C. S. Jeffery

Time-resolved spectral analysis of the pulsating helium star V652 Her

A series of 59 moderate-resolution high signal-to-noise spectra of the pulsating helium star V652 Her covering 1.06 pulsation cycles was obtained with the William Herschel Telescope. These have been supplemented by archival ultraviolet and visual spectrophotometry and used to make a time-dependent study of the properties of V652 Her throughout the pulsation cycle. This study includes the following features: the most precise radial velocity curve for V652 Her measured so far, new software for the automatic measurement of effective temperature, surface gravity and projected rotation velocities from moderate-resolution spectra, self-consistent high-precision measurements of effective temperature and surface gravity around the pulsation cycle, a demonstration of excessive line-broadening at minimum radius and evidence for a pulsation-driven shock front, a new method for the direct measurement of the radius of a pulsating star using radial velocity and surface gravity measurements alone, new software for the automatic measurement of chemical abundances and microturbulent velocity, updated chemical abundances for V652 Her compared with previous work (Paper IV), a reanalysis of the total flux variations (cf. Paper II) in good agreement with previous work, and revised measurements of the stellar mass and radius which are similar to recent results for another pulsating helium star, BX Cir. Masses measured without reference to the ultraviolet fluxes turn out to be unphysically low (∼0.18 M ). The best estimate for the dimensions of V652 Her averaged over the pulsation cycle is given by: 〈Teff〉 = 22 930 ± 10 K and 〈log g〉 = 3.46 ± 0.05 (ionization equilibrium), 〈Teff〉 = 20 950 ± 70 K (total flux method), 〈R〉 = 2.31 ± 0.02 R , 〈L〉 = 919 ± 14 L , M = 0.59 ± 0.18 M and d = 1.70 ± 0.02 kpc. Two significant problems were encountered. The line-blanketed hydrogen-deficient model atmospheres used yield effective temperatures from the optical spectrum (ionization equilibrium) and visual and UV photometry (bolometric flux) that are inconsistent. Secondly, the IUE spectra are poorly distributed in phase and have low signal-to-noise. These problems may introduce systematic errors of up to 0.1 M .

The binary properties of the pulsating subdwarf B eclipsing binary PG 1336-018 (NY Virginis)

Aims. We present an unbiased orbit solution and mass determination of the components of the eclipsing binary PG1336−018 as a critical test for the formation scenarios of subdwarf B stars. Methods. We obtained high-resolution time series VLT/UVES spectra and high-speed multicolour VLT/ULTRACAM photometric observations of PG1336−018, a rapidly pulsating subdwarf B star in a short period eclipsing binary. Results. Combining the radial velocity curve obtained from the VLT/UVES spectra with the VLT/ULTRACAM multicolour lightcurves, we determined numerical orbital solutions for this eclipsing binary. Due to the large number of free parameters and their strong correlations, no unique solution could be found, only families of solutions. We present three solutions of equal statistical significance, two of which are compatible with the primary having gone through a core He-flash and a common-envelope phase described by the α-formalism. These two models have an sdB primary of 0.466 M and 0.389 M, respectively. Finally, we report the detection of the Rossiter-McLaughlin effect for PG1336−018.

Gravity-mode pulsations in subdwarf B stars: a critical test of stellar opacity

The identification of non-radial g-mode oscillations as the cause of variability in cool subdwarf B stars (PG1716 variables) has been frustrated by a 5000K discrepancy between the observed and theoretical blue edge of the instability domain (Fontaine et al. 2003). A major component in the solution to this problem has been identified by (a) using updated OP instead of OPAL opacities and (b) considering an enhancement of nickel, in addition to that of iron, in the driving zone. The reason for this success is that, in OP, the “Fe-bump” contributions from iron and nickel occur at higher temperatures than in OPAL. As well as pointing to a solution of an important problem in stellar pulsation theory, this result provides a critical test for stellar opacities and the atomic physics used to compute them.

Physical parameters of helium-rich subdwarf B stars from medium resolution optical spectroscopy

Most subdwarf B (sdB) stars have spectra that show helium (He) abundances depleted to a lesser or greater extent. This has been attributed to diffusion and gravitational settling. However a small but significant number of stars with similar temperatures and gravities show relatively strong helium spectra. The question is how these helium-rich sdB (He-sdB) stars are related to their He-deficient counterparts. We present physical parameters for sample of He-sdB stars using line-blanketed LTE models. From the analysis we show that the He-sdB stars are quite distinct from the He-poor sdB. We also explore the idea of a possible link between some extreme helium (EHe) stars and He-rich subdwarf O (He-sdO) stars.

An extremely peculiar hot subdwarf with a 10 000-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.

Fe-bump instability: the excitation of pulsations in subdwarf B and other low-mass stars

We consider the excitation of radial and non-radial oscillations in low-mass B stars by the iron-bump opacity mechanism. The results are significant for the interpretation of pulsations in subdwarf B stars, helium-rich subdwarfs and extreme helium stars, including the EC14026 and PG1716 variables. We demonstrate that, for radial oscillations, the driving mechanism becomes effective by increasing the contrast between the iron-bump opacity and the opacity from other sources. This can be achieved either by increasing the iron abundance or by decreasing the hydrogen abundance. The location of the iron-bump instability boundary is found to depend on the mean molecular weight in the envelope and also on the radial order of the oscillation. A bluer instability boundary is provided by increasing the iron abundance alone, rather than the entire metal component, and is required to explain the observed EC14026 variables. A bluer instability boundary is also provided by higher radial order oscillations. Using data for observed and theoretical period ranges, we show that the coolest EC14026 variables may vary in the fundamental radial mode, but the hottest variables must vary in modes of higher radial order. In considering non-radial oscillations, we demonstrate that g-modes of high radial order and low spherical degree (l < 4) may be excited in some blue horizontal branch stars with near-normal composition (Z = 0.02). Additional iron enhancement extends the g-mode instability zone to higher effective temperatures and also creates a p-mode instability zone. The latter is essentially contiguous with the radial instability zone. With sufficient iron, the p- and g-mode instability zones overlap, allowing a small region where the EC14026- and PG1716type variability can be excited simultaneously. The overlap zone is principally a function of effective temperature and only weakly a function of luminosity. However, its location is roughly 5000 K, too low compared with the observed boundary between EC14026 and PG1716 variables. The discrepancy cannot be resolved by simply increasing the iron abundance.

Discovery of pulsation in a helium-rich subdwarf B star

Helium-rich subdwarf B (He-sdB) stars form a very small inhomogeneous group of subluminous stars showing varying degrees of helium enrichment. They have been found in the field of our galaxy as well as in globular clusters. Here we report the first discovery of pulsation in a He-sdB star – LS IV-14°116. Two pulsation periods can be clearly identified (1950 and 2900 s) and are more likely to be due to high-order non-radial g -mode oscillations than to radial or non-radial p -modes.

Stellar archaeology: the evolving spectrum of FG Sagittae

Over an interval of 120 years, the extraordinary object FG Sge has been transformed from a hot post-AGB star to a very luminous cool supergiant. Theoretically, this evolution has been associated with the reignition of a helium-shell during the post-AGB stage. A series of studies of the chemical composition of the photosphere have suggested that this evolution has been associated with a dramatic increase of approximately 3 dex in the abundances of s-process elements between about 1960 and 1995. The problem with this apparent change is that it occurred at a time when the surface convection zone, which is governed by the stars effective temperature, could not have developed sufficiently to dredge processed material from the stellar interior to the surface. We have reviewed the chemical evolution of FG Sge by means of modelling the time-varying spectrum under a range of assumptions. By comparing these models with published observational data, a self-consistent picture has emerged. In particular we find that surface hydrogen has been depleted during the interval in question. In contrast, the s-process abundances have generally maintained a steady enhancement of around

An MK-like system of spectral classification for hot subdwarfs

1{-}2

KIC 1718290: A HELIUM-RICH V1093-Her-LIKE PULSATOR ON THE BLUE HORIZONTAL BRANCH

dex, although some modest changes may have occurred since 1950. This implies that FG Sge has not just completed dredging up freshly-produced s-process isotopes. However, there remains a contradiction between the observed hydrogen-depletion, the age of the associated planetary nebula, and current evolutionary models for a pre-white dwarf suffering a late thermal pulse.