C. Karl

High-resolution UVES/VLT spectra of white dwarfs observed for the ESO SN Ia progenitor survey (SPY). I

We have started a large survey for radial velocity variations in white dwarfs (PI R. Napiwotzki) with the aim of nding close double degenerates, which could be precursor systems for SNe Ia. The UVES spectrograph at the ESO VLT is used to obtain high resolution spectra with good S=N. During this project 1500 white dwarfs will be observed. This unique data set will also allow to derive atmospheric parameters and masses for the largest sample of white dwarfs ever analyzed in a homogenous way. In this paper we present a catalog of objects and report results for the rst sample of about 200 white dwarfs, many of which are spectroscopic conrmations of candidates from the HE, MCT, and EC surveys. Among the peculiar spectra we identify two new magnetic DA, one previously known magnetic DA, several DA with emission cores, in some cases due to a late-type companion, and two new DBA.

Close binary EHB stars from SPY

We present the results of a radial velocity (RV) survey of 46 subdwarf B (sdB) and 23 helium-rich subdwarf O (He-sdO) stars. We detected 18 (39%) new sdB binary systems, but only one (4%) He-sdO binary. Orbital parameters of nine sdB and sdO binaries, derived from follow-up spectroscopy, are presented. Our results are compared with evolutionary scenarios and previous observational investigations.

Binaries discovered by the SPY project. IV. Five single-lined DA double white dwarfs

We present results from our ongoing follow-up observations of double white dwarf binaries detected in the ESO SN Ia Progenitor SurveY (SPY). We discuss our observing strategy and data analysis and present the orbital solutions of five close double white dwarf binaries: HE0320−1917, HE1511−0448, WD0326−273, WD1013−010 and WD1210+140. Their periods range from 0.44 to 3.22 days. In none of these systems we find any spectral lines originating from the companion. This rules out main sequence companions and indicates that the companion white dwarfs are significantly older and cooler than the bright component. Infrared photometry suggests the presence of a cool, helium-rich white dwarf companion in the binary WD 0326−273. We briefly discuss the consequences of our findings for our understanding of the formation and evolution of double white dwarfs.

Search for progenitors of supernovae type Ia with SPY

Institute of Astronomy of the Russian Academy of Sciences, 48 Pyatnitskaya Str., 109017 Moscow, RussiaReceived date; accepted dateAbstract. We have started a large survey for double degenerate (DD) binaries as potential progenitors of type Ia supernovaewith the UVES spectrograph at the ESO VLT (ESO SNIa Progenitor surveY – SPY). About 400 white dwarfs were checkedfor radial velocity variations during the first 15 months of t his project, twice the number of white dwarfs investigated duringthe last 20 years. We give an overview of the SPY project and present first results Fifty four new DDs have been discovered,seven of them double lined (only 18 and 6 objects of these groups were known before, respectively). The final sample isexpected to contain 150 to 200 DDs. Eight new pre-cataclysmic binaries were also detected. SPY is the first DD surveywhich encompasses also non-DA white dwarfs. SPY produces animmense, unique sample of very high resolution whitedwarf spectra, which provides a lot of spin-off opportunities. We describe our projects to exploit the SPY sample for thedetermination of basic parameters, kinematics, and rotational velocities of white dwarfs. A catalogue with a first subs et ofour white dwarf data has already been published by Koester et al. (2001).

Six detached white-dwarf close binaries

We determine the orbits of four double-degenerate systems (DDs), composed of two white dwarfs and two white-dwarf-M-dwarf binaries. The four DDs, WD1022+050, WD1428+373, WD1824+040 and WD2032+188, show orbital periods of 1.157155(5), 1.15674(2), 6.26602(6) and 5.0846(3) d, respectively. These periods combined with estimates for the masses of the brighter component, based on their effective temperatures, allow us to constrain the masses of the unseen companions. We estimate that the upper limit for the contribution of the unseen companions to the total luminosity in the four DDs ranges between 10 and 20 per cent. In the case of the two white-dwarf-M-dwarf binaries, WD 1042-690 and WD2009+622, we calculate the orbital parameters by fitting simultaneously the absorption line from the white dwarf and the emission core from the M dwarf. Their orbital periods are 0.337083(1) and 0.741 226(2) d, respectively. We find signatures of irradiation on the inner face of the companion to WD2009+622. We calculate the masses of both components from the gravitational redshift and the mass-radius relationship for white dwarfs and find masses of 0.75-0.78 and 0.61-0.64 M ○. for WD 1042-690 and WD2009+622, respectively. This indicates that the stars probably reached the asymptotic giant branch in their evolution before entering a common envelope phase. These two white-dwarf-M-dwarf binaries will become cataclysmic variables, although not within a Hubble time, with orbital periods below the period gap.

High resolution spectroscopy of bright subdwarf B stars. I. Radial velocity variables

Radial velocity curves for 15 bright subdwarf B binary systems have been measured using high precision radial velocity measurements from high S/N optical high-resolution spectra. In addition, two bright sdB stars are discovered to be radial velocity variable but the period could not yet be determined. The companions for all systems are unseen. The periods range from about 0.18 days up to more than ten days. The radial velocity semi amplitudes are found to lie between 15 and 130 km/s. Using the mass functions, the masses of the unseen companions have been constrained to lower limits of 0.03 up to 0.55 M_sun, and most probable values of 0.03 up to 0.81 M_sun. The invisible companions for three of our program stars are undoubtedly white dwarfs. In the other cases they could be either white dwarfs or main sequence stars. For two stars the secondaries could possibly be brown dwarfs. As expected, the orbits are circular for most of the systems. However, for one third of the program stars we find slightly eccentric orbits with small eccentricities of e~0.02-0.06. This is the first time that non-circular orbits have been found in sdB binaries. No correlation with the orbital period can be found.

Binaries discovered by the SPY project. III: HE 2209-1444: A massive, short period double degenerate

In the course of our search for double degenerate (DD) binaries as potential progenitors of type Ia supernovae with the UVES spectrograph at the ESO VLT (ESO S NI aProgenitor surveY - SPY) we discovered HE 2209−1444 to be a double-lined system consisting of two DA white dwarfs. From the analysis of the radial velocity curve we determined the period of the system to be P = 6 h 38 m 47 s . The semi-amplitudes for both individual components are 109 km s −1 each. A model atmosphere analysis enabled us to derive individual temperatures for both components (8490 K and 7140 K, resp.) and masses of 0.58 Mfor each component. The total mass of the system is 1.15 ± 0.07 M� . The system will lose angular momentum due to gravitational wave radiation and therefore will merge within 5 Gyrs - less than a Hubble time. HE 2209−1444 is the second massive, short period double degenerate detected by SPY. Its total mass is about 20% below the Chandrasekhar mass limit and therefore it does not qualify as a potentional SN Ia progenitor. However, together with our previous detections it supports the view that Chandrasekhar mass systems do exist.

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.

HS 2333+3927: A new sdB+dM binary with a large reflection effect

We report the discovery of a binary, HS 2233 + 3927, consisting of an sdB star with a faint companion. From its lightcurve the orbital period of 14,844 s, the mass ratio, the inclination, and other system parameters are derived. The companion does not contribute to the optical light of the system except through a strong reflection effect. The semi-amplitude of the radial velocity curve K1= 89.6 km/s−1 and a mass function of f(m) = 0.013 M⊙ are determined. A preliminary spectroscopic analysis of the blue spectra using NLTE model atmospheres results in Teff= 36 500 K, log g= 5.70, and log(nHe/nH) =−2.15. These parameters are typical for sdB stars, the companion is probably an M dwarf.

Identification of a DO white dwarf and a PG 1159 star in the ESO SN Ia progenitor survey (SPY)

We present high-resolution VLT spectra of a new helium-rich DO white dwarf (HE1314+0018) and a new PG 1159 star (HE1429−1209), which we identified in the ESO SPY survey. We performed NLTE model atmosphere anal- yses and found that the PG 1159 star is a low-gravity, extremely hot (Teff = 160 000 K, log g = 6) star, having a carbon-helium dominated atmosphere with considerable amounts of oxygen and neon (He = 38%, C = 54%, O = 6%, Ne = 2% by mass). It is located within the planetary nebula nuclei instability strip, hence, future searches for an associated PN as well as for stel- lar pulsations might be successful. The DO white dwarf is a unique object. From the relative strength of neutral and ionized helium lines we found Teff ≈ 60 000 K, however, the He  lines are extraordinarily strong and cannot be fitted by any model. Similar problems were encountered with hot subdwarfs and white dwarfs showing signatures of a super-hot wind. The reason is unknown.