Astrometric Radial Velocities from HIPPARCOS

Details Astrometric Radial Velocities from HIPPARCOS Astrometric Radial Velocities from HIPPARCOS

Aug 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997esasp.402..733d&link_type=abstract

Proceedings of the ESA Symposium `Hipparcos - Venice '97', 13-16 May, Venice, Italy, ESA SP-402 (July 1997), p. 733-738

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35

Scientific paper

Space astrometry now permits accurate determinations of stellar radial motion, without using spectroscopy. Using Hipparcos data, this is possible for stars in nearby moving clusters, where all stars share nearly the same space velocity. A maximum-likelihood method has been developed to yield kinematic cluster parameters (including the internal velocity dispersion) purely from parallaxes and proper motions. The deduced astrometric radial velocities of the Ursa Major open cluster and the Hyades have inaccuracies of 0.3 and 0.4 km/s, respectively, and the internal cluster velocity dispersions are found to be 0.66 +/- 0.10 and 0.25 +/- 0.04 km/s (consistent with random stellar motions). Remaining errors arise from uncertainties in excluding binary stars. The errors get worse for the more distant Coma Berenices cluster. The fitting of cluster parameters includes all individual stellar distances. The constraint of a uniform average cluster velocity markedly improves the parallax precisions (roughly by a factor two), compared with Hipparcos data for individual stars. The HR diagram for the Hyades now reveals a very narrow main sequence line (not band), even suggesting some wiggles in it. Discrepancies between astrometric and spectroscopic radial velocities reveal effects (other than stellar motion) that affect wavelength positions of spectral lines. Such are caused by stellar pulsation, surface convection, and by gravitational redshifts. A parallel programme is obtaining and analysing high-precision spectroscopic radial velocities for different classes of spectral lines in these programme stars.

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