arXiv subjects

S. Dreizler

Publications and source records attributed to S. Dreizler.

At least 199 records · Page 11Linked to original sources

arXiv2004

Subluminous O stars from the ESO Supernova Progenitor Survey - Observation versus Theory

The ESO Supernova Progenitor Survey (SPY) has identified 58 (mostly helium-rich) subluminous O stars. We use the Balmer line strength to distinguish sdO from He-sdO (no Balmer lines) and present the results of the analyses of high resolution optical VLT-UVES spectra using an extensive grid of NLTE atmosphere models covering a large range in Teff, log g and helium abundances. The stellar atmospheric parameters are derived from line profile fits using a chi**2 technique. The resulting distribution in the (Teff, log g) diagram as well as the luminosity function are discussed in the context of stellar evolution scenarios. By combining our results with those for the sdB stars from SPY (Lisker et al. 2004) we discuss the implications for binary population synthesis models of Han et al. (2003). Models with a low CEE efficiency and a constant mass ratio distribution provide a reasonable explanation of the observed properties of the SPY sample of sdB and sdO stars indicating that the sdO stars form the hot and luminous extension of the sdB sequence. However, for the He-sdO stars none of the considered evolution scenarios are in agreement with the measured parameters of our programme stars. We conclude that He-sdO stars are formed by a different process than the sdB and sdO stars.

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arXiv2004

Discovery of a Long-Period Photometric Variation in the V361 Hya Star HS 0702+6043

We report the discovery of a long-period g-mode oscillation in the previously known short-period p-mode sdB pulsator HS 0702+6043. This makes this star an extraordinary object, unique as a member of the family of sdB pulsators, and one of the very few known pulsating stars overall exhibiting excited modes along both the acoustic and gravity branches of the nonradial pulsation spectrum. Because p-modes and g-modes probe different regions of a pulsating star, HS 0702+6043 holds a tremendous potential for future detailed asteroseismological investigations.

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arXiv2004

Metal Abundances in Hot DA White Dwarfs

We compare measured element abundances in hot DA white dwarfs from UV observations to predictions from our self-consistent non-LTE model atmosphere diffusion calculations.

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arXiv2004

HST and FUSE Spectroscopy of Hot Hydrogen-Rich Central Stars of Planetary Nebulae

High-resolution UV spectra, obtained with HST and FUSE, enable us to analyse hot hydrogen-rich central stars in detail. Up to now, optical hydrogen and helium lines have been used to derive temperature and surface gravity. Those lines, however, are rather insensitive; in particular, neutral helium lines have completely vanished in the hottest central stars. Therefore, we have concentrated on ionization balances of metals, which have a rich line spectrum in the UV, to establish a new temperature scale for our sample. Furthermore, we have determined abundances of light metals, which had been poorly known before. They show considerable variation from star to star. We present results of quantitative spectral analyses performed with non-LTE model atmospheres.

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arXiv2004

Iron Abundance in Hydrogen-Rich Central Stars of Planetary Nebulae

We report on an on-going analysis of high-resolution UV spectra of hot hydrogen-rich central stars of planetary nebulae (CSPN), obtained with the Hubble Space Telescope and FUSE. Since UV spectra of many CSPN are dominated by Fe and Ni lines, we intend to use them as temperature indicators to check the CSPN temperature scale we have derived earlier from CNO ionization balances. Furthermore, the observed line strengths of heavy metals show large variations between different objects suggesting a possible spread in abundances. We will determine abundances of iron group elements by quantitative spectral analyses with non-LTE model atmospheres.

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arXiv2004

On Possible Oxygen/Neon White Dwarfs: H1504+65 and the White Dwarf Donors in Ultracompact X-ray Binaries

We discuss the possibility to detect O/Ne white dwarfs by evidence for Ne overabundances. The hottest known WD, H1504+65, could be the only single WD for which we might be able to proof its O/Ne nature directly. Apart from this, strong Ne abundances are known or suspected only from binary systems, namely from a few novae, and from a handful of LMXBs. We try to verify the hypothesis that the latter might host strongly ablated O/Ne WD donors, by abundance analyses of the accretion disks in these systems. In any case, to conclude on O/Ne WDs just by strong Ne overabundances is problematic, because Ne enrichment also occurs by settling of this species into the core of C/O WDs.

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arXiv2004

AcDc - A new code for the NLTE spectral analysis of accretion discs: application to the helium CV AM CVn

We present a recently developed code for detailed NLTE calculations of accretion disc spectra of cataclysmic variables and compact X-ray binaries. Assuming a radial structure of a standard alpha-disc, the disc is divided into concentric rings. For each disc ring the solution of the radiation transfer equation and the structure equations, comprising the hydrostatic and radiative equilibrium, the population of the atomic levels as well as charge and particle conservation, is done self-consistently. Metal-line blanketing and irradiation by the central object are taken into account. As a first application, we show the influence of different disc parameters on the disc spectrum for the helium cataclysmic variable AM CVn.

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arXiv2004

HS2237+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 HS2237+8154, identified as a blue variable star from the Hamburg Quasar Survey. Ellipsoidal modulation observed in the R band as well as the Halpha radial velocity variations measured from time-resolved spectroscopy determine the orbital period to be Porb=178.10+-0.08min. The optical spectrum of HS2237+8154 is well described by a combination of a Teff=11500+-1500K white dwarf (assuming log g=8) and a dM3.5+-0.5 secondary star. The distance implied from the flux scaling factors of both stellar components is d=105+-25pc. Combining the constraints obtained from the radial velocity of the secondary and from the ellipsoidal modulation, we derive a binary inclination of i~50deg-70deg and stellar masses of Mwd=0.47-0.67Msun and Msec=0.2-0.4Msun. All observations imply that the secondary star must be nearly Roche-lobe filling. Consequently, HS2237+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.

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arXiv2003

Modeling of Oxygen-Neon Dominated Accretion Disks in Ultracompact X-ray Binaries: 4U1626-67

We report on first results of computing synthetic spectra from H/He-poor accretion disks in ultracompact LMXBs. We aim at the determination of the chemical composition of the very low-mass donor star, which is the core of a former C/O white dwarf. The abundance analysis allows to draw conclusions on gravitational settling in WDs which is an important process affecting cooling times and pulsational g-mode periods.

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arXiv2003

Spectroscopic Analyses of the Blue Hook Stars in NGC 2808: A More Stringent Test of the Late Hot Flasher Scenario

Recent UV observations of the globular cluster NGC2808 (Brown et al. 2001) show a significant population of hot stars fainter than the zero-age horizontal branch (``blue hook'' stars), which cannot be explained by canonical stellar evolution. Their results suggest that stars which experience unusually large mass loss on the red giant branch and which subsequently undergo the helium core flash while descending the white dwarf cooling curve could populate this region. Theory predicts that these ``late hot flashers'' should show higher temperatures than the hottest canonical horizontal branch (HB) stars and should have He- and C-rich atmospheres. As a test of this late hot flasher scenario, we have obtained and analysed medium resolution spectra of a sample of blue hook stars in NGC2808 to derive their atmospheric parameters. Using the same procedures, we have also re-analyzed our earlier spectra of the blue hook stars in omega Cen (Moehler et al. 2002) for comparison with the present results for NGC2808. The blue hook stars in these two clusters are both hotter (Teff > 35,000 K) and more helium-rich than canonical extreme HB stars in agreement with the late hot flasher scenario. Moreover, we find indications for C enhancement in the three most He-enriched stars in NGC2808. However, the blue hook stars still show some H in their atmospheres, perhaps indicating that some residual H survives a late hot flash and then later diffuses to the surface during the HB phase. We note that the presence of blue hook stars apparently depends mostly on the total mass of the globular cluster and not so much on its HB morphology.

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arXiv2003

Helium-rich EHB Stars in Globular Clusters

Recent UV observations of the most massive Galactic globular clusters show a significant population of hot stars below the zero-age HB (``blue hook'' stars), which cannot be explained by canonical stellar evolution. Stars which suffer unusually large mass loss on the red giant branch and thus experience the helium-core flash while descending the white dwarf cooling curve could populate this region. They should show higher temperatures than the hottest canonical HB stars and their atmospheres should be helium-rich and probably C/N-rich. We have obtained spectra of blue hook stars in omega Cen and NGC 2808 to test this possibility. Our analysis shows that the blue hook stars in these clusters reach effective temperatures well beyond the hot end of the canonical EHB and have higher helium abundances than canonical EHB stars. These results support the hypothesis that the blue hook stars arise from stars which ignite helium on the white dwarf cooling curve.

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arXiv2003

MSST observations of the pulsating sdB star PG1605+072

We present the first results from the MultiSite Spectroscopic Telescope (MSST) observations of the sdBV star PG1605+072. Pulsating sdB stars (also known as EC14026 stars) offer the chance to gain new insights into the formation and evolution of extreme Horizontal Branch stars using the tools of asteroseismology. PG1605+072 is an outstanding object in its class, with the richest frequency spectrum, the longest periods, and the largest variations. The MSST campaign took place in May/June 2002 immediately following the Whole Earth Telescope Xcov22 run, which observed PG1605+072 as an alternate target. We will first give an overview of the project and its feasibility, after which we will present the massive data set, made up of 399 hours of photometry and 151 hours of spectroscopy. The overall aims of the project are to examine light/velocity amplitude ratios and phase differences, changes in equivalent width/line index, and wavelength-dependence of photometric amplitudes, and to use these properties for mode identification.

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arXiv2003

2MASSJ0516288+260738: Discovery of the first eclipsing late K + Brown dwarf system?

We report the discovery of a new eclipsing system less than one arcminute south of the pulsating DB white dwarf KUV 05134+2605. The object could be identified with the point source 2MASSJ0516288+260738 published by the Two Micron All Sky Survey. We present and discuss the first light curves as well as some additional colour and spectral information. The eclipse period of the system is 1.29d, and, assuming this to be identical to the orbital period, the best light curve solution yields a mass ratio of m2/m1=0.11, a radius ratio of r2/r1~1 and an inclination of 74 deg. The spectral anaylsis results in a Teff=4200K for the primary. On this basis, we suggest that the new system probably consists of a late K + Brown dwarf (which would imply a system considerably younger than ~0.01 Gyr to have r2/r1~1), and outline possible future observations.

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arXiv2003

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 (<500m/s) 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 M_jup, R<1.6 R_jup, and an orbital separation of about 5 R_sol, 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.

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arXiv2003

Simultaneous time-series spectroscopy and multi-band photometry of the sdBV PG 1605+072

We present time-series spectroscopy and multi-band photometry of the sdBV PG 1605+072 carried out simultaneously at the Calar Alto 2.2m and 3.5m telescopes. The periodogram analysis of the radial velocity curves reveals three frequencies at 2.078, 2.756, and 1.985 mHz for Hbeta and at 2.076, 2.753, and 1.978 mHz for Hgamma. The corresponding radial velocity amplitudes are 12.7, 8.0, and 7.9 km/s for Hbeta and 14.3, 6.5, and 7.2 km/s for Hgamma. Furthermore, we found five frequencies that are present in all wavelength bands of the BUSCA photometer. The frequencies detected in the radial velocity curves are recovered by the photometric measurements. Moreover, additional frequencies were present in the periodograms which could not be identified in all four bands simultaneously. The comparison of the amplitudes presented here with previous results from radial velocity and photometric observations of PG 1605+072 shows a significant change or even switch in the power of the modes within short time scales, i. e. about one year. No changes in frequency were registered and the phases of the modes show no wavelength dependency within our multi-band photometry.

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arXiv2003

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 observations of candidates from the Hamburg Quasar Survey. Fundamental parameters (effective 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 forpossible trends. We discovered a correlation between the helium abundance and the effective temperature: the larger the temperature, the larger the photospheric helium content of sdB stars. Additionally, a separation into two sequences of sdB stars in the effective 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 partlybe explained by NLTE effects. Three apparently normal B stars were discovered, which could be massive stars far away from the galactic plane (7-19kpc). Radial velocities were measured for 23 stars from which we discovered a new radial velocity variable sdB star.

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arXiv2003

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 232 000 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 796s. 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 during the 1996 observations, most of the pulsation energy was transferred into the radial order k=8 mode, which displayed a sinusoidal pulse shape in spite of the large amplitude. The multiplet structure of the individual modes changes from year to year, and during the 2000 observations only the k=9 mode displays clear normal triplet structure. Even though the pulsation amplitudes change on timescales of days and years, the eigenfrequencies remain essentially the same, showing the stellar structure is not changing on any dynamical timescale.

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arXiv2002

Towards asteroseismology of the non-radial pulsating sdB star PG 1605+072

The recently discovered new class of sdB pulsators (sdBV) offers a powerful possibility for the investigation of their interior and thus their evolutionary history. The first step towards applying asteroseismologic tools is the identification of pulsation modes. We reoport on simultaneous spectroscopic and multi-band photometric time series observations of PG 1605+072 and analyse its radial velocity and light curve.

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