S. Dreizler

Spectral analysis of sdB stars from the Hamburg Quasar Survey

We present the results of a spectral analysis of a large sample of subdwarf B stars selected from follow-up obser- vations of candidates from the Hamburg Quasar Survey. Fundamental parameters (eective temperature, gravity, and helium abundance) were determined by matching synthetic line profiles calculated from model atmospheres to all hydrogen and helium absorption lines present in the observed optical spectra. The derived helium abundances are compared with the atmospheric parameters to search for possible trends. We discovered a correlation between the helium abundance and the eective temper- ature: the larger the temperature, the larger the photospheric helium content of sdB stars. Additionally, a separation into two sequences of sdB stars in the eective temperature - helium abundance plane is detected. We compared our analysis results with data from the literature. The stars from our sample are found to be somewhat more luminous. This can only partly be explained by NLTE eects. Three apparently normal B stars were discovered, which could be massive stars far away from the galactic plane (7-19 kpc). Radial velocities were measured for 23 stars from which we discovered a new radial velocity variable sdB star.

HS 2237+8154: On the onset of mass transfer or entering the period gap?

We report follow-up observations of a new white dwarf/red dwarf binary HS 2237+8154, identified as a blue variable star from the Hamburg Quasar Survey. Ellipsoidal modulation observed in the R band as well as the radial velocity variations measured from time-resolved spectroscopy determine the orbital period to be Porb = 178.10 +- 0.08 min. The optical spectrum of HS 2237+8154 is well described by a combination of a Teff = 11500 +- 1500 K white dwarf (assuming log g = 8) and a dM 3.5 +- 0.5 secondary star. The distance implied from the flux scaling factors of both stellar components is d = 105 +- 25 pc. Combining the constraints obtained from the radial velocity of the secondary and from the ellipsoidal modulation, we derive a binary inclination of i = 50-70 and stellar masses of and Mwd = 0.47-0.67 M and Msec = 0.2-0.4 M. All observations imply that the secondary star must be nearly Roche-lobe filling. Consequently, HS 2237+8154 may be either a pre-cataclysmic variable close to the start of mass transfer, or - considering its orbital period - a cataclysmic variable that terminated mass transfer and entered the period gap, or a hibernating nova.

Detection of pulsations in three subdwarf B stars

We report the detection of short period oscillations in the sdB stars HS 0815+4243, HS 2149+0847 and HS 2201+2610 from time-series photometry made at the Nordic Optical Telescope (NOT) from a sample of 31 candidates. Hence these three hot subdwarfs are new members of the EC 14026 class of pulsating sdB stars. One short period is detected for HS 0815+4243 ( P ∼ 126 s; A ∼ 7 mma) and two short periods are seen for HS 2149+0847 ( P ∼ 142, 159 s; A ∼ 11, 7 mma), whereas the single oscillation detected for HS 2201+2610 has a considerably longer period ( P ∼ 350 s; A ∼ 11 mma). Our NLTE model atmosphere analysis of the time-averaged optical spectra indicate that HS 0815+4243 has T eff = 33 700 K and , HS 2149+0847 has K and log , and HS 2201+2610 has T eff = 29 300 K and log . This places the former two at the hot end and the latter at the cool end of the theoretical sdBV instability strip.

PG 1325+101 and PG 2303+019: Two new large amplitude subdwarf B pulsators

We report the detection of short period oscillations in the sdB stars PG 1325+101 (B = 13.8) and PG 2303+019 (alias HS 2303+0152, B = 16.0) from time-series photometry made at the Nordic Optical Telescope of a sample of 21 candidates. Both stars are multi-mode pulsators with at least three distinct periods in the range 100-140 s, and relatively large amplitudes up to 2.6 and 1.6% respectively. Moreover PG 1325+101 shows the shortest pulsation period ever registered among sdBV stars, 68.9 s, which corresponds to the first harmonic of the main signal. Following previous temperature and gravity determinations for PG 1325+101 (T e f f = 34500 K, log g = 6.1) and our NLTE model atmosphere analysis for PG 2303+019 (T e f f = 35 200 K, log g = 5.7), both stars are well inside the theoretical sdBV instability strip.

Four new subdwarf B pulsators

We report the detection of short period oscillations in the sdB stars HS 0039+4302, HS 0444+0408, HS 1824+5745 and HS 2151+0857 from time-series photometry made at the Nordic Optical Telescope (NOT) of a sample of 55 candidates. Hence these four hot subdwarfs are new members of the EC 14026 class of pulsating sdB stars. HS 0039+4302 is a multi-mode pulsator with at least four distinct periods in the range between 182 and 234 s, and amplitudes up to 8 mma. HS 0444+0408 shows one dominant pulsation at 137 s (A 12 mma) and a second weaker pulsation at 170 s (A 3 mma). For HS 1824+5745 we nd a single period of 139 s with an amplitude of about 5 mma. HS 2151+0857 shows four periods in the range 129{151 s with amplitudes between 2 and 5 mma. Our NLTE model atmosphere analysis of the time-averaged optical spectra place all stars well within the theoretical sdBV instability strip.

Two new extremely iron-rich hot DA white dwarfs and the nature of the EUV opacity

We have obtained IUE echelle spectra of two bright extreme ultraviolet (EUV) sources discovered by the ROSAT wide field camera all-sky survey. These stars, RE 2214-492 and RE 0623-377, are previously uncataloged hot DA white dwarfs with respective apparent visual magnitudes of 11.71 and 12.09 and exhibit short-wavelength EUV cutoffs steeper than most other DAs. The IUE echelle spectra are rich in absorption lines due to a large number of Fe V and Fe IV features, in addition to the highly ionized C, N, O ions frequently seen in other hot DA white dwarfs. No He is observed in either star

Rotation velocities of white dwarfs. III. DA stars with convective atmospheres

The sharp Hα NLTE line cores of hydrogen-rich (DA) white dwarfs allow their projected rotational velocities to be determined. High resolution optical spectra of 22 stars obtained with the Keck I telescope are matched by synthetic spectra computed from a grid of NLTE model atmospheres. In this paper, the third in a series on white dwarf rotation, we concentrate preferentially on DA white dwarfs with convective atmospheres, i.e. with T e f f < 14000 K. Previous analyses found DA white dwarfs hotter than 14000 K to be very slow rotators and rarely show any spectroscopically detectable rotation. For 19 of our programme stars we were able to derive projected rotational velocities or upper limits. No rotation could be detected for seven stars in our sample. However twelve stars show significant line broadening. In the case of the ZZ Ceti star G 117-B15A, the observed Hα line profile cannot be matched by a rotationally broadened profile as its line core is too narrow. Combining our results with those from two similar studies, we have obtained information on the rotation or other line broadening mechanisms (such as caused by magnetic fields) of 56 DA white dwarfs. The fraction of rotating DA white dwarfs whose line profiles can be matched for a vanishing projected rotation velocity is high for hot white dwarfs with radiative atmospheres (25 out of 28). Amongst the cool white dwarfs with presumedly convective atmospheres, only for a few stars (8 out of 22) has no additional line broadening to be invoked to explain their observed Hα line profiles. We conjecture that the physics of Hα line formation in convective DA while dwarf atmospheres is not yet sufficiently well understood and additional observational and theoretical efforts have to be made.

OGLE-TR-3: A possible new transiting planet ?

Recently, 59 low-luminosity object transits were reported from the Optical Gravitational Lensing Experiment (OGLE). Our follow-up low-resolution spectroscopy of 16 candidates provided two objects, OGLE-TR-3 and OGLE-TR-10, which have companions with radii compatible with those of gas-giant planets. Further high-resolution spectroscopy revealed a very low velocity variation (<500 m s 1 ) of the host star OGLE-TR-3 which may be caused by its unseen companion. An analysis of the radial velocity and light curve results in M< 2:5 MJup, R< 1:6 RJup, and an orbital separation of about 5 R, which makes it the planet with the shortest period known. This allows to identify the low-luminosity companion of OGLE-TR-3 as a possible new gas-giant planet. If confirmed, this makes OGLE-TR-3 together with OGLE-TR-56 the first extrasolar planets detected via their transit light curves.

The CARMENES search for exoplanets around M dwarfs: Nine new double-line spectroscopic binary stars

Context. The CARMENES spectrograph is surveying ~300 M dwarf stars in search for exoplanets. Among the target stars, spectroscopic binary systems have been discovered, which can be used to measure fundamental properties of stars. Aims. Using spectroscopic observations, we determine the orbital and physical properties of nine new double-line spectroscopic binary systems by analysing their radial velocity curves. Methods. We use two-dimensional cross-correlation techniques to derive the radial velocities of the targets, which are then employed to determine the orbital properties. Photometric data from the literature are also analysed to search for possible eclipses and to measure stellar variability, which can yield rotation periods. Results. Out of the 342 stars selected for the CARMENES survey, 9 have been found to be double-line spectroscopic binaries, with periods ranging from 1.13 to ~8000 days and orbits with eccentricities up to 0.54. We provide empirical orbital properties and minimum masses for the sample of spectroscopic binaries. Absolute masses are also estimated from mass-luminosity calibrations, ranging between ~0.1 and ~0.6 Msol . Conclusions. These new binary systems increase the number of double-line M dwarf binary systems with known orbital parameters by 15%, and they have lower mass ratios on average.

NEW RESULTS ON PG 1159 STARS AND ULTRAHIGH-EXCITATION DO WHITE DWARFS

We address four topics describing our latest progress since the last white dwarf workshop. 1. We report on new HST UV spectra of selected PG 1159 stars which were taken for fine analyses to constrain the GW Vir instability strip. 2. We describe the results of an analysis of a new ROSAT detected PG 1159 star which turns out to be the hottest known object of this spectral type. 3. Latest HST observations of the so-called ultra-high ionization white dwarfs give surprising insights into the “O VIII” phenomenon. 4. Finally we report on results of a recent effort to search for faint nebulae around hot white dwarfs.