations - causes isotropic three-dimensional charge transfer. This is presumably a general mechanism for phase transitions and tailored properties of dichalcogenides with charge density waves.
Y1  - 2019
U6  - https://doi.org/10.1038/s41598-018-36637-0
SN  - 2045-2322
VL  - 9
IS  - 488
PB  - Nature Publ. Group
CY  - London
ER  - 
TY  - JOUR
A1  - Hörmann, Ulrich
A1  - Zeiske, Stefan
A1  - Park, Soohyung
A1  - Schultz, Thorsten
A1  - Kickhoefel, Sebastian
A1  - Scherf, Ullrich
A1  - Blumstengel, Sylke
A1  - Koch, Norbert
A1  - Neher, Dieter
T1  - Direct observation of state-filling at hybrid tin oxide/organic interfaces
JF  - Applied physics letters
N2  - Electroluminescence (EL) spectra of hybrid charge transfer states at metal oxide/organic type-II heterojunctions exhibit bias-induced spectral shifts. The reasons for this phenomenon have been discussed controversially and arguments for either electric field-induced effects or the filling of trap states at the oxide surface have been put forward. Here, we combine the results of EL and photovoltaic measurements to eliminate the unavoidable effect of the series resistance of inorganic and organic components on the total voltage drop across the hybrid device. For SnOx combined with the conjugated polymer [ladder type poly-(para-phenylene)], we find a one-to-one correspondence between the blue-shift of the EL peak and the increase of the quasi-Fermi level splitting at the hybrid heterojunction, which we unambiguously assign to state filling. Our data are resembled best by a model considering the combination of an exponential density of states with a doped semiconductor. Published under license by AIP Publishing.
Y1  - 2019
U6  - https://doi.org/10.1063/1.5082704
SN  - 0003-6951
SN  - 1077-3118
VL  - 114
IS  - 18
PB  - American Institute of Physics
CY  - Melville
ER  - 
TY  - JOUR
A1  - Benbow, W.
A1  - Bird, R.
A1  - Brill, A.
A1  - Brose, Robert
A1  - Chromey, A. J.
A1  - Daniel, M. K.
A1  - Feng, Q.
A1  - Finley, J. P.
A1  - Fortson, L.
A1  - Furniss, A.
A1  - Gillanders, G. H.
A1  - Giuri, C.
A1  - Gueta, O.
A1  - Hanna, D.
A1  - Halpern, J. P.
A1  - Hassan, Tarek
A1  - Holder, J.
A1  - Hughes, G.
A1  - Humensky, T. B.
A1  - Joyce, Amy M.
A1  - Kaaret, P.
A1  - Kar, P.
A1  - Kelley-Hoskins, N.
A1  - Kertzman, M.
A1  - Kieda, D.
A1  - Krause, M.
A1  - Lang, M. J.
A1  - Lin, T. T. Y.
A1  - Maier, Gernot
A1  - Matthews, N.
A1  - Moriarty, P.
A1  - Mukherjee, R.
A1  - Nieto, D.
A1  - Nievas-Rosillos, M.
A1  - Ong, R. A.
A1  - Park, N.
A1  - Petrashyk, A.
A1  - Pohl, Martin
A1  - Pueschel, Elisa
A1  - Quinn, John
A1  - Ragan, K.
A1  - Reynolds, P. T.
A1  - Richards, G. T.
A1  - Roache, E.
A1  - Rulten, C.
A1  - Sadeh, Iftach
A1  - Santander, M.
A1  - Sembroski, G. H.
A1  - Shahinyan, K.
A1  - Sushch, Iurii
A1  - Wakely, S. P.
A1  - Wells, R. M.
A1  - Wilcox, P.
A1  - Wilhelm, Alina
A1  - Williams, David A.
A1  - Williamson, T. J.
T1  - Direct measurement of stellar angular diameters by the VERITAS Cherenkov telescopes
JF  - Nature astronomy
N2  - The angular size of a star is a critical factor in determining its basic properties1. Direct measurement of stellar angular diameters is difficult: at interstellar distances stars are generally too small to resolve by any individual imaging telescope. This fundamental limitation can be overcome by studying the diffraction pattern in the shadow cast when an asteroid occults a star2, but only when the photometric uncertainty is smaller than the noise added by atmospheric scintillation3. Atmospheric Cherenkov telescopes used for particle astrophysics observations have not generally been exploited for optical astronomy due to the modest optical quality of the mirror surface. However, their large mirror area makes them well suited for such high-time-resolution precision photometry measurements4. Here we report two occultations of stars observed by the Very Energetic Radiation Imaging Telescope Array System (VERITAS)5 Cherenkov telescopes with millisecond sampling, from which we are able to provide a direct measurement of the occulted stars’ angular diameter at the ≤0.1 mas scale. This is a resolution never achieved before with optical measurements and represents an order of magnitude improvement over the equivalent lunar occultation method6. We compare the resulting stellar radius with empirically derived estimates from temperature and brightness measurements, confirming the latter can be biased for stars with ambiguous stellar classifications.
Y1  - 2019
U6  - https://doi.org/10.1038/s41550-019-0741-z
SN  - 2397-3366
VL  - 3
IS  - 6
SP  - 511
EP  - 516
PB  - Nature Publ. Group
CY  - London
ER  -