Manganese abundances in mercury-manganese stars

We use exact curve-of-growth analysis and spectral synthesis to deduce the abundance of Mn from high signal-to-noise ratio visible-region echelle spectra of selected Mn i and MnII lines in 24 HgMn stars. The results are compared with the Mn abundances derived from UV resonance lines by Smith & Dworetsky. We find excellent agreement for several unblended Mn lines and confirm the temperature dependence of the Mn abundance found by Smith & Dworetsky. The MnII lines at λλ 4206 and 4326 are much stronger than one would predict from the mean Mn abundances. The lack of agreement is greatest for stars with the strongest MnII lines. Using ad hoc multicomponent fits to the profiles of sharp-lined stars, we show that most of the discrepancies can be explained by hyperfine structure that desaturates the lines, with full widths of the order of 0.06--0.09 Å

EUV, X-ray and optical studies of white dwarf binary systems

This thesis describes an EUV, X-ray and optical study of white dwarfs, concentrating on those in binary systems with a late-type companion star. In the introductory chapter I outline the types of binary systems known to include white dwarf components and the observed characteristics of binary and single white dwarfs. Chapter 2 discusses WFC-discovered white dwarfs which appear isolated. I derive magnitudes and temperatures both from optical spectra and EUV data, and compare this population to that of known field white dwarfs. I also report on a search for any further 'hidden' white-dwarf/red-dwarf systems. Chapter 3 presents extensive work on RE1016-05, a white-dwarf/red-dwarf candidate pre-cataclysmic binary, discovered in the ROSAT all-sky-survey. EUV, X-ray and optical data are analysed to determine the parameters and nature of the system. In chapters 4 and 5 I describe work undertaken on other white-dwarf/red dwarf systems: several discovered with the ROSAT WFC ; and several pre-cataclysmic systems known before the all-sky-survey. EUV and X-ray observations (obtained during the ROSAT survey and pointed phases) and optical follow-up spectra are analysed to determine parameters of both components in the systems and extract information about their evolutionary state and the structure and composition of the white dwarf atmospheres. The main results and conclusions to be drawn from this thesis are summarised in chapter 6. Appendix A describes software I wrote to aid in the reduction of the large volumes of spectral data accumulated by the ROSAT Optical Identification Programme. Appendix B contains a brief description of the ROSAT spacecraft and our (MSSL) participation in the WFC survey data reduction

Abundances and search for vertical stratification in the atmospheres of four HgMn stars

Using high resolution, high-S/N archival UVES spectra, we have performed a detailed spectroscopic analysis of 4 chemically peculiar HgMn stars (HD 71066, HD 175640, HD 178065 and HD 221507). Using spectrum synthesis, mean photospheric chemical abundances are derived for 22 ions of 16 elements. We find good agreement between our derived abundances and those published previously by other authors. For the 5 elements that present a sufficient number of suitable lines, we have attempted to detect vertical chemical stratification by analyzing the dependence of derived abundance as a function of optical depth. For most elements and most stars we find no evidence of chemical stratification with typical 3\sigma upper limits of \Delta\log N_elem/N_tot~0.1-0.2 dex per unit optical depth. However, for Mn in the atmosphere of HD 178065 we find convincing evidence of stratification. Modeling of the line profiles using a two-step model for the abundance of Mn yields a local abundance varying approximately linearly by ~0.7 dex through the optical depth range log \tau_5000=-3.6 to -2.8.Comment: 11 figures, 9 tables, table 6-9 (online material), accepted by MNRA

Abundance analysis of prime B-type targets for asteroseismology II. B6--B9.5 stars in the field of view of the CoRoT

The CoRoT satellite is collecting precise time-resolved photometry for tens of asteroseismology targets. To ensure a correct interpretation of the CoRoT data, the atmospheric parameters, chemical compositions, and rotational velocities of the stars must be determined. The main goal of the ground-based seismology support program for the CoRoT mission was to obtain photometric and spectroscopic data for stars in the fields monitored by the satellite. These ground-based observations were collected in the GAUDI archive. High-resolution spectra of more than 200 B-type stars are available in this database, and about 45% of them is analysed here. To derive the effective temperature of the stars, we used photometric indices. Surface gravities were obtained by comparing observed and theoretical Balmer line profiles. To determine the chemical abundances and rotational velocities, we used a spectrum synthesis method, which consisted of comparing the observed spectrum with theoretical ones based on the assumption of LTE. Atmospheric parameters, chemical abundances, and rotational velocities were determined for 89 late-B stars. The dominant species in their spectra are iron-peak elements. The average Fe abundance is 7.24+/-0.45 dex. The average rotational velocity is 126 km/sec, but there are 13 and 20 stars with low and moderate Vsin i values, respectively. The analysis of this sample of 89 late B-type stars reveals many chemically peculiar (CP) stars. Some of them were previously known, but at least 9 new CP candidates, among which at least two HgMn stars, are identified in our study. These CP stars as a group exhibit Vsin i values lower than the stars with normal surface chemical composition.Comment: 21 pages, 13 figures, accepted to Astronomy and Astrophysic

Magnetism, chemical spots, and stratification in the HgMn star phi Phoenicis

Mercury-manganese (HgMn) stars have been considered as non-magnetic and non-variable chemically peculiar (CP) stars for a long time. However, recent discoveries of the variability in spectral line profiles have suggested an inhomogeneous surface distribution of chemical elements in some HgMn stars. From the studies of other CP stars it is known that magnetic field plays a key role in the formation of surface spots. All attempts to find magnetic fields in HgMn stars have yielded negative results. In this study, we investigate the possible presence of a magnetic field in phi Phe (HD 11753) and reconstruct surface distribution of chemical elements that show variability in spectral lines.We also test a hypothesis that a magnetic field is concentrated in chemical spots and look into the possibility that some chemical elements are stratified with depth in the stellar atmosphere. Our analysis is based on high-quality spectropolarimetric time-series observations, covering a full rotational period of the star. Spectra were obtained with the HARPSpol at the ESO 3.6-m telescope. Combining information from all suitable spectral lines, we set an upper limit of 4 G on the mean longitudinal magnetic field. For chemical spots, an upper limit on the longitudinal field varies between 8 and 15 G. We confirmed the variability of Y, Sr, and Ti and detected variability in Cr lines. Stratification analysis showed that Y and Ti are not concentrated in the uppermost atmospheric layers. Our spectropolarimetric observations rule out the presence of a strong, globally-organised magnetic field in phi Phe. This implies an alternative mechanism of spot formation, which could be related to a non-equilibrium atomic diffusion. However, the typical time scales of the variation in stratification predicted by the recent time-dependent diffusion models exceed significantly the spot evolution time-scale reported for phi Phe.Comment: Accepted for publication in Astronomy & Astrophysics, 15 pages, 15 figure

Mass-loss and diffusion in subdwarf B stars and hot white dwarfs: do weak winds exist?

The effect of diffusion on the chemical composition of subdwarf B (sdB) stars and of hot white dwarfs strongly depends on the presence of weak winds. In the paper, for stars with half a solar mass, for various effective temperatures between 25000 K and 50000 K, and for various metallicities between solar and 1/100 solar, the mass loss rates are predicted as a function of the surface gravity. With assumptions similar to the original theory of radiatively driven winds, the method of solution of the momentum equation has been modified, because the usual parametrization of the line force multiplier leads to complications in the case of weak winds. According to the results, weak winds with mass-loss rates of the order E-11 solar masses per year may exist only for the most luminous sdB stars. For the more compact ones, the decoupling of the metals from the bulk matter (hydrogen and helium) is expected in the wind region, because the momentum exchange via Coulomb collisions is not effective enough. Thus multicomponent effects are of great importance. For the case of white dwarfs no wind solution exists at all, if multicomponent effects are neglected.Comment: 19 pages, 13 figure

The age, life expectancy, and space density of Post Common Envelope Binaries

We present a sample of 30 well observed Post Common Envelope Binaries (PCEBs). Deriving the cooling age of the white dwarfs, we show that the PCEB population is dominated by young systems. Having calculated the orbital evolution of the systems under the assumption of two different prescriptions for the angular momentum loss, we find that most of the systems have not yet completed a significant fraction of their PCEB life time. We therefore predict the existence of a large population of old PCEBs containing cold white dwarfs (T_eff<15000 K). Our calculations show that nearly half of the PCEBs in our sample will evolve into a semi-detached configuration and start mass transfer in less than the Hubble-time. These systems are thus representative for progenitors of the current CV population. Only one of them (V471 Tau) will evolve into a long-period (Porb>4h) CV, and a significant fraction of the systems will start mass transfer in the period gap. Having estimated the distances of the PCEBs in the sample, we derive a space density of ~6-30x1e-6 [pc-3], depending on the assumed angular momentum loss prescription. Taking into account the evolutionary time scales we compute a lower limit for the CV space density, predicted by the currently known PCEB population of ~10 x 1e-5 [pc-3]. Finally, we discuss possible observational selection effects and conclude that the observed PCEB population is probably highly incomplete.Comment: 18 pages including 11 figures; accepted for publication in A&