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Astron. Astrophys. 327, 577-586 (1997)

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1. Introduction

Hot subdwarfs are blue, horizontal-branch like stars, representing the late stages of evolution of stars having started with less than about 2.5 M [FORMULA] on the main sequence. The hotter subdwarf B (sdB) stars form a well defined group. Their luminosity is of the order of 10 L [FORMULA] and their surface temperatures are roughly between 2 104 and 3 104 K. The age of the sdB stars does not follow from the stellar properties. The abundance of metals in subdwarf star atmospheres is possibly affected by upward convection of processed material but more likely by gravitational downward diffusion of the heavy elements. Thus the normally low metal abundance seen in sdB stars is not an indicator for their age.

Our studies of hot subdwarf stars have two main goals: one is to determine the parameters and the evolutionary state of the subdwarf stars in the framework of stellar evolution; the other is to investigate the structure of the Milky Way using these stars. A relation between these two goals can be found in the connection between the age of subdwarf stars and their spatial distribution. Old stars will more likely be distributed in a thicker disk, due to the heating up of their average kinematics through interactions with other stars. Young stars, on the other hand, are rather moving inside the thin gaseous galactic disk where they originated. Still, the present day location of a star is, in general, no indication for its age.

In this paper we will investigate the kinematic behaviour exemplified by the orbit. The characteristics of the orbits may contain information about the formation epoch and place of the stars, information not accessible through the metal content. Our investigations will show whether the star is in its motion confined in the thin disk of the Milky Way, or that it may reach to large z -distances similar to objects of the halo population.

A first sample of data addressing the population nature of subdwarf stars has been presented by Colin et al.  (1994). They showed that indeed most stars in their (small) sample had disk orbits but 1 star had an orbit with z -distance maxima ranging from 8 to 20 kpc. Evidence for more stars with halo orbits was given by de Boer et al.  (1995).

We have investigated 41 stars for their kinematic behaviour. The choice of stars was solely determined by the availability of the parameters necessary to calculate orbits. The stars selected are listed in Table 1. The sample includes the stars already investigated by Colin et al.  (1994).


Table 1. Observational data for the stars for which orbits are calculated

For 12 stars new absolute and accurate proper motions have been determined (see Sect. 2.1) allowing to evaluate the difference between the various astrometric catalogues and improving on the data for some of the Colin et al.  stars.

After having presented the orbits we analyse their shapes and try to identify the oldest objects of the sample (Sect. 3). The range of z -values the stars reach is then used to investigate the spatial distribution of sdB stars in the Milky Way (Sect. 4).

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© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998