Atmospheres Of Hot White Dwarfs Research Articles

Search for trans-iron elements in hot, helium-rich white dwarfs with the HST Cosmic Origins Spectrograph

The metal abundances in the atmospheres of hot white dwarfs (WDs) entering the cooling sequence are determined by the preceding Asymptotic Giant Branch (AGB) evolutionary phase and, subsequently, by the onset of gravitational settling and radiative levitation. In this paper, we investigate three hot He-rich WDs, which are believed to result from a late He-shell flash. During such a flash, the He-rich intershell matter is dredged up and dominates the surface chemistry. Hence, in contrast to the usual H-rich WDs, their spectra allow direct access to s-process element abundances in the intershell that were synthesized during the AGB stage. In order to look for trans-iron group elements (atomic number Z > 29), we performed a non-local thermodynamic equilibrium model atmosphere analysis of new ultraviolet spectra taken with the Cosmic Origins Spectrograph aboard the Hubble Space Telescope. One of our program stars is of PG 1159 spectral type; this star, PG 1707+427, has effective temperature Teff = 85 000 K, and surface gravity logg = 7.5. The two other stars are DO white dwarfs: WD 0111+002 has Teff = 58 000 K and log g = 7.7, and PG 0109+111 has Teff = 70 000 K and log g = 8.0. These stars trace the onset of element diffusion during early WD evolution. While zinc is the only trans-iron element we could detect in the PG 1159 star, both DOs exhibit lines from Zn, Ga, Ge, Se; one additionally exhibits lines from Sr, Sn, Te, and I and the other from As. Generally, the trans-iron elements are very abundant in the DOs, meaning that radiative levitation must be acting. Most extreme is the almost six orders of magnitude oversolar abundance of tellurium in PG 0109+111. In terms of mass fraction, it is the most abundant metal in the atmosphere. The two DOs join the hitherto unique hot DO RE 0503−289, in which 14 trans-iron elements had even been identified.

The supersoft X-ray source in V5116 Sagittarii

Classical novae occur on the surface of an accreting white dwarf in a binary system. After ejection of a fraction of the envelope and when the expanding shell becomes optically thin to X-rays, a bright source of supersoft X-rays arises, powered by residual H burning on the surface of the white dwarf. While the general picture of the nova event is well established, the details and balance of accretion and ejection processes in classical novae are still full of unknowns. The long-term balance of accreted matter is of special interest for massive accreting white dwarfs, which may be promising supernova Ia progenitor candidates. V5116 Sgr was observed as a bright and variable supersoft X-ray source by XMM-Newton 610~days after outburst. The light curve showed a periodicity consistent with the orbital period. During one third of the orbit the luminosity was a factor of seven brighter than during the other two thirds of the orbital period. In the present work we aim to disentangle the X-ray spectral components of V5116 Sgr and their variability. We present the high resolution spectra obtained with XMM-Newton RGS and Chandra LETGS/HRC-S in March and August 2007. The grating spectrum during the periods of high-flux shows a typical hot white dwarf atmosphere dominated by absorption lines of N VI and N VII. During the low-flux periods, the spectrum is dominated by an atmosphere with the same temperature as during the high-flux period, but with several emission features superimposed. Some of the emission lines are well modeled with an optically thin plasma in collisional equilibrium, rich in C and N, which also explains some excess in the spectra of the high-flux period. No velocity shifts are observed in the absorption lines, with an upper limit set by the spectral resolution of 500 km/s, consistent with the expectation of a non-expanding atmosphere so late in the evolution of the post-nova.

X‐Ray Spectroscopy of the 2006 Outburst of RS Ophiuchi

We present X-ray grating spectra of the recurrent nova RS Ophiuchi during its 2006 outburst, obtained with XMM-Newton and Chandra. For the first month after optical maximum, the X-ray spectrum was hard and dominated by emission lines of H-like and He-like ions. The X-ray luminosity was 2.4 × 1036 ergs s−1 in the 0.33-10 keV range. The spectra indicate a collisionally dominated plasma with a broad range of temperatures and an energy-dependent velocity structure. During an observation obtained in week 4, a soft X-ray flare occurred in which a new system of soft, higher velocity emission lines appeared in the spectrum. Then, during weeks 6-10, the supersoft continuum of the hot white dwarf atmosphere was the dominant emission component. The X-ray luminosity reached at least 9 × 1037 ergs s−1 in the 0.2-1 keV range, while the intrinsic nebular absorption decreased by a factor of 5 since the first observation. Preliminary model fitting indicates a white dwarf temperature of ~800,000 K, and a mass of at least 1.2 M☉. Therefore, RS Oph may be an important Type Ia supernova progenitor. We show that the data are consistent with mass loss ending before day 54 of the outburst, and nuclear burning ending around day 69. A rapid decay in X-ray luminosity followed after week 10. The X-ray luminosity 5, 7, and 8 months after optical maximum dropped by more than 2 orders of magnitude. The spectra do not appear to be consistent with emission from an accretion disk.

Supersoft X-ray sources. parameters of stellar atmospheres

ROSAT spectra of 11 supersoft X-ray sources are approximated with theoretical spectra obtained in LTE models for the atmospheres of hot white dwarfs with line blanketing. The confidence intervals of parameters derived from these approximations—Teff, log, g, NH, and R2/d2—are determined. The results are compared with predictions for a model with stable/recurrent thermonuclear burning on the white-dwarf surface.

The re-analysis of theROSATdata of Nova Mus 1983 using white dwarf atmosphere emission models

The analyses of X-ray emission from classical novae during the outburst stage have shown that the soft X-ray emission below 1 keV, which is thought to originate from the photosphere of the white dwarf, is inconsistent with the simple blackbody model of emission. Thus, $ROSAT$ Position Sensitive Proportional Counter (PSPC) archival data of the classical novae GQ Mus 1983 (GQ Mus) have been reanalyzed in order to understand the spectral development in the X-ray wavelengths during the outburst stage. The X-ray spectra are fitted with the hot white dwarf atmosphere emission models developed for the remnants of classical novae near the Eddington luminosity. The post-outburst X-ray spectra of the remnant white dwarf is examined in the context of evolution on the Hertzsprung-Russell diagram using C-O enhanced atmosphere models. The data obtained in 1991 August (during the ROSAT All Sky Survey) indicate that the effective temperature is kT_e<54 eV (<6.2x10^5 K). The 1992 February data show that the white dwarf had reached an effective temperature in the range 38.3-43.3 eV (4.4-5.1x10^5 K) with an unabsorbed X-ray flux (i.e., $\sim$ bolometric flux) between 2.5x10^-9 and 2.3x10^-10 erg s^-1 cm^-2. We show that the H burning at the surface of the WD had most likely ceased at the time of the X-ray observations. Only the 1991 August data show evidence for ongoing H burning.

The sphericity effects in model atmospheres of central stars of Planetary Nebulae

The model atmospheres of central stars of planetary nebulae are often calculated under the assumption of the plane-parallel geometry. This assumption seems to be reasonable for stars with relatively thin atmospheres like white dwarfs, where the thickness of the atmosphere is only few kilometers while the corresponding radius is about several thousands of kilometers. Nevertheless, calculations of a grid of pure hydrogen model atmospheres of hot white dwarfs demonstrated that this assumption may fail even for thin atmospheres (Kubát 1995). We found small differences between line profiles (about 1%) calculated under the assumption of plane-parallel and spherically symmetric atmospheres. Such differences are detectable by contemporary observational technique, and, consequently, may not be neglected. In addition, sphericity effects are important also in stars of other types (Kubát 1996). We decided to test the assumption of the plane parallel geometry also for sample model atmospheres of central stars of planetary nebulae. We assumed static atmospheres in radiative, hydrostatic, and statistical equilibrium (NLTE) consisting of hydrogen and helium. We calculated spherically symmetric and plane parallel model atmospheres of two central stars, namely LoTr4 and K1-27. Basic atmospheric parameters of these stars were adopted from Rauch et al. (1996, 1994), respectively. The atmospheric parameters for LoTr4(Teff = 120000K, log g = 5.5, M = 0.65M⊙, n(H)/n(He) = 0.5 yield a model with an extension (r(τR = 10–5)/r(τR = 2/3)) of 1.028. For K1-27, the atmospheric parameters (Teff = 100000K, log g = 6.5, M = 0.52M⊙, n(H)/n(He) = 0.2 produce only a small extension of 1.0035. The differences in temperature structure of our model atmospheres are more pronounced for a star with lower gravity and, consequently, larger extension. Continuum flux is lower for spherical atmospheres. This difference is larger for LoTr4, i.e. a star with more extended atmosphere, and almost negligible for K1-27. Paschen and Pickering He ii lines also show differences. These differences are quite large (several per cent) for α lines, and they decrease towards higher series members. Using plane-parallel model atmospheres instead of spherically symmetric ones introduces a systematic error into results. This error is present also in highly sophisticated NLTE line blanketed models. Due to the above mentioned differences one should avoid using lower series members (e.g. He ii 4686 å) for determination of atmospheric parameters (Teff, log g, abundances,…) from plane-parallel atmospheres. Details of our calculations will be presented in Astronomy and Astrophysics.

Hervy Element Abundances Predicted by Radiative Support Theory in the Atmospheres of Hot White Dwarfs

The surface composition of a white dwarf evolves as a result of the interaction of several mechanisms, the most important of which being gravitational settling. In the early phases of the evolution, theory shows that selective radiative levitation can occasionally defeat settling and, thus, prevent the formation of a pristine pure hydrogen (helium) atmospheric layer in a hot DA (non-DA) white dwarf (Fontaine and Michaud 1979; Vauclair, Vauclair, and Greenstein 1979). The exciting discovery of sharp metallic features in the ultraviolet spectra of several hot DA and non-DA stars alike resulting from the work of several investigators has provided the essential motivation for further theoretical investigations of radiative levitation in the atmospheres of white dwarfs. Additionnal impetus comes from the continuing investigations of hot DA white dwarfs carried out by Bruhweiler and Kondo which have already revealed a most interesting observational pattern of heavy elements in these stars (Bruhweiler 1985). Moreover the recent availability of theoretical equivalent widths of selected astrophysically important ultraviolet metal lines in hot DA white dwarfs (Henry, Shipman, and Wesemael 1985) makes a comparison between theory and observations -in at least this type of stars- a timely and useful exercise.