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 < v 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.

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. To identify molybdenum lines in the ultraviolet (UV) spectra of the DA-type white dwarf G191−B2B and the DO-type white dwarf RE 0503−289 and, to determine their photospheric Mo abundances, reliable Mo iv-vii oscillator strengths are used. Methods. We newly calculated Mo iv-vii oscillator strengths to consider their radiative and collisional bound-bound transitions in detail in our NLTE stellar-atmosphere models for the analysis of Mo lines exhibited in high-resolution and high S/N UV observations of RE 0503−289.Results. We identified 12 Mo v and 9 Mo vi lines in the UV spectrum of RE 0503−289 and measured a photospheric Mo abundance of 1.2−3.0 × 10 −4 (mass fraction, 22 500−56 400 times the solar abundance). In addition, from the As v and Sn iv resonance lines, we measured mass fractions of arsenic (0.5−1.3 × 10 −5 , about 300−1200 times solar) and tin (1.3−3.2 × 10 −4 , about 14 300−35 200 times solar). For G191−B2B, upper limits were determined for the abundances of Mo (5.3 × 10 −7 , 100 times solar) and, in addition, for Kr (1.1 × 10 −6 , 10 times solar) and Xe (1.7 × 10 −7 , 10 times solar). The arsenic abundance was determined (2.3−5.9 × 10 −7 , about 21−53 times solar). A new, registered German Astrophysical Virtual Observatory (GAVO) service, TOSS, has been constructed to provide weighted oscillator strengths and transition probabilities. Conclusions. Reliable measurements and calculations of atomic data are a prerequisite for stellar-atmosphere modeling. Observed Mo v-vi line profiles in the UV spectrum of the white dwarf RE 0503−289 were well reproduced with our newly calculated oscillator strengths. For the first time, this allowed the photospheric Mo abundance in a white dwarf to be determined.

No abstract

We report on the analysis of 34 years of photometric observations of the pulsating helium atmosphere white dwarf GD358. The complete data set includes archival data from 1982-2006, and 1195.2 hours of new observations from [2007][2008][2009][2010][2011][2012][2013][2014][2015][2016]. From this data set, we extract 15 frequencies representing g-mode pulsation modes, adding 4 modes to the 11 modes known previously. We present evidence that these 15 modes are ℓ = 1 modes, 13 of which belong to a consecutive sequence in radial overtone k. We perform a detailed asteroseismic analysis using models that include parameterized, complex carbon and oxygen core composition profiles to fit the periods. Recent spectroscopic analyses place GD358 near the red edge of the DBV instability strip, at 24,000 ± 500 K and a log g of 7.8 ± 0.08 dex.The surface gravity translates to a mass range of 0.455 to 0.540 M ⊙ . Our best fit model has a temperature of 23,650 K and a mass of 0.5706 M ⊙ . That is slightly more massive than suggested by most the recent spectroscopy. We find a pure helium layer mass of 10 −5.50 , consistent with the result of previous studies and the outward diffusion of helium over time. Subject headings: Stars: oscillations -Stars: variables: general -white dwarfs spectroscopic temperature (T eff = 24000 ± 500 K) and log g = 7.8 places GD358 near the red edge of the instability strip. GD358's pulsation spectrum contains a series of independent radial overtones, and many have complex frequency structure. For one epoch of data taken during the Whole Earth Telescope (WET) run XCOV25, models involving magnetic fields and oblique rotation are proposed to explain such structure (Montgomery et al. 2010). Since the XCOV25 WET run reported in Provencal et al. (2009), we have maintained an active observing program of this complex star. These new observations have successfully identified additional periods in GD358's pulsation spectrum, bringing the total known independent radial overtones to 15. Thirteen of these modes belong to a consecutive ℓ = 1 sequence, the longest sequence observed in a DBV. Paradoxically, among the DBVs with enough detected periods to be fitted asteroseismically, GD358 is the only one that has not been analyzed using the complex C/O profiles adapted and parameterized from stellar evolution calculations (e.g. Salaris et al. (1997); Althaus et al. (2005)). The most recent fits of GD358 (Metcalfe et al. 2003b) wereperformed using 11 observed modes and simple models where the oxygen abundance drops linearly from its central value to zero. This study was plagued by a symmetric asteroseismic signature from the core and the envelopes in the models and was subsequently unable to derive a unique fit to the period spectrum. We present here a new detailed asteroseismic analysis employing more sophisticated interior chemical profiles.With these profiles, we are able to remove the degeneracy in the best fit parameters and better constrain the asteroseismic fits.The present analysis also allows us to place GD358 in...

Context. In the line-of-sight toward the DO-type white dwarf RX J0503.9−2854, the density of the interstellar medium (ISM) is very low, and thus the contamination of the stellar spectrum almost negligible. This allows to identify many metal lines in a wide wavelength range from the extreme ultraviolet to the near infrared. Aims. In previous spectral analyses, many metal lines in the ultraviolet spectrum of RX J0503.9−2854 have been identified. A complete line list of observed and identified lines is presented here. Methods. We compared synthetic spectra that had been calculated from model atmospheres in non-local thermodynamical equilibrium, with observations. Results. In total, we identified 1272 lines (279 of them were newly assigned) in the wavelength range from the extreme ultraviolet to the near infrared. 287 lines remain unidentified. A close inspection of the EUV shows that still no good fit to the observed shape of the stellar continuum flux can be achieved although He, and Ba are included in the stellar atmosphere models. Conclusions. There are two possible reasons for the deviation between observed and synthetic flux in the EUV may have two reasons. Opacities from hitherto unconsidered elements in the model-atmosphere calculationmay be missing, and/or the effective temperature is slightly lower than previously determined.