Stellar Atmospheres

A knowledge of stellar abundances is crucial to our understanding of stellar and galactic evolution. While most stars have solar abundances or are metal-deficient with respect to the Sun, more pronounced abundance anomalies are also found. A few stars, for instance, have photospheres composed of 99% helium and 1% carbon (by numbers) with all other elements (including hydrogen) present only in trace amounts. Detailed abundance studies of stars in external galaxies (beyond the Magellanic Clouds) are becoming feasible using the HST and 10 metre class ground-based facilities. Improved techniques for the analysis of stellar spectra are under development; these take advantage of better computer hardware, numerical methods and (most important of all) recent advances in atomic and molecular physics. Synthesis of whole spectral regions with inclusion of all likely spectral features is now a viable method of approach.

Subdwarf-B (sdB) star research is of interest to investigations of stellar mass-loss, as these stars appear to represent extreme cases of near complete envelope loss during the later stages of stellar evolution. If most stars evolve through the sdB star stage, following evolution up the giant branch, understanding the envelope loss which results in sdB star formation is crucial to understanding the late stages of stellar evolution; it would also be the key to understanding chemical evolution of a galaxy or star cluster over several stellar generations. A recently published better understanding of sdB star evolution should help determine the contribution of sdB stars to the observed ultraviolet upturn in giant elliptical galaxies, with the view to its calibration as an age indicator. Following the exciting discovery of low-amplitude pulsation in seventeen sdB stars, the techniques of asteroseismology are being used to constrain stellar evolution models through a determination of envelope structure. The intention is to establish sdB star envelope structure by determining the dispersion relation of acoustic or gravity waves in surface layers, where wave scattering can be accurately computed.

We have recently helped to explain the strange spectral energy distributions of cool white dwarfs via the inclusion of the HeH⁺ ion in their atmospheres (see Harris, Lynas-Gray, Miller and Tennyson, 2004).