FUSE Observations of EC14026 Stars

We present a FUSE abundance analysis of EC14026 stars. We compare the abundances of heavy elements in the atmospheres of EC14026 stars to non-pulsating stars with similar atmospheric parameters, and investigate whether weak stellar winds could explain the coexistence of variable and non-variable sdB stars in the log g – T_eff diagram. We also present preliminary results on time-dependent diffusion calculations of iron in presence of radiative levitation and mass loss, and show how weak stellar winds can affect the diffusive equilibrium between gravitational settling and radiative support.     

Mass-loss predictions for Subdwarf B stars

We present the results of Monte Carlo mass-loss computations for hot low-mass stars, specifically for subdwarf B (sdB) stars. It is shown that the mass-loss rates on the Horizontal Branch (HB) computed from radiative line-driven wind models are not high enough to create sdB stars. We argue, however, that mass loss plays a role in the chemical abundance patterns observed both in field sdB stars, as well as in cluster HB stars. The derived mass loss recipe for these (extremely) hot HB stars may also be applied to other groups of hot low-mass stars, such as post-HB (AGB-manqué, UV-bright) stars, over a range in effective temperatures between ?10 000 and 50 000 K. Finally, we present preliminary spectral synthesis on the more luminous sdB stars for which emission cores in Hα have been detected (Heber, U., et al.: 2003, in:Stellar Atmosphere Modeling, ASP Conference Proceedings, p. 251). We find that these line profiles can indeed be interpreted as the presence of a stellar wind with mass loss on the order of 10^?11?M _⊙ yr ^?1.     

A search for magnetic fields in cool sdB stars

Hot cluster horizontal branch (HB) stars and field subdwarf B (sdB) stars are core helium burning stars that exhibit abundance anomalies that are believed to be due to atomic diffusion. Diffusion can be effective in these stars because they are slowly rotating. In particular, the slow rotation of the hot HB stars (T_eff > 11000 K), which show abundance anomalies, contrasts with the fast rotation of the cool HB stars, where the observed abundances are consistent with those of red giants belonging to the same cluster. The reason why sdB stars and hot HB stars are rotating slowly is unknown. In order to assess the possible role of magnetic fields on abundances and rotation, we investigated the occurrence of such fields in sdB stars with T_eff < 30 000 K, whose temperatures overlap with those of the hot HB stars. We conclude that large‐scale organised magnetic fields of kG order are not generally present in these stars but at the achieved accuracy, the possibility that they have fields of a few hundred Gauss remains open. We report the marginal detection of such a field in SB 290; further observations are needed to confirm it (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

HST/STIS observations of sdBV stars: testing diffusion and pulsation theory

We present the initial results of an abundance analysis of echelle UV spectra of five hot subdwarf B (sdB) stars. These stars have been identified as core helium burning objects on the extreme Horizontal Branch. Around 5% of sdBs show short-period acoustic-mode oscillations. Models predict that these oscillations are due to an opacity bump caused by the ionisation of iron group elements. The necessary metal abundance has to be maintained by diffusive equilibrium between gravitational settling and radiative levitation. However, analyses of high-resolution optical spectra has revealed that we cannot discriminate between pulsating and non-pulsating sdBs on the basis of the surface iron abundance. We have therefore obtained HST/STIS observations of three pulsators and two non-pulsators in the near- and far-UV to measure the surface abundance of elements that are unobservable from the ground. The overall aim of our study is to test diffusion and pulsation calculations by searching for significant differences between these surface abundances.     

Metal abundances of subdwarf B stars—the extended sample

The formation of sdBs as well as the chemical composition of their atmospheres is still puzzling. While helium and other light elements are depleted relative to solar values, heavy elements are highly enriched. Diffusion processes in the hot, radiative atmosphere of these stars are the most likely explanation. Although several attempts have been made, it has not yet been possible to model all the observed features of sdB atmospheres. A drawback of most prior studies was the small sample size. We present a detailed abundance analysis of 139 sdBs. A general trend of enrichment was found with increasing temperature for most of the heavier elements. The lighter elements like carbon, oxygen and nitrogen are depleted and less affected by temperature. Although there is considerable scatter from one star to the other, the general abundance patterns in most sdBs are similar. An interplay between gravitational settling, radiative levitation and weak winds is most likely responsible. About 3% of the analysed stars show an enrichment in carbon and helium, which cannot be explained in the framework of diffusion alone. Nuclear processed material must have been transported to the surface somehow.     

Progress on the Front of Pulsating Subdwarf B Stars

We briefly review the recent advances that have been made on the front of pulsating subdwarf B (sdB) stars. The first family of sdB pulsators, the EC 14026 stars, was discovered a few years ago and consists of short-period (～100?200 s) p-mode variables. The second type of pulsating sdB’s consists of the PG 1716+426 stars, a group of variables showing long-period (～1 h) g-mode pulsations. The existence of the latter was first reported less than a year ago. While the two types of sdB pulsators differ markedly in their observational characteristics, we recently found a unifying property in the sense that the observed modes in these objects are excited through the same driving process, a classic kappa mechanism associated with the radiative levitation of iron in the stellar envelope.     

Hot subdwarfs in binary systems and the nature of their unseen companions

A large fraction of the sdB stars reside in short period binaries. It is therefore clear that binary evolution plays an important role in the still unsolved problem of hot subdwarf formation. Here we present new results from different projects devoted to the analysis of sdBs in close binaries. The nature and masses of the unseen companions of 31 sdBs have been constrained by an analysis of high resolution spectra. In the course of this study candidate systems with massive compact companions have been discovered. The HYPERMUCHFUSS project aims at finding such systems making use of the huge spectral database of SDSS. A multi-site follow-up campaign of promising radial velocity variable sdBs started in 2009 and preliminary results are shown here. The most recent discovery of a substellar companion to the bright sdB HD?149382 may provide new evidence for the decisive role of low mass companions for sdB formation in general. A mysterious IR-excess has been detected, which may be caused by this otherwise invisible companion. Another low mass companion has been found to orbit the sdB star EGB?5 within 16.5 days. The space mission CoRoT is performing wide field and high precision photometry. First preliminary results from a spectroscopic survey of the COROT fields are also reported.     

Pulsating sdB Stars: A New Approach to Probing their Interiors

Horizontal branch stars should show significant differential rotation with depth. Models that assume systematic angular momentum exchange in the convective envelope and local conservation of angular momentum in the core produce HB models that preserve a rapidly rotating core. A direct probe of core rotation is available. The nonradial pulsations of the EC14026 stars frequently show rich pulsation spectra. Thus their pulsations probe the internal rotation of these stars, and should show the effects of rapid rotation in their cores. Using models of sdB stars that include angular momentum evolution, we explore this possibility and show that some of the sdB pulsators may indeed have rapidly rotating cores.     

Radial velocity curves of nine sdB binary stars

Radial velocity curves have been measured for nine sdB stars from high resolution optical spectra and are found to be sinusoidal indicating that they are binary stars with circular orbits. Their periods range from ≈12 h to more than 8 days. The companions are invisible in optical light. Minimum companion masses are derived from the mass functions, assuming the mass of the sdB primary to be half a solar mass. We argue that the companions to UVO 1735+22, Feige 108, HD 188112, and HD 171858 are white dwarfs, since their (statistically) most likely mass is above ≈0.7M _⊙. Five of the systems have not been investigated before, for the others our results agree with previously published ones.     

The HYPER-MUCHFUSS project—target selection and analysis

The HYPER-MUCHFUSS project targets a population of high velocity subluminous B stars to discover either close binaries with massive unseen companions or hyper-velocity stars. Our starting point is the enormous database of SDSS. We preselected sdO/B candidates by colour and classified them by visual inspection of their spectra. We measured the radial velocity from the coadded SDSS spectra, which serves as first epoch measurement. Stars with high Galactic rest-frame velocities were selected and second epoch observations were obtained starting in 2007 at several sites. For the brighter targets we also included the SDSS individual spectra as additional information. In the course of our survey we observed 88 out of 265 stars from our target list. We discovered 39 HVS candidates as well as 49 close binaries. In addition we analysed all single spectra of sdBs from SDSS and found 120 close binaries. For the targets with constant RVs we performed a proper motion analysis with the highest possible accuracy from the available digitised photographic plates. Together with the analysed spectra and the calculation of the spectroscopic distance, we calculated complete trajectories and deduced the origins of these stars. Targets with high RV variability on short timescales were selected for follow-up. Numerical simulations based on the period and companion mass distribution of the known sdB binary sample were carried out to optimise the target selection and single out candidate binaries with massive companions. The follow-up campaign using WHT/ISIS and CAHA-3.5m/TWIN started in 2009.     

UV observations of sdB stars and prospects of WSO–UV mission for such studies

The hot subdwarf B (sdB) stars are considered to be core He-burning stars with surface temperatures T _eff up to 40?000 K and log?g≥5. They are UV bright and ultraviolet observatories have a significant impact on studies of these objects. Using the technical characteristics of the instruments of the World Space Observatory–Ultraviolet (WSO–UV) mission and data from previous UV studies of sdBs we estimate the limiting apparent magnitudes for these stars that can be observed with the signal-to-noise ratio required in fine spectroscopic analysis. WSO–UV is an international space observatory for observation in UV spectral range 100–350 nm, that is beyond the reach of ground-based instruments but where most of astrophysical processes can be efficiently studied with unprecedented capability. The WSO–UV project is currently funded by national space agencies of Russia and Spain with participation of Germany, Ukraine and China. The WSO–UV consists of a 1.7 m aperture telescope (under responsibility of Russia) with instrumentation designed to carry out high resolution spectroscopy, long-slit low resolution spectroscopy and direct sky imaging. The WSO–UV Ground Segment is under development by Spain and Russia. They will coordinate the Mission and Science Operations and provide the satellite tracking stations for the project. The WSO–UV will work as a targeted scientific observatory. The scientific program of the observatory is open to excellent scientific projects from the world-wide community and occupies up to 40% of total observational time.     

How asteroseismology can constrain the global parameters of solar-like star models

In the previous years, p-mode oscillations (pressure oscillations stochastically excited by convection) have been detected in several solar-like stars thanks to the ground-based spectroscopic and space spectroscopic and photometric observations. We study the importance of seismic constraints on stellar modeling and the impact of their accuracy on reducing the uncertainties of global stellar parameters (i.e. mass, age, etc.). We use the Singular Value Decomposition (SVD) method to analyze the sensitivity of stellar models to seismic constraints. In this context, we construct a grid of evolutionary sequences for solar-like stars with varying age and mass. Around each model of this grid, we evaluate the partial derivatives with respect to a large set of free parameters: mass ?, age τ, mixing-length parameter α, initial helium abundance Y ₀, and initial metallicity Z/X ₀. Masses between 0.9 and 1.55 M _⊙ and central hydrogen abundances from Xc=0.7 to 0.05 have been considered in this study.     

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. Twenty four objects show spectral signs of a cool companion, being either companion absorption lines or a flux contribution at Hα. Five stars with relatively high luminosity show peculiar Hα profiles, possibly indicating stellar winds. Our results are compared to recent theoretical simulations by Han et al. [MNRAS, 341, 669] 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.     

Lithium Abundances in 11 Solar-type Stars

We present determinations of fundamental parameters and lithium abundances in eleven solar-type stars through observations of the Li I λ6707.8Å. The correlations between the abundance of lithium and that of other elements (Ca, K and Fe) are also discussed. The analysis of our data indicates that the maximum lithium abundance decreases with decreasingT _eff, and also decreases with increasing age. The sun is just one of the stars with low lithium abundance. One of the sample stars shows a high lithium abundance of as much as 2.34 dex. The plot of lithium abundance versus [Ca/H] is similar to that versus [Fe/H]. Lithium seems depleted more quickly than potassium in the cool solar-type stars. The correlation between the lithium abundance and the other stellar characteristics, such as absolute visual magnitude, does not seem very strong. The large scatter present at each color cannot be uniquely attributed to different initial compositions or to pure age effect. Other complex mechanisms may exist to provide different amounts of lithium depletion for stars with properties similar to the sun.     

The masses of sdB and sdO stars

Mass is a fundamental parameter, but the masses are not well known for most hot subdwarfs. We propose a method of determining the masses of hot subdwarfs. Using this method, we studied the masses of hot subdwarfs from the ESO supernova Ia progenitor survey and the Hamburg quasar survey. The study shows that most of the subdwarf B stars have masses between 0.42 and 0.54 M _⊙, whilst most sdO stars are in the range 0.40～0.55 M _⊙. Comparing our study to the theoretical mass distributions of Han et al. (Mon. Not. R. Astron. Soc. 341:669, 2003), we found that sdO stars with mass less than ～0.5 M _⊙ may evolve from sdB stars, whilst most high-mass (>0.5 M _⊙) sdO stars result from mergers directly.     

Probability distribution of terrestrial planets in habitable zones around host stars

With more and more exoplanets being detected, it is paid closer attention to whether there are lives outside solar system. We try to obtain habitable zones and the probability distribution of terrestrial planets in habitable zones around host stars. Using Eggleton’s code, we calculate the evolution of stars with masses less than 4.00 M _⊙. We also use the fitting formulae of stellar luminosity and radius, the boundary flux of habitable zones, the distribution of semimajor axis and mass of planets and the initial mass function of stars. We obtain the luminosity and radius of stars with masses from 0.08 to 4.00 M _⊙, and calculate the habitable zones of host stars, affected by stellar effective temperature. We achieve the probability distribution of terrestrial planets in habitable zones around host stars. We also calculate that the number of terrestrial planets in habitable zones of host stars is 45.5 billion, and the number of terrestrial planets in habitable zones around K type stars is the most, in the Milky Way.     

Spectroscopic study of the peculiar galaxy UGC 5119

We present the results of our study of the stellar kinematics in the elliptical galaxy UGC 5119, which has previously been suspected to be a polar-ring galaxy. We have detected a rapidly rotating disk in the central region (r ≤ 3.2 kpc) of the galaxy’s main body and found a radial velocity gradient along its minor axis (in the putative ring). We conclude that UGC 5119 is a medium-luminosity elliptical galaxy with a rapidly rotating disk component and a stellar (probably polar) ring. We have calculated the Lick indices of the Hβ, Mggb, Fe 5270, and Fe 5335 absorption lines and compared them with evolutionary synthesis models. Differences in the [Mg/Fe] ratios, metallicities, and ages of the stars have been found: the young stellar population with a solar [Mg/Fe] ratio and a high metallicity dominates in the circumnuclear region (r ≤ 1 kpc), while the old one with a low metal abundance dominates in the ring.     

HS 2333 + 3927: a new sdB 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 K ₁= 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 T_eff= 36 500 K, log g= 5.70, and log(n _He/n _H) =?2.15. These parameters are typical for sdB stars, the companion is probably an M dwarf.     

Strömgren 4-Color Photometry of Very Metal-Poor Stars

Strömgren uvby photometry has been observed for an additional 140 very metal-poor stars from the survey of Beers et al. (1992). These Galactic stars of very-low metallicity provide crucial information for the investigation of the formation and evolution of the Galaxy, as well as on the nature of the early Universe. The Strömgren uvby-β system allows the measurement of stellar atmospheric parameters as a prelude to detailed abundance studies which will make use of high-resolution spectroscopy and the new generation of large telescopes. The photometric techniques developed by Schuster et al. (1996) are used not only to classify these very metal-poor stars but also to derive effective temperatures, surface gravities, and improved estimates for their interstellar reddenings. In particular, photometric diagrams such as [c ₁], [m ₁] and c ₀, (b-y)₀ are used to classify these stars, especially those near the main-sequence turnoff, where contamination from slightly-evolved subgiants, lower surface-gravity horizontal-branch stars, and even a few supergiant or AGB candidates is found.