H. Drechsel

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.

HS 0705+6700: A new eclipsing sdB binary ?;??

We report the discovery of an eclipsing binary { HS 0705+6700 { being an sdB star with a faint companion. From its light curve the orbital period of 8263.87 s, the mass ratio of the system q =0 :28, the inclination of 84:4 and other system parameters are derived. The companion does not contribute to the optical light of the system except through a strong reflection eect. The semi-amplitude of the radial velocity curve K1 =8 5: 8k m s 1 and a mass function of f(m )=0 :00626 M are determined. A spectroscopic analysis of the blue spectra results in Te = 28 800 K, log g =5 :40, and log(nHe=nH )= 2:68. These characteristics are typical for sdB stars, as is its mass of 0.48 M. According to its mass (0.13 M) and radius (0.19 R), the companion is an M dwarf. The primary is in a core helium burning phase of evolution, and the system must have gone through a common envelope stage when the primary was near the tip of the red giant branch.

BINARIES DISCOVERED BY THE MUCHFUSS PROJECT: SDSS J08205+0008-AN ECLIPSING SUBDWARF B BINARY WITH A BROWN DWARF COMPANION

Hot subdwarf B stars (sdBs) are extreme horizontal branch stars believed to originate from close binary evolution. Indeed about half of the known sdB stars are found in close binaries with periods ranging from a few hours to a few days. The enormous mass loss required to remove the hydrogen envelope of the red-giant progenitor almost entirely can be explained by common envelope ejection. A rare subclass of these binaries are the eclipsing HW Vir binaries where the sdB is orbited by a dwarf M star. Here, we report the discovery of an HW Vir system in the course of the MUCHFUSS project. A most likely substellar object ({approx_equal}0.068 M{sub sun}) was found to orbit the hot subdwarf J08205+0008 with a period of 0.096 days. Since the eclipses are total, the system parameters are very well constrained. J08205+0008 has the lowest unambiguously measured companion mass yet found in a subdwarf B binary. This implies that the most likely substellar companion has not only survived the engulfment by the red-giant envelope, but also triggered its ejection and enabled the sdB star to form. The system provides evidence that brown dwarfs may indeed be able to significantly affect late stellar evolution.

Binaries discovered by the SPY project I. HE 1047{0436: A subdwarf B + white dwarf system ?;??

In the course of our search for double degenerate binaries as potential progenitors of type Ia supernovae with the UVES spectrograph at the ESO VLT (ESO S NI aProgenitor surveY { SPY) we discovered that the sdB star HE 1047 0436 is radial velocity variable. The orbital period of 1.213253 d, a semi-amplitude of 94 km s 1 , and a minimum mass of the invisible companion of 0:44M are derived from the analysis of the radial velocity curve. We use an upper limit on the projected rotational velocity of the sdB star to constrain the system inclination and the companion mass to M> 0:71M, bringing the total mass of the system closer to the Chandrasekhar limit. However, the system will merge due to loss of angular momentum via gravitational wave radiation only after several Hubble times. Atmospheric parameters and metal abundances are also derived. The resulting values are typical for sdB stars.

Binaries discovered by the SPY project. II. HE 1414-0848: A double degenerate with a mass close to the Chandrasekhar limit

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 that the white dwarf HE 1414 0848 is a double-lined DA+DA binary with an orbital period of P =1 2 h 25 m 39 s . Semi-amplitudes of 128 km s 1 and 100 km s 1 are derived for the individual components. The amplitude ratio and the measured dierence in gravitational redshift is used to estimate the masses of the individual components: 0:55 M and 0:71 M. Hence the total mass of the HE 1414 0848 system is 1:26 M, only 10% below the Chandrasekhar limit. The results of a model atmosphere analysis are consistent with our mass estimated from the orbit. Temperatures of the individual components are also determined. Possible scenarios for the formation of this system are discussed. The system will merge due to loss of angular momentum via gravitational wave radiation after two Hubble times. HE 1414 0848 does not qualify as an SN Ia progenitor, but it is the most massive close DD known today.

Tomographic separation of composite spectra. x. the massive close binary hd 101131

We present the first orbital elements for the massive close binary HD 101131, one of the brightest objects in the young open cluster IC 2944. This system is a double-lined spectroscopic binary in an elliptical orbit with a period of 9.64659 ± 0.00012 days. It is a young system of unevolved stars (approximately 2 million yr old) that are well within their critical Roche surfaces. We use a Doppler tomography algorithm to reconstruct the individual component optical spectra, and we apply well-known criteria to arrive at classifications of O6.5 V((f)) and O8.5 V for the primary and secondary, respectively. We compare the reconstructed spectra of the components to single-star spectrum standards to determine a flux ratio of f2/f1 = 0.55 ± 0.08 in the V band. Both components are rotating faster than synchronously. We estimate the temperatures and luminosities of the components from the observed spectral classifications, composite V magnitude, and cluster distance modulus. The lower limits on the masses derived from the orbital elements and the lack of eclipses are 25 and 14 M☉ for the primary and secondary, respectively. These limits are consistent with the somewhat larger masses estimated from the positions of the stars in the Hertzsprung-Russell diagram and evolutionary tracks for single stars.

EC 10246−2707: an eclipsing subdwarf B + M dwarf binary

We announce the discovery of a new eclipsing hot subdwarf B + M dwarf binary, EC 10246-2707, and present multi-colour photometric and spectroscopic observations of this system. Similar to other HW Vir-type binaries, the light curve shows both primary and secondary eclipses, along with a strong reflection effect from the M dwarf; no intrinsic light contribution is detected from the cool companion. The orbital period is 0.1185079936 +/- 0.0000000009 days, or about three hours. Analysis of our time-series spectroscopy reveals a velocity semi-amplitude of K_1 = 71.6 +/- 1.7 km/s for the sdB and best-fitting atmospheric parameters of Teff = 28900 +/- 500 K, log g = 5.64 +/- 0.06, and log[N(He)/N(H)] = -2.5 +/- 0.2. Although we cannot claim a unique solution from modeling the light curve, the best-fitting model has an sdB mass of 0.45 Msun and a cool companion mass of 0.12 Msun. These results are roughly consistent with a canonical-mass sdB and M dwarf separated by a ~ 0.84 Rsun. We find no evidence of pulsations in the light curve and limit the amplitude of rapid photometric oscillations to 7.2 x 10^(-12). If EC 10246-2707 evolves into a cataclysmic variable, its period should fall below the famous CV period gap.

Early evolution of the extraordinary Nova Delphini 2013 (V339 Del)

We determine the temporal evolution of the luminosity L(WD), radius R(WD) and effective temperature Teff of the white dwarf (WD) pseudophotosphere of V339 Del from its discovery to around day 40. Another main objective was studying the ionization structure of the ejecta. These aims were achieved by modelling the optical/near-IR spectral energy distribution (SED) using low-resolution spectroscopy (3500 - 9200 A), UBVRcIc and JHKLM photometry. During the fireball stage (Aug. 14.8 - 19.9, 2013), Teff was in the range of 6000 - 12000 K, R(WD) was expanding non-uniformly in time from around 66 to around 300 (d/3 kpc) R(Sun), and L(WD) was super-Eddington, but not constant. After the fireball stage, a large emission measure of 1.0-2.0E+62 (d/3 kpc)**2 cm**(-3) constrained the lower limit of L(WD) to be well above the super-Eddington value. The evolution of the H-alpha line and mainly the transient emergence of the Raman-scattered O VI 1032 A line suggested a biconical ionization structure of the ejecta with a disk-like H I region persisting around the WD until its total ionization, around day 40. It is evident that the nova was not evolving according to the current theoretical prediction. The unusual non-spherically symmetric ejecta of nova V339 Del and its extreme physical conditions and evolution during and after the fireball stage represent interesting new challenges for the theoretical modelling of the nova phenomenon.

A new bright eclipsing hot subdwarf binary from the ASAS and SuperWASP surveys

We report the discovery of a bright (mV = 11.6 mag) eclipsing hot subdwarf binary of spectral type B with a late main sequence companion from the All Sky Automated Survey (ASAS 102322-3737.0). Such systems are called HW Vir stars after the prototype. The lightcurve shows a grazing eclipse and a strong reflectio n effect. An orbital period of P = 0.13927 d, an inclination of i = 65.86 ◦ , a mass ratio q = 0.34, a radial velocity semiamplitude K1 = 81.0 km s −1 , and other parameters are derived from a combined spectroscopic and photometric analysis. The short period can only be explained by a common envelope origin of the system. The atmospheric parameters (Teff = 28 400 K, log g = 5.60) are consistent with a core helium-burning star located on the extreme horizontal branch. In agreement with that we derived the most likely sdB mass to be MsdB = 0.46M⊙, close to the canonical mass of such objects. The companion is a late M-dwarf with a mass of Mcomp = 0.16 M⊙. ASAS 102322-3737.0 is the third brightest of only 12 known HW Virginis systems, which makes it an ideal target for detailed spectroscopic studies and long term photometric monitoring to search for period variations, e.g. caused by a substellar companion.

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.