The analysis of the galactic orbits of 41 stars shows that the frequency distribution of finding these stars at a given z can be given by an exponential in z with a scale height of 1.0 0.1 kpc. Our results are, as various tests with variations of the input parameters have shown, robust and reliable with the indicated error. That value is in agreement with the notion that sdB stars are part of the older disk population. The asymmetric drift value supports this conclusion.
Since sdB stars most likely are the end product of evolution of stars with mass ranging from 0.8 to at most 3 M , stars of different age are present in the sample. That evolutionary time from the zero age main-sequence to sdB state ranges from 0.5 Gyr to over 12 Gyr. In that time the number of gravitational interactions with other stars has apparently been large enough to erase most traces of their origin.
Our sample contains 10 stars (one out of four) with orbits reaching to kpc, the extreme being the star reaching 5 kpc. Several stars have highly eccentric orbits, a small angular momentum , or orbits with a large normalised z -extent. These all are likely the older stars in the sample.
© European Southern Observatory (ESO) 1997
Online publication: April 6, 1998