TY - JOUR A1 - Reindl, Nicole A1 - Bainbridge, M. A1 - Przybilla, Norbert A1 - Geier, Stephan Alfred 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 - JOUR A1 - Rauch, Thomas A1 - Quinet, P. A1 - Hoyer, D. A1 - Werner, K. A1 - Richter, Philipp A1 - Kruk, J. W. A1 - Demleitner, M. T1 - VII. New Kr IV - VII oscillator strengths and an improved spectral analysis of the hot, hydrogen-deficient DO-type white dwarf RE 0503-289 JF - Plant physiology : an international journal devoted to physiology, biochemistry, cellular and molecular biology, biophysics and environmental biology of plants N2 - Context. For the spectral analysis of high-resolution and high signal-to-noise (S/N) spectra of hot stars, state-of-the-art non-local thermodynamic equilibrium (NLTE) model atmospheres are mandatory. These are strongly dependent on the reliability of the atomic data that is used for their calculation. Aims. New Kr IV-VII oscillator strengths for a large number of lines enable us to construct more detailed model atoms for our NLTE model-atmosphere calculations. This enables us to search for additional Kr lines in observed spectra and to improve Kr abundance determinations. Methods. We calculated Kr IV-VII oscillator strengths to consider radiative and collisional bound-bound transitions in detail in our NLTE stellar-atmosphere models for the analysis of Kr lines that are exhibited in high-resolution and high S/N ultraviolet (UV) observations of the hot white dwarf RE 0503-289. Results. We reanalyzed the effective temperature and surface gravity and determined T-eff = 70 000 +/- 2000 K and log (g/cm s(-2)) = 7.5 +/- 0.1. We newly identified ten Kr V lines and one Kr vi line in the spectrum of RE 0503-289. We measured a Kr abundance of 3.3 +/- 0.3 (logarithmic mass fraction). We discovered that the interstellar absorption toward RE 0503-289 has a multi-velocity structure within a radial-velocity interval of -40 km s(-1) < upsilon(rad) < +18 km s(-1). Conclusions. Reliable measurements and calculations of atomic data are a prerequisite for state-of-the-art NLTE stellar-atmosphere modeling. Observed Kr V-VII line profiles in the UV spectrum of the white dwarf RE 0503-289 were simultaneously well reproduced with our newly calculated oscillator strengths. KW - atomic data KW - line: identification KW - stars: abundances KW - stars: individual: RE 0503-289 KW - virtual observatory tools KW - stars: individual: RE 0457-281 Y1 - 2016 U6 - https://doi.org/10.1051/0004-6361/201628131 SN - 1432-0746 VL - 590 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Reindl, Nicole A1 - Rauch, Thomas A1 - Miller Bertolami, Marcelo Miguel A1 - Todt, Helge Tobias A1 - Werner, K. T1 - Breaking news from the HST BT - the central star of the Stingray Nebula is now returning towards the AGB JF - Monthly notices of the Royal Astronomical Society N2 - SAO 244567 is a rare example of a star that allows us to witness stellar evolution in real time. Between 1971 and 1990, it changed from a B-type star into the hot central star of the Stingray Nebula. This observed rapid heating has been a mystery for decades, since it is in strong contradiction with the low mass of the star and canonical post-asymptotic giant branch (AGB) evolution. We speculated that SAO 244567 might have suffered from a late thermal pulse (LTP) and obtained new observations with Hubble Space Telescope (HST)/COS to follow the evolution of the surface properties of SAO 244567 and to verify the LTP hypothesis. Our non-LTE spectral analysis reveals that the star cooled significantly since 2002 and that its envelope is now expanding. Therefore, we conclude that SAO 244567 is currently on its way back towards the AGB, which strongly supports the LTP hypothesis. A comparison with state-of-the-art LTP evolutionary calculations shows that these models cannot fully reproduce the evolution of all surface parameters simultaneously, pointing out possible shortcomings of stellar evolution models. Thereby, SAO 244567 keeps on challenging stellar evolution theory and we highly encourage further investigations. KW - stars: AGB and post-AGB KW - stars: atmospheres KW - stars: evolution Y1 - 2016 U6 - https://doi.org/10.1093/mnrasl/slw175 SN - 0035-8711 SN - 1365-2966 VL - 464 SP - L51 EP - L55 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Kusterer, D. -J. A1 - Nagel, T. A1 - Hartmann, S. A1 - Werner, K. A1 - Feldmeier, Achim T1 - Monte Carlo radiation transfer in CV disk winds: application to the AM CVn prototype JF - Astronomy and astrophysics : an international weekly journal N2 - Context. AMCVn systems are ultracompact binaries in which a (semi-) degenerate star transfers helium-dominated matter onto a white dwarf. They are effective gravitational-wave emitters and potential progenitors of Type Ia supernovae. Aims. To understand the evolution of AMCVn systems it is necessary to determine their mass-loss rate through their radiation-driven accretion-disk wind. We constructed models to perform quantitative spectroscopy of P Cygni line profiles that were detected in UV spectra. Methods. We performed 2.5D Monte Carlo radiative transfer calculations in hydrodynamic wind structures by making use of realistic NLTE spectra from the accretion disk and by accounting for the white dwarf as an additional photon source. Results. We present first results from calculations in which LTE opacities are used in the wind model. A comparison with UV spectroscopy of the AMCVn prototype shows that the modeling procedure is potentially a good tool for determining mass-loss rates and abundances of trace metals in the helium-rich wind. KW - radiative transfer KW - stars: winds, outflows KW - stars: individual: AM CVn KW - accretion, accretion disks Y1 - 2014 U6 - https://doi.org/10.1051/0004-6361/201321438 SN - 0004-6361 SN - 1432-0746 VL - 561 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Reindl, Nicole A1 - Rauch, Thomas A1 - Parthasarathy, M. A1 - Werner, K. A1 - Kruk, J. W. A1 - Hamann, Wolf-Rainer A1 - Sander, Andreas Alexander Christoph A1 - Todt, Helge Tobias T1 - The rapid evolution of the exciting star of the Stingray nebula JF - Astronomy and astrophysics : an international weekly journal N2 - Context. SAO 244567, the exciting star of the Stingray nebula, is rapidly evolving. Previous analyses suggested that it has heated up from an effective temperature of about 21 kK in 1971 to over 50 kK in the 1990s. Canonical post-asymptotic giant branch evolution suggests a relatively high mass while previous analyses indicate a low-mass star. Aims. A comprehensive model-atmosphere analysis of UV and optical spectra taken during 1988-2006 should reveal the detailed temporal evolution of its atmospheric parameters and provide explanations for the unusually fast evolution. Methods. Fitting line profiles from static and expanding non-LTE model atmospheres to the observed spectra allowed us to study the temporal change of effective temperature, surface gravity, mass-loss rate, and terminal wind velocity. In addition, we determined the chemical composition of the atmosphere. Results. We find that the central star has steadily increased its effective temperature from 38 kK in 1988 to a peak value of 60 kK in 2002. During the same time, the star was contracting, as concluded from an increase in surface gravity from log g = 4.8 to 6.0 and a drop in luminosity. Simultaneously, the mass-loss rate declined from log(M/M-circle dot yr(-1)) = -9.0 to -11.6 and the terminal wind velocity increased from v(infinity) = 1800 km s(-1) to 2800 km s(-1). Since around 2002, the star stopped heating and has cooled down again to 55 kK by 2006. It has a largely solar surface composition with the exception of slightly subsolar carbon, phosphorus, and sulfur. The results are discussed by considering different evolutionary scenarios. Conclusions. The position of SAO 244567 in the log T-eff-log g plane places the star in the region of sdO stars. By comparison with stellar-evolution calculations, we confirm that SAO 244567 must be a low-mass star (M < 0.55 M-circle dot). However, the slow evolution of the respective stellar evolutionary models is in strong contrast to the observed fast evolution and the young planetary nebula with a kinematical age of only about 1000 years. We speculate that the star could be a late He-shell flash object. Alternatively, it could be the outcome of close-binary evolution. Then SAD 244567 would be a low-mass (0.354 M-circle dot) helium pre-white dwarf after the common-envelope phase, during which the planetary nebula was ejected. KW - stars: abundances KW - stars: evolution KW - stars: AGB and post-AGB KW - stars: individual: SAO 244567 KW - stars: fundamental parameters KW - planetary nebulae: individual: Stingray nebula (Henize 3-1357) Y1 - 2014 U6 - https://doi.org/10.1051/0004-6361/201323189 SN - 0004-6361 SN - 1432-0746 VL - 565 PB - EDP Sciences CY - Les Ulis ER -