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

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Proper motions of the hot subdwarfs

The kinematic population membership of the sdB *

P. Thejll 1, C. Flynn 1, 3, R. Williamson 2 and R. Saffer 2

1 Nordita, Blegdamsvej 17, DK-2100 Copenhagen, Denmark
2 Space Telescope Science Institute, Baltimore, USA
3 Tuorla Observatory, Piikkiö, FIN-21210, Finland

Received 22 May 1996 / Accepted 6 June 1996

Abstract

** We report on first results from an ongoing program to measure and analyze proper motions for hot subdwarf and white dwarf stars, with the aim of determining to which stellar population sdB stars (in the V magnitude range 10.5 to 14.5) belong. Ours is the largest sample of hot subdwarf proper motions measured to date. Our kinematic analysis suggests that the parent population of these hydrogen-rich sdB stars is as old or older than the old disk. We measure the absolute magnitude of the sdB in the field as [FORMULA], providing independent confirmation of absolute V magnitude estimates for these stars from clusters and spectroscopic analyses.

Provided it can be shown that the sdO stars evolve from the sdB, then the sdO absolute V magnitude distribution is about 1 magnitude brighter than for the sdB, and 1 magnitude wider.

Key words: astrometry – stars: subdwarf – fundamental parameters – Galaxy: stellar content

* Based on observations made with the Carlsberg Automatic Meridian Circle operated on the island of La Palma by the Copenhagen University Observatory, Royal Greenwich Observatory and Real Instituto y Observatorio de la Armada en San Fernando.
** The Tables 5 to 11 are only available in electronic form at the CDS via anonymous ftp 130.79.128.5 or http://cdsweb.u-strasbg.fr/Abstract.html.

Send offprint requests to: Peter Thejll, email address: thejll@nordita.dk

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998
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