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T. Lisker

Publications and source records attributed to T. Lisker.

27 records · Page 2Linked to original sources
arXiv2006

Hot subdwarfs from the ESO Supernova Ia Progenitor Survey: II. Atmospheric parameters of subdwarf O stars

We address the origin and evolutionary status of hot subdwarf stars by studying the optical spectral properties of 58 subdwarf O (sdO) stars. Combining them with the results of our previously studied subdwarf B (sdB) stars, we aim at investigating possible evolutionary links. We analyze high-resolution ESO VLT UVES spectra from the ESO Supernova Ia Progenitor Survey (SPY). Effective temperatures, gravities, and helium abundances are determined simultaneously by fitting the profiles of H and He lines using dedicated synthetic spectra in NLTE. Evidence for cool companions to 8 sdOs as well as a binary consisting of two sdO stars is found. A correlation between He abundances and the presence of carbon and/or nitrogen lines emerges: below solar He abundance, no sdO shows C or N lines. In contrast, C and/or N lines are present in ALL sdOs with super- solar He abundance. We thus use the solar He abundance to divide our sample into He-deficient and He-enriched sdOs. While He-deficient sdOs are scattered in a wide range of the Teff-log(g)-diagram, most of the He-enriched sdOs cluster in a narrow region at Teff = 40,000 ... 50,000K and log(g)=5.5 ... 6.0. An evolu- tionary link between sdBs and sdOs appears plausible only for the He-deficient sdOs indicating that they are the likely successors to sdBs. The properties of He-enriched sdOs cannot be explained with canonical single star evolutionary models. Alternative scenarios (late hot flasher) as well as for binary evolution (white dwarf merger; post-RGB evolution) are tested. While we regard the post-RGB scenario as inappropriate, the white dwarf merger and the late hot flasher scenarios remain viable to explain the origin of He-enriched sdOs.

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arXiv2005

GOODS, UDF, and the evolution of early-type galaxies

The HST/ACS images of GOODS-South have been used to select a sample of early-type galaxies, based on morphology and on the Kormendy relation. The classification scheme does not use galaxy colours, hence it does not bias against young stellar populations. The 249 galaxies (i_AB<24) paint a complex formation picture. Their stellar populations show gradients which readily rule out a large range of ages among and within galaxies. On the other hand, there is a decrease in the comoving number density, which suggests a strong bias when comparing local and distant early-type galaxies. This bias can be caused either by a significant fraction of non early-type progenitors or by a selection effect (e.g. dust-enshrouded progenitors). The deep images of the UDF were used to determine the structural properties of some of these galaxies. Regarding the distribution of disky/boxy isophotes, we do not find large differences with respect to local systems. However, some early-types with the standard features of a red and dead galaxy reveal interesting residuals, possible signatures of past merging events.

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arXiv2005

Evolution of field early-type galaxies: The view from GOODS/CDFS

(Abridged) We explore the evolution of field early-type galaxies on a sample extracted from GOODS/CDFS. The galaxies are selected by means of a non-parametric analysis followed by visual inspection. We exclude those galaxies which are not consistent with an evolution into the Kormendy relation. The final set comprises 249 galaxies with a median redshift z=0.7. The distribution of number counts versus apparent magnitude suggests a substantial decrease of the comoving number density with redshift. The majority of the galaxies feature passively evolving old stellar populations. One third of those in the upper half of the redshift distribution have blue colors, in contrast to only 10% in the lower redshift subsample. An adaptive binning of the color maps is performed to explore the internal color distribution. We find that most blue galaxies in our sample feature blue cores whereas most of the red early-types are passively evolving stellar populations with red cores. The color gradients and scatter do not evolve with redshift and are compatible with the observations at z=0 assuming a radial dependence of the metallicity within each galaxy. This work emphasizes the need for a careful sample selection, as we found that most of those galaxies which were visually classified as early types -- but then rejected based on the Kormendy relation -- feature blue colors characteristic of recent star formation.

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arXiv2005

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.

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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

Hot subdwarfs from the ESO Supernova Ia Progenitor Survey - I. Atmospheric parameters and cool companions of sdB stars

We present the analysis of a high-resolution, high-quality sample of optical spectra for 76 subdwarf B (sdB) stars from the ESO Supernova Ia Progenitor Survey (SPY). Effective temperature, surface gravity, and photospheric helium abundance are determined simultaneously by fitting the profiles of hydrogen and helium lines using synthetic spectra calculated from LTE and NLTE model atmospheres. We perform a detailed comparison of our measurements with theoretical calculations, both for single star evolution and for binary population synthesis models of close binary evolution. The luminosity evolution given by the standard EHB evolutionary tracks from Dorman et al. (1993) shows an overall agreement in shape with our observations, although a constant offset in luminosity exists. The various simulation sets for binary formation channels of sdB stars calculated by Han et al. (2003) are compared individually to our data for testing our current understanding of sdB formation processes and the physical effects involved. The best-matching sets manage to reproduce the observed sdB distribution in the temperature-gravity-plane well. However, they do not match the observed cumulative luminosity function, indicating that theoretical improvement is necessary. We also investigate composite-spectrum objects showing clear signatures of a cool companion with optical and infrared photometry. These stars have cool main sequence companions of spectral types F to K. Typical helium abundances of composite and non-composite sdB stars do not differ.

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arXiv2004

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.

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arXiv2003

Spectroscopic analysis of sdB stars from the ESO Supernova Ia Progenitor Survey

We report on the analysis of high-resolution optical spectra for 77 subdwarf B (sdB) stars from the ESO Supernova Ia Progenitor Survey. Effective temperature, surface gravity, and photospheric helium abundance are determined simultaneously by spectral line profile fitting of hydrogen and helium lines, and are found to be in agreement with previous studies of sdB stars. 24 objects show spectral signs of a cool companion, being either companion absorption lines or a flux contribution at Halpha. Five stars with relatively high luminosity show peculiar Halpha profiles, possibly indicating stellar winds. Our results are compared to recent theoretical simulations by Han et al. (2003) for the distribution in effective temperature and surface gravity, and are found to agree very well with these calculations. Finally we present a binary system consisting of two helium-rich hot subdwarfs.

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