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Astron. Astrophys. 317, 689-693 (1997)

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1. Introduction

The hot subdwarf stars are found in the Color-Magnitude diagram on and above the very blue end of the horizontal branch, and are seen both in the field and cluster populations. The hydrogen rich subdwarf B (sdB) stars lie on the extended blue horizontal branch (EHB) and are currently identified with the helium-core burning stage of stellar evolution. The neighboring (sdO) stars lie above the EHB and are not as well understood. Beyond knowing that the sdO are hot (40,000K to 80,000K) and helium-rich (photospheres are 10% to 100% He, by number), little else is accurately known. The surface gravity, found in NLTE spectral analysis, is known to lie in the range log(g)=5.0 to 7.0. Theories for the formation and evolution of sdB and sdO stars fall in several categories based on whether they involve single star evolution or require binary scenarios. Canonical single star evolution theory, in the form of model evolution calculations (e.g. Caloi 1989) describes the sdB stars as evolving into sdO stars. One test of this is whether the spatial kinematics for sdB and sdO stars are similar. We are engaged in a large project to collect such data, but before we can analyze the proposition we have to focus on an important sub-problem - namely that of determining the population membership of the sdB stars themselves. This is important because the sdB stars are better understood than the sdO and have more complete data available, which makes an analysis possible now. Progress on the sdO will require solution of various problems related to difficulties in spectral analysis and in gathering kinematical data for enough sdO stars - there are not very many sufficiently bright sdO stars known, compared to the large number of sdB stars. We have gathered kinematical data for the sdB test and report on the results of a simple analysis of these in this paper.

We also attempt to estimate the absolute magnitude of the sdB, since this can be used to set limits to the sdB mass. Since all spectroscopic analyses of field sdB stars have used the assumption of a typical EHB stellar mass near 0.5 [FORMULA] in order to derive the absolute magnitude from atmospheric parameters, for the first time we can test this assumption from field stars. For cluster sdB stars, other possibilities exist for analysis, due to the (in principle) known distance to the cluster, but separate problems exist in interpreting the cluster sdB data (see Moehler et al. 1995, for a review of this problem).

Reviews of the hot subdwarf literature and summaries of the present knowledge of these objects can be found in Heber (1986, 1992), Saffer&Liebert (1994) and Thejll (1995), and references therein.

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998