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Astron. Astrophys. 327, 577-586

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Hot subdwarf stars: galactic orbits and distribution perpendicular to the plane

K.S. de Boer1, Y. Aguilar Sánchez1, M. Altmann1, M. Geffert1, M. Odenkirchen1,2, J.H.K. Schmidt1, and J. Colin2

1Sternwarte, Univ. Bonn, Auf dem Hügel 71, D-53121 Bonn, Germany
2Observatoire de Bordeaux, CNRS/INSU, F-33270 Floirac, France

Received 24 April 1997 / Accepted 24 June 1997


The spatial distribution and the population nature of subdwarf B type stars in the galaxy is investigated based on the kinematics of these stars. With new and available proper motions, radial velocities, and distances, the orbits of 41 stars have been calculated using a galactic mass model. The orbits are well behaved and 10 stars reach to FORMULA kpc. Many orbits are very eccentric, reaching in to just 2 kpc from the galactic centre, or veering out to beyond 20 kpc. None of the stars can be identified uniquely as classical Population II objects.

The average eccentricity ecc of the orbits of our sample is 0.24, the average normalised z-extent nze of the orbits is 0.16, and the asymmetric drift of our sample is -36 km s-1. This suggests that our sample of sdB stars is part of a population of thick disk stars.

A statistical analysis of the orbits shows that the subdwarf stars have a spatial distribution in z compatible with an exponential one with a scale height hz of about 1.0 kpc. However, since only few stars reach to large z the spatial distribution is only well defined to FORMULA kpc.

The distribution in z shows a relative minimum near z=0 pc and has maxima near z = 300 pc. This reflects the smaller probability to find the stars in the disk than away from the disk, as expected for any orbit reaching to larger z. Scale height studies based on limited samples of stars in specified directions can therefore easily be flawed when they do not reach to large enough distances to overcome this aspect of the z-distribution.

Key words: astrometry - stars: kinematics - stars: horizontal branch - stars: Population II - Galaxy: halo - Galaxy: structure

Send offprint requests to: deboer@astro.uni-bonn.de

© European Southern Observatory (ESO) 1997

Online publication: October 14, 1997
Last change: April 6, 1998