Extensive study of HD 25558, a long-period double-lined binary with two SPB components

We carried out an extensive observational study of the Slowly Pulsating B (SPB) star, HD 25558. The ≈2000 spectra obtained at different observatories, the ground-based and MOST satellite light curves revealed that this object is a double-lined spectroscopic binary with an orbital period of about nine years. The observations do not allow the inference of an orbital solution. We determined the physical parameters of the components, and found that both lie within the SPB instability strip. Accordingly, both show line-profile variations due to stellar pulsations. 11 independent frequencies were identified in the data. All the frequencies were attributed to one of the two components based on pixel-by-pixel variability analysis of the line profiles. Spectroscopic and photometric mode identification was also performed for the frequencies of both stars. These results suggest that the inclination and rotation of the two components are rather different. The primary is a slow rotator with ≈6 d period, seen at ≈60° inclination, while the secondary rotates fast with ≈1.2 d period, and is seen at ≈20° inclination. Spectropolarimetric measurements revealed that the secondary component has a magnetic field with at least a few hundred Gauss strength, while no magnetic field can be detected in the primary

Diffusion Calculations in the Atmospheres of Subdwarf B Stars

The effects of diffusion in the atmospheres of subdwarf B stars have been investigated using a model atmosphere code which includes diffusion self-consistently. The impact of helium abundance profiles on the atmospheres of sdBs are presented. An examination of helium spectral lines in the models revealed the sensitivity of the profiles to abundance gradients. Possible diagnostics for detecting helium stratification in the atmospheres of sdBs are discussed.¨rpceedo,g pthe the conference on "Hot Subdwarf Stars and Related Objects", held 23-27 July, 2007, Otto-Friedrich-Universität, Bamberg (Germany). Edited by Ulrich Heber, C. Simon Jeffery and Ralf Napiwotzki.info:eu-repo/semantics/publishe

New model atmospheres for chemically peculiar stars

As stars evolve, their surfaces may be altered by several different mechanisms. These introduce distinctly unusual chemical peculiarities in their atmospheres. Hydrogen-deficient stars and subdwarf B stars are examples of such objects. These stars present a challenge for constructing reliable models. as their atmospheric structure is particularly sensitive to opacity sources, and may be stratified by diffusion. In order to interpret accurately the properties of these stars, theoretical models must address these issue~. Such models are presented in this work. New model atmospheres have been constructed using up-to-date opacity data. Photoionization crosssections from the Opacity Project (OP) and the IRON Project have been incorporated into the continuous opacity calculations. An opacitysampling approach has been implemented to treat the line opacity, replacing the existing opacity distribution function method which was restricted in composition. Significant increases in the carbon and nitrogen opacities of hydrogen-deficient stars were.found when using the OP data, compared to models computed with opacity data from Peach (1970. MNRAS. 73, 1), and are reflected in the flux distributions and temperature structures of these stars. SUbsequently, systematically cooler effective temperatures were measured for hot extreme helium stars and He-sdB stars compared to previous analyses. Self-consistent model atmospheres incorporating microscopic diffusion have been computed. Based on these models, abundances predictions were made for helium, carbon, nitrogen and silicon in the atmospheres of sdB stars. An examination of helium spectral lines in the models revealed a possible diagnostic for detecting elemental stratification in subdwarf B stars. For a model atmosphere stratified by diffusion, helium abundance measurements will differ from line to line based on the depths of formation, and some line asymmetries may be observed. This was confirmed observationally in two sdBs -- Feige 48 and JL36. An analysis of helium line profiles present in the observed spectra of these stars strongly indicates that the atmospheres have been stratified by diffusion.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Model Atmospheres for Hot Evolved Stars: Revising the Treatment of Continuous Opacities

The model atmosphere code Sterne, first developed in the 1970s to study hydrogen-deficient stars, has evolved considerably over the years. Most recently, attention has turned to revising the treatment of both continuous and line opacities. We have introduced continuous opacities from the Opacity Project and an opacity sampling technique is being investigated to account for contributions of up to 106 atomic lines. Changes due to the improvements in the continuous opacities are examined. The principal discovery is that additional C+ opacity around 1000 Å substantially modifies the flux distribution of hydrogen-deficient atmospheresProceedings of the 14th European Workshop on White Dwarfs, edited by Koester, Detlev and Moehler, Sabineinfo:eu-repo/semantics/publishe

LTE model atmospheres with new opacities: I. Methods and general properties

We describe an opacity sampled version of the LTE model atmosphere code STERNE with revised continuous opacities. In particular, we examine the results of replacing the existing treatment of the continuous opacities which followed the method of Kurucz (1970, SAOSR, No. 309) and Peach (1970, MNRAS, 73, 1), with photoionization cross-sections from the Opacity Project and the IRON Project, and the effects of substituting an opacity distribution function method for treating the line opacities with an opacity sampling method. In studying the application of these new models to atmospheres with hydrogen-rich and extreme chemical compositions, we find the new opacity treatment increases the temperature in the line forming region by up to 3000 K for extremely hydrogen-poor stars and up to 1250 K for hydrogen-rich stars with T eff = 30 000 K and log g = 3.00. This translates into systematically cooler effective temperature measurements for these types of stars. © ESO 2006.info:eu-repo/semantics/publishe

New model atmospheres for hydrogen-deficient stars: Continuous and line opacities

The model atmosphere code STERNE has evolved considerably over the years. Most recently, attention has turned to revising the treatment of both continuous and line opacities. Opacity Project photoionization cross-sections have been incorporated and an opacity sampling technique has been implemented. The new opacities and methods are described, while the principal consequences for the models are presented. In studying the application of these new models to atmospheres with extreme chemical compositions, we find the new opacity treatment increases the temperature in the line forming region by 1000-3000 K for extremely hydrogen-poor stars with Teff around 30000 K. This implies cooler effective temperature measurements for stars of this type.info:eu-repo/semantics/publishe

Self-Consistent Diffusion in Low Mass Hot Stars

The effects of diffusion in the atmospheres of low mass hot stars have been investigated using a model atmosphere code which includes diffusion self-consistently. The atmospheric structures associated with non-uniform composition are compared to models with homogeneous abundances. We compare preliminary results obtained using these models to observed abundances of chemically peculiar subdwarf B stars.Proceedings of the 15th European Workshop on White Dwarfs, held 7-11 August, 2006 in Leicester, UK. Edited by Napiwotzki, Ralf and Burleigh, Matthew R.info:eu-repo/semantics/publishe

Discovery of extremely lead-rich subdwarfs: Does heavy metal signal the formation of subdwarf B stars?

Hot subdwarfs represent a group of low-mass helium-burning stars formed through binarystar interactions and include some of the most chemically peculiar stars in the Galaxy. Stellar evolution theory suggests that they should have helium-rich atmospheres but, because radiation causes hydrogen to diffuse upwards, a majority are extremely helium poor. Questions posed include: when does the atmosphere become chemically stratified and at what rate? The existence of several helium-rich subdwarfs suggests further questions: are there distinct subgroups of hot subdwarf, or do hot subdwarfs change their surface composition in the course of evolution? Recent analyses have revealed remarkable surface chemistries amongst the helium-rich subgroup. In this paper, we analyse high-resolution spectra of nine intermediate helium-rich hot subdwarfs. We report the discovery that two stars, HE 2359-2844 and HE 1256-2738, showan atmospheric abundance of lead which is nearly 10 000 times that seen in the Sun. This is measured from optical Pb IV absorption lines never previously seen in any star. The lead abundance is 10 to 100 times that measured in normal hot-subdwarf atmospheres from ultraviolet spectroscopy. HE 2359-2844 also shows zirconium and yttrium abundances similar to those in the zirconium star LS IV-14°116. The new discoveries are interpreted in terms of heavily stratified atmospheres and the general picture of a surface chemistry in transition from a new-born helium-rich subdwarf to a normal helium-poor subdwarf. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

An extremely peculiar hot subdwarf with a 10000-fold excess of zirconium, yttrium and strontium

Helium-rich subdwarf B (He-sdB) stars represent a small group of low-mass hot stars with luminosities greater than those of conventional sdB stars, and effective temperatures lower than those of subdwarf O (sdO) stars. By measuring their surface chemistry, we aim to explore the connection between He-sdB stars, He-rich sdO stars and normal sdB stars. LSIV-14°116 is a relatively intermediate He-sdB star, also known to be a photometric variable. High-resolution blue-optical spectroscopy was obtained with the Anglo-Australian Telescope. Analysis of the spectrum shows LSIV-14°116 to have effective temperature T eff= 34000 ± 500K, surface gravity logg= 5.6 ± 0.2 and surface helium abundance n He= 0.16 ± 0.03 by number. This places the star slightly above the standard extended horizontal branch, as represented by normal sdB stars. The magnesium and silicon abundances indicate the star to be metal poor relative to the Sun. A number of significant but unfamiliar absorption lines were identified as being due to germanium, strontium, yttrium and zirconium. After calculating oscillator strengths (for Ge, Y and Zr), the photospheric abundances of these elements were established to range from 3 dex (Ge) to 4 dex (Sr, Y and Zr) above solar. The most likely explanation is that these overabundances are caused by radiatively driven diffusion forming a chemical cloud layer in the photosphere. It is conjectured that this cloud formation could be mediated by a strong magnetic field. © 2010 The Authors Monthly Notices of the Royal Astronomical Society © 2010 RAS.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Effective temperatures of cool metal-poor stars derived from the analysis of 3D Balmer lines

Balmer lines are recognized as accurate indicators of the effective temperature of late- type stars. The influence of convection on the shape of Balmer line profiles has been investigated using LTE 3D hydrodynamical model atmospheres and 'classical' LTE 1D stellar atmospheres, where convection is modeled within the simplistic picture of mixing-length theory. Models and line profiles computed with the CO5 BOLD and Linfor3D codes have been used to determine the effective temperatures of the Sun and three well known metal-poor stars HD84937, HD74000, and HD140283. Our 3D fit provides the best fit thus far for the solar Ha temperature using the Barklem theory. The resulting (3D-1D) Teff biases related to the different treatment of convection in the 1D and 3D models are presented. © 2009 American Institute of Physics.15th Cambridge Workshop on Cool Stars, Stellar Systems and the Sun; St. Andrews; United Kingdom; 21 July 2008 through 25 July 2008info:eu-repo/semantics/publishe