A. E. Lynas-Gray

EUVE spectroscopy of epsilon Canis Majoris (B2 II) from 70 to 730 A

We present spectra of the brightest stellar source of extreme ultraviolet (EUV) radiation longward of 400 A, the B2 II star, epsilon CMa. These data were taken with the three spectrometers aboard the NASA Extreme Ultraviolet Explorer satellite (EUVE) during the first cycle of pointed observations. We report on our initial studies of the continuum and line spectrum of the stellar photosphere in the 320 to 730 A region, and on the wind emission lines observed in the 170-375 A region. This is the first EUV spectrum of an early-type star, and thus makes epsilon CMa the most comprehensively observed B star from the X-ray to infrared regimes. The radiation in both the H Lyman continuum and He I continuum (shortward of 504 A) are found to be significantly greater than predicted by both Local Thermodynamic Equilibrium (LTE) and non-LTE model atmospheres. Since epsilon CMa also exhibits a mid-infrared excess, this points to the outer layers being warmer than the models indicate. The anomalously large Lyman continuum flux, combined with the very low column density measured in the direction toward this star implies that it is the dominant source of hydrogen ionization of the local interstellar medium in the immediate vicinity of the sun. All of the lines predicted to be strong from model atmospheres are present and several wind absorption features are also identified. We have detected emission lines from highly ionized iron that are consistent with the ROSAT Position Sensitive Proportional Counter (PSPC) observations if a multi-temperature emission model is used, and the assumption is made that there is significant absorption beyond that of the neutral phase of the ISM. The spectrum shows strong O III 374 A line emission produced by the Bowen flourescence mechanism, which has not previously been observed in the spectra of hot stars.

The role of HeH+ in cool helium-rich white dwarfs

HeH+ is found to be the dominant positive ion over a wide range of temperatures and densities relevant to helium-rich white dwarfs. The inclusion of HeH+ in ionization equilibrium computations increases the abundance of free electrons by a significant factor. For temperatures below 8000 K, He- free-free absorption is increased by up to a factor of 5 by the inclusion of HeH+. Illustrative model atmospheres and spectral energy distributions are computed, which show that HeH+ has a strong effect on the density and pressure structure of helium-rich white dwarfs with Teff < 8000 K. The inclusion of HeH+ significantly reddens spectral energy distributions and broadband color indices for models with Teff < 5500 K. This has serious implications for existing model atmospheres, synthetic spectra, and cooling curves for helium-rich white dwarfs.

New VIC photometry of the sdOB binary AA Dor and an improved photometric model

New VI CCD photometry, obtained with integration times of 20s, of the sdOB+degenerate-dwarf eclipsing binary system AA Dor has provided new complete light curves with an rms scatter about a mean curve of +/-0.004 mag. These data are analysed with an improved Light2 light curve synthesis code to yield more accurate determinations of the radii of both stars, the orbital inclination, and the flux ratio between the two components. These radii are only a little different from the values derived 25 years ago from less complete data, but the uncertainties on these values are improved by a factor of two. The apparent discrepancy remains between the surface gravity of the sdOB primary star obtained from the light-curve solution with the published spectroscopic orbit and that obtained from NLTE analysis of high-resolution spectra of the sdOB star. The substantial reflection effect in the system is adequately represented by the Light2 code with a bolometric albedo of unity in light curves extending from 0.35 microns to 2.2 microns. However there are differences at individual wavelengths in the derived albedo, which may indicate redistribution of flux from shorter wavelengths into the V and I passbands.

The Effect of the Electron Donor H+3 on the Pre-Main-Sequence and Main-Sequence Evolution of Low-Mass, Zero-Metallicity Stars

H has been shown (1991 work of Lenzuni and coworkers) to be the dominant positive ion in a zero-metallicity gas at low temperature and intermediate to high density. It therefore affects both the number of free electrons and the opacity of the gas. The most recent H partition function (1995 work of Neale & Tennyson) is an order of magnitude larger at 4000 K than all previous partition functions, implying that H is a more important electron donor than previously thought. Here we present new Rosseland mean opacities for a hydrogen-helium gas of 1000 K ≤ T ≤ 9000 K and -14 ≤[ρ (g cm-3)] ≤ -2. In the calculation of these opacities, we have made use of the latest collision-induced absorption data as well as the most recent H partition function and line opacity data. It is shown that these updated and new sources of opacity give rise to a Rosseland mean opacity for a hydrogen-helium gas that is, in general, greater than that calculated in earlier works. The new opacity data are then used to model the evolution of low-mass (0.15-0.8 M☉), zero-metallicity stars, from pre-main-sequence collapse to main-sequence turnoff. To investigate the effect of H on the evolution of low-mass, zero-metallicity stars, we repeat our calculations neglecting H as a source of electrons and line opacity. We find that H can have an effect on the structure and evolution of stars of mass ~0.5 M☉ or less. A gray atmosphere is used for the calculation, which is sufficient to demonstrate that H affects the evolution of very low mass stars to a greater degree than previously believed.

Non-grey hydrogen burning evolution of subsolar mass Population III stars

The primordial elements, H, He and Li, are included in a low-temperature equation of state and monochromatic opacity function. The equation of state and opacity function are incorporated into the stellar evolution code NG-ELMS, which makes use of a non-grey model atmosphere computed at runtime. NG-ELMS is used to compute stellar evolution models for primordial and lithium-free element mixtures and for stars in the subsolar mass range 0.8‐0.15 M� . We find that lithium has little or no effect upon the structure and observable properties of stars in this mass range. Furthermore, lithium is completely destroyed by fusion before the main sequence in stars of mass less than∼0.7 M� . We find that on the red giant branch and Hayashi track, the use of a non-grey model atmosphere to provide the upper boundary conditions for the stellar evolution calculation results in significantly cooler less-luminous stars, across the mass range.

An Historical Overview

Studies in extragalactic astronomy, galactic structure and the late stages of stellar evolution provide ample motivation for surveys of fields in the Galactic Halo. Apart from white dwarfs, blue stars had been regarded as luminous objects confined to star-forming regions in the Galactic Plane; finding them at high galactic latitudes attracted immediate interest, because their luminosities were intermediate between those of white dwarfs and blue Main Sequence stars. The study of blue stars away from the Galactic Plane was initiated by Greenstein; in due course effective temperatures (Teff), surface gravities (log g) and abundances showed these stars form what appeared to be a blue extension of the known Horizontal Branch (HB) in the Hertzsprung–Russell Diagram. Extended Horizontal Branch (EHB) stars were identified with Extreme Horizontal Branch stars in globular clusters. It was realised that HB and EHB stars must have formed as a consequence of mass-loss on the Giant Branch, either at or before the helium flash. Mass-loss on the Giant Branch leading to the formation of EHB stars was considered more likely for stars in binary systems. The scene was then set for three decades of EHB star research.

The EC14026 Stars: A New Class of Pulsating Star

Hot subdwarfs comprise typically 50 per cent of faint blue star surveys at high galactic latitude (e.g. the Palomar-Green (PG) survey: Green, Schmidt & Liebert 1986; the Edinburgh-Cape (EC) survey: Stobie et al. 1996a, Kilkenny et al. 1996a). In contrast to the PG survey, the EC survey has found that a large fraction (~ 15 per cent) of its hot subdwarf sample is comprised of sdB stars showing the Can K line. This implies that the hot subdwarf has a relatively luminous companion of spectral type F or G.

Binary Stars as the Source of the Far-UV Excess in Elliptical Galaxies

The discovery of an excess of light in the far-ultraviolet (UV) spectrum in elliptical galaxies was a major surprise in 1969. While it is now clear that this UV excess is caused by an old population of hot helium-burning stars without large hydrogen-rich envelopes rather than young stars, their origin has remained a mystery. Here we show that these stars most likely lost their envelopes because of binary interactions, similar to the hot subdwarf population in our own Galaxy. This has major implications for understanding the evolution of the UV excess and of elliptical galaxies in general. In particular, it implies that the UV excess is not a sign of age, as had been postulated previously, and predicts that it should not be strongly dependent on the metallicity of the population.