Jochen L. Deetjen

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.

The everchanging pulsating white dwarf GD358

We report 323 hours of nearly uninterrupted time series photometric observations of the DBV star GD 358 acquired with the Whole Earth Telescope (WET) during May 23rd to June 8th, 2000. We acquired more than 232000 independent measurements. We also report on 48 hours of time-series photometric observations in Aug 1996. We detected the non-radial g-modes consistent with degree l = 1 and radial order 8 to 20 and their linear combinations up to 6th order. We also detect, for the first time, a high amplitude l = 2 mode, with a period of 796 s. In the 2000 WET data, the largest amplitude modes are similar to those detected with the WET observations of 1990 and 1994, but the highest combination order previously detected was 4th order. At one point

Observations of the pulsating subdwarf B star Feige 48: Constraints on evolution and companions

Since pulsating subdwarf B (sdBV or EC14026) stars were first discovered, observational efforts have tried to realize their potential for constraining the interior physics of extreme horizontal branch stars. Difficulties encountered along the way include uncertain mode identifications and a lack of stable pulsation mode properties. Here we report on Feige 48, an sdBV star for which follow-up observations have been obtained spanning more than four years. These observations show some stable pulsation modes. We resolve the temporal spectrum into five stable pulsation periods in the range 340–380 s with amplitudes less than 1 per cent, and two additional periods that appear in one data set each. The three largest amplitude periodicities are nearly equally spaced, and we explore the consequences of identifying them as a rotationally split l= 1 triplet by consulting a representative stellar model. The general stability of the pulsation amplitudes and phases allows us to use the pulsation phases to constrain the time-scale of evolution for this sdBV star. Additionally, we are able to place interesting limits on any stellar or planetary companion to Feige 48.

Iron abundance in hot hydrogen-deficient central stars and white dwarfs from FUSE, HST, and IUE spectroscopy

We present a rst systematic investigation of the iron abundance in very hot (Te 50 000 K) hydrogen- decient post-AGB stars. Our sample comprises 16 PG 1159 stars and four DO white dwarfs. We use recent FUSE observations as well as HST and IUE archival data to perform spectral analyses with line blanketed NLTE model atmospheres. Iron is not detected in any PG 1159 star. In most cases this is compatible with a solar iron abundance due to limited quality of HST and IUE data, although the tendency to an iron underabundance may be recognized. However, the absence of iron lines in excellent FUSE spectra suggests an underabundance by at least 1 dex in two objects (K 1-16, NGC 7094). A similar result has been reported recently in the (WC)-PG 1159 transition object Abell 78 (Werner et al. 2002). We discuss dust fractionation and s-process neutron-captures as possible origins. We also announce the rst identication of sulfur in PG 1159 stars.

HS0702+6043 – A new large amplitude sdB variable at the cool end of the instability region

We report on time series photometry of hot sdB stars from the Hamburg Schmidt survey carried out at the Calar Alto 1.23 m and 2.2 m telescopes. Among 14 spectroscopically selected candidates we detected oscillations in the sdB star HS0702+6043 with a period of 363 s and an amplitude of 29 mmag which is among the largest amplitude observed in sdBV stars. A second period of 382 s with a signicantly smaller amplitude (3.8 mmag) might be present. Our NLTE model atmosphere analysis of the time-averaged optical spectra, obtained at the Calar Alto 3.5 m telescope, indicates that HS0702+6043 has Te = 28 400 K and log g =5 :35. This places the star at the cool end of the theoretical sdBV instability strip (EC 14026 stars). Among the other thirteen sdB stars we conrmed the variablity of the previously discovered sdBV HS0815+4243 and identied twelve stars to be stable, for which we give upper limits for light variations.

Preliminary Results from XCOV 17: PG 1336-018

We present a preliminary analysis of the data collected during the 1999/1 (XCOV17) Whole Earth Telescope (WET) campaign to observe the pulsating subdwarf B star PG 1336-018. Since PG 1336-018 is an HW Vir type eclipsing binary, our hope is to use the known orbital period, assume a tidally locked system, and search for rotationally split modes. To this end, we can already report some success.

Metal Abundances in PG1159 Stars from Chandra and Fuse Spectroscopy

We investigate FUSE spectra of three PG1159 stars and do not find any evidence for iron lines. From a comparison with NLTE models we conclude a deficiency of 1-1.5 dex. We speculate that iron was transformed into heavier elements. A soft X-ray Chandra spectrum of the unique H- and He-deficient star H1504+65 is analyzed. We find high neon and magnesium abundances and confirm that H1504+65 is the bare core of either a C-O or a O-Ne-Mg white dwarf.

Temperature Scale and Iron Abundances of Very Hot Central Stars of Planetary Nebulae

The determination of effective temperatures of very hot central stars (Teff>70000K) by model atmosphere analyses of optical H and He line profiles is afflicted with considerable uncertainty, primarily due to the lack of neutral helium lines. Ionization balances of metals, accessible only with UV lines, allow more precise temperature estimates. The potential of iron lines is pointed out. At the same time iron and other metal abundances, hardly investigated until today, may be derived from UV spectra. We describe recent HST spectroscopy performed for this purpose. A search for iron lines in FUV spectra of the hottest H-deficient central stars (PG1159-type, Teff>100000K) taken with FUSE was unsuccessful. The derived deficiency is interpreted in terms of iron depletion due to n-capture nucleosynthesis in intershell matter, which is now exposed at the stellar surface as a consequence of a late He shell flash.

Preliminary Results of the wet XCOV22 Campaign at Calar alto Observatory

The Whole Earth Telescope had been initiated as a collaboration of midsize telescopes, equipped with high-speed photometers, covering different ranges of longitudes as completely as possible (Nather et al., 1990). To resolve the frequency spectrum of a star rapidly pulsating in multiple modes, it is necessary to avoid the severe problem of aliasing (introduced at single sites by the day-night cycle in the frequency range of interest) by uninterrupted monitoring. Since CCD cameras as an alternative instrumentation have also started to contribute to these world-wide coordinated observing campaigns, the Calar Alto 1.2 m telescope has repeatedly been used to participate in WET runs. The WET Xcov22 campaign in May 2002 marked the last opportunity where this was possible since the telescope will in the future not be available to the German-Spanish astronomical community as a whole any more.

Analysis Of Fuse and IUE Spectra of the SDOB Star EC11481-2303

SdOB stars are associated with the extreme horizontal branch (EHB). They by-pass AGB evolution because of their low H-envelope mass and they may be progenitors of white dwarfs with peculiar chemical abundances. Recently, Stys et al. (2000) have announced the detection of an unusual object which optically resembles the spectrum of an sdOB star. From a Balmer and Hell line fit they derive T eff = 41790K, log g=5.84 and a subsolar helium abundance n(He)/n(H) = 0.014.