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Subluminous B Stars

Hot subluminous stars (or subdwarfs) are low-mass stars that are less luminous than massive main-sequence stars of a similar temperature. They are all evolved stars, most being about 0.5M$_{\odot}$ with helium-burning cores. We are concerned with questions about their origin and to study the physics of their interior.

Drilling and Jeffery worked towards completing a spectral classification scheme for hot subdwarfs. By describing objects phenomenologically, classification can provide a model-independent description which may be used repeatedly in large-scale surveys to identify targets of interest. For stars, detailed classification schemes have been developed for the main-sequence and above, but with insufficient precision for hot stars below the main sequence. The problem is compounded by additional parameters, so that spectral-type, hydrogen line strength, hydrogen/helium ratio and carbon-line strength must be included in the classification. Roughly speaking, these are proxies for effective temperature, luminosity and chemical composition. In the first instance, it is necessary to establish manually a network of `standard' stars against which all others may be measured. As a result of our collaboration, this network of standards has now been established. It will enable (see below), automatic procedures trained using these standards to establish spectral types from large-scale surveys.

Ahmad and Jeffery completed a study of the temperature distribution of helium-rich subdwarf B (He-sdB) stars using ultraviolet and ground-based spectrophotometry. The results supported a previous spectroscopic study, and these studies together showed that the majority of helium-rich sdB stars lay on the evolutionary track for the merger of helium white dwarfs. However, the subsequent serendipitous discovery that the prototype He-sdB is a short-period double-lined spectroscopic binary contradicts such a hypothesis (see highlight). Ahmad and Jeffery also made a preliminary exploration of the pulsational stability of He-sdBs. Two possible variables were identified during a week of photometry at the South African Astronomical Observatory (SAAO) 1.0m telescope; follow-up observations will be made in 2004.

Substantial work was also carried out on observations and theory related to other aspects of subluminous B stars, including pulsations (with Ramachandran and O'Toole) and binaries. Some is reported elsewhere, other work is ongoing.