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

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5. Summary and discussion

We have observed and analyzed the largest sample of hot subdwarf (sdB and sdO) proper motions to date. We presented first an analysis of the space velocities for that set of sdB stars which had previously published radial velocities combined with our new proper motions. We found firstly that the space velocities are similar to those of populations intermediate to the Old and Thick disks, and secondly using a modified statistical parallax technique that the absolute magnitude of the sdB is [FORMULA]. This is in good agreement with what is known about sdB absolute V magnitudes from other investigations, theoretical and observational (and this constitutes the first independent observational evidence from field stars). This indicates that the common assumption of a 0.5 [FORMULA] mass for the sdB is warranted.

We then examined a much larger data set of sdB and sdO for which we have our new proper motions only. We showed that the reduced proper motion distributions of sdB and sdO imply that the absolute magnitudes are different, or that the kinematics are different, or both. If the sdO follow from the sdB by simple single-star evolution, then the kinematics will be the same and then we can infer that the absolute V magnitude distribution of the sdO is about 1 magnitude brighter, in the mean, and 1 magnitude broader, than the sdB distribution. This prediction is open to experimental test when direct or indirect distances are found from trigonometric data (e.g. HIPPARCOS astrometric data), or analysis of non-interacting binary systems with hot subdwarfs, or interstellar absorption and reddening analysis of hot subdwarf spectral features.

A population origin in the old part of the Galaxy would support those evolutionary scenarios that require great age for the hot subdwarfs to form. Such scenarios include single star evolution with strong mass loss on the first ascent of the red giant branch, and such binary scenarios that act to coalesce orbiting stellar cores (He white dwarfs, really) on a long time scale.

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998