Péter Németh

A selection of hot subluminous stars in the GALEX survey - II. Subdwarf atmospheric parameters

We present an update of our low-resolution spectroscopic follow-up and model atmosphere analysis of hot subdwarf stars from the Galaxy Evolution Explorer (GALEX) survey. Targets were selected on the basis of colour indices calculated from the GALEX GR6 NUV, Guide Star Catalogue (GSC2.3.2) V and the Two Micron All Sky Survey (2MASS) J and H photometry. High signal-to-noise ratio spectra were obtained at the European Southern Observatory (ESO) and the Kitt Peak National Observatory (KPNO) over the course of three years. Detailed H, He and CNO abundance analysis helped us improve our Teff, log g and He abundance determination and to constrain CNO abundances. We processed 191 observations of 180 targets and found 124 sdB and 42 sdO stars in this sample while some blue horizontal branch stars were also found in this programme. With quantitative binary decomposition of 29 composite spectra we investigated the incidence of A, F and G type companions. The incidence of late G and K type companions and their effects on subdwarf atmospheric parameters were also examined.

A selection of hot subluminous stars in the GALEX survey – I. Correlation with the Guide Star Catalog

We assembled a catalogue of bright, hot subdwarf and white dwarf stars extracted from a joint ultraviolet, optical and infrared source list. The selection is secured using colour criteria that correlate well with effective temperatures T eff ≳ 12 000 K. We built a N UV - V versus V -J diagram for ≳60000 bright sources using the Galaxy Evolution Explorer (GALEX) N UV magnitude (N UV < 14), the associated Guide Star Catalog (GSC2.3.2) photographic quick-V magnitude and the Two-Micron All-Sky Survey J and H magnitudes. This distillation process delivered a catalogue of ≈700 sources with N UV - V < 0.5 comprising ~160 known hot subdwarf stars and another ~60 known white dwarf stars. A reduced proper-motion diagram built using the proper-motion measurements extracted from the Naval Observatory Merged Astrometric Dataset allowed us to identify an additional ~120 new hot subdwarf candidates and ~10 hot white dwarf candidates. We present a spectroscopic study of a subset of 52 subdwarfs, 48 of them analysed here for the first time, and with nine objects brighter than V ~ 12. Our sample of spectroscopically confirmed hot subdwarfs comprises 10 sdO-type stars and 42 sdB-type stars suitable for pulsation and binary studies. We also present a study of 50 known white dwarfs selected in the GALEX survey and six new white dwarfs from our catalogue of subluminous candidates. Ultraviolet, optical and infrared synthetic magnitudes employed in the selection and analysis of white dwarf stars are listed in appendix.

The heavily polluted atmosphere of the DAZ white dwarf GALEX J193156.8+011745

We report on the discovery of a new heavily polluted white dwarf. The DAZ white dwarf GALEX J193156.8+011745 was identified in a joint Galaxy Evolution Explorer (GALEX)/GSC survey of ultraviolet-excess objects. Optical spectra obtained at ESO New Technology Telescope show strong absorption lines of magnesium and silicon, and a detailed abundance analysis based on Very Large Telescope-Kueyen UVES spectra reveal super-solar abundances of silicon and magnesium and near-solar abundances of oxygen, calcium and iron. The overall abundance pattern bears the signature of on-going accretion on to the white dwarf atmosphere. The infrared spectral energy distribution shows an excess in the H and K bands likely associated with the accretion source.

Pressure shifts and abundance gradients in the atmosphere of the DAZ white dwarf GALEX J193156.8+011745

We present a detailed model atmosphere analysis of high-dispersion and high signal-to-noise ratio spectra of the heavily polluted DAZ white dwarf GALEX J1931+0117. The spectra obtained with the Very Large Telescope (VLT)-Kueyen/UV-Visual Echelle Spectrograph show several well-resolved Si II spectral lines enabling a study of pressure effects on line profiles. We observed large Stark shifts in silicon lines in agreement with theoretical predictions and laboratory measurements. Taking into account Stark shifts in the calculation of synthetic spectra, we reduced the scatter in individual line radial velocity measurements from ∼ 3t o 1k m s −1 . We present revised abundances of O, Mg, Si, Ca and Fe based on a critical review of line-broadening parameters and oscillator strengths. The new measurements are generally in agreement with our previous analysis with the exception of magnesium with a revised abundance of a factor of 2 lower than previously estimated. The magnesium, silicon and iron abundances exceed solar abundances, but the oxygen and calcium abundances are below solar. Also, we compared the observed line profiles to synthetic spectra computed with variable accretion rates and vertical abundance distributions assuming diffusive steady state. The inferred accretion rates vary from u M = 2 × 10 6 for calcium to 2 × 10 9 gs −1 for oxygen. We find that the accretion flow must be oxygen rich while being deficient in calcium relative to solar abundances. The lack of radial velocity variations between two measurement epochs suggests that GALEX J1931+0117 is probably not in a close binary and that the source of the accreted material resides in a debris disc.

Two new hot subdwarf binaries in the GALEX survey

We report the discovery of two new hot, hydrogen-rich subdwarfs (sdB) in close binary systems. The hot subdwarfs, GALEX J0321+4727 and GALEX J2349+3844, were selected from a joint optical–ultraviolet catalogue of hot subluminous stars based on GSC2.3.2 and the Galaxy Evolution Explorer all-sky survey. Using high-dispersion spectra of the Hα core, obtained using the 2-m telescope at Ondy Observatory, we measured the radial velocities of the sdB primaries and determined orbital periods of 0.265 84±0.000 04 d and 0.462 49±0.000 07 d for GALEX J0321+4727 and GALEX J2349+3844, respectively. The time series obtained from the Northern Sky Variability Survey with an effective wavelength near the R-band show that GALEX J0321+4727 is a variable star (�m = 0.12 mag), while no significant variations are observed in GALEX J2349+3844. The period of variations in GALEX J0321+4727 coincides with the orbital period and the variability is probably caused by a reflection effect on a late-type secondary star. Lack of photometric variations in GALEX J2349+3844 probably indicates that the companion is a white dwarf star. Using all available photometry and spectroscopy, we measured the atmospheric properties of the two sdB stars and placed limits on the mass and luminosity of the companion stars.

Abundance Analysis of DAZ White Dwarfs

We present an abundance analysis of a sample of 33 hydrogen‐rich (DA) white dwarfs. We have used archival high‐resolution spectra to measure abundances of calcium, magnesium and iron in a set of 30 objects. In addition, we present preliminary calcium abundances in three new white dwarfs based on low‐dispersion spectra. We investigate some abundance ratios (Mg/Ca, Fe/Ca) that may help uncover the composition of the accretion source.

Composite hot subdwarf binaries – I. The spectroscopically confirmed sdB sample

Hot subdwarf-B (sdB) stars in long-period binaries are found to be on eccentric orbits, even though current binary-evolution theory predicts that these objects are circularized before the onset of Roche lobe overflow (RLOF). To increase our understanding of binary interaction processes during the RLOF phase, we started a long-term observing campaign to study wide sdB binaries. In this paper, we present a sample of composite binary sdBs, and the results of the spectral analysis of nine such systems. The grid search in stellar parameters (gssp) code is used to derive atmospheric parameters for the cool companions. To cross-check our results and also to characterize the hot subdwarfs, we used the independent XTgrid code, which employs Tlusty non-local thermodynamic equilibrium models to derive atmospheric parameters for the sdB component and phoenix synthetic spectra for the cool companions. The independent gssp and XTgrid codes are found to show good agreement for three test systems that have atmospheric parameters available in the literature. Based on the rotational velocity of the companions, we make an estimate for the mass accreted during the RLOF phase and the minimum duration of that phase. We find that the mass transfer to the companion is minimal during the subdwarf formation.

Accretion and Diffusion in the DAZ White Dwarf GALEX J1931+0117

We present an analysis of high‐dispersion and high signal‐to‐noise ratio spectra of the DAZ white dwarf GALEX J1931+0117. The spectra obtained with the VLT‐Kueyen/UV‐Visual Echelle Spectrograph show several well‐resolved Si II spectral lines enabling a study of pressure effects on line profiles. We observed large Stark shifts in silicon lines in agreement with laboratory measurements. A model atmosphere analysis shows that the magnesium, silicon and iron abundances exceed solar abundances, while the oxygen and calcium abundances are below solar. Also, we compared the observed line profiles to synthetic spectra computed with variable accretion rates and vertical abundance distributions assuming diffusion steady‐state. The inferred accretion rates vary from Ṁ = 2×106 for calcium to 2×109 g s−1 for oxygen and indicate that the accretion flow is dominated by oxygen, silicon and iron while being deficient in carbon, magnesium and calcium. The lack of radial velocity variations between two measurement epochs su...

A FEROS Survey of Hot Subdwarf Stars

Abstract We have completed a survey of twenty-two ultraviolet-selected hot subdwarfs using the Fiber-fed Extended Range Optical Spectrograph (FEROS) and the 2.2-m telescope at La Silla. The sample includes apparently single objects as well as hot subdwarfs paired with a bright, unresolved companion. The sample was extracted from our GALEX catalogue of hot subdwarf stars. We identified three new short-period systems (P = 3.5 hours to 5 days) and determined the orbital parameters of a long-period (P = 62d.66) sdO plus G III system. This particular system should evolve into a close double degenerate system following a second common envelope phase.We also conducted a chemical abundance study of the subdwarfs: Some objects show nitrogen and argon abundance excess with respect to oxygen. We present key results of this programme.

On the nature of the Be star HR 7409 (7 Vul)

HR 7409 (7 Vul) is a newly identified Be star, possibly part of the Gould Belt, and is the massive component of a 69-d spectroscopic binary. The binary parameters and properties of the Be star measured using high-dispersion spectra obtained at Ondy Observatory and at Rozhen Observatory imply the presence of a low-mass companion (≈0.5–0.8 M� ). If the pair is relatively young (<50–80 Myr), then the companion is a K V star, but, following another, older evolutionary scenario, the companion is a horizontal branch star or possibly a white dwarf star. In the latter scenario, a past episode of mass transfer from an evolved star on to a less massive dwarf star would be responsible for the peculiar nature of the present-day, fast-rotating Be star.