Parallaxes And The Hertzsprung-Russell Diagram From The Preliminary Hipparcos Solution H30

We outline the principles by which absolute trigonometric parallaxes have been derived from the first 30 months of data acquired by the Hipparcos astrometry satellite. Distributions of the parallaxes and formal errors of more than one hundred thousand stars are presented, indicating median standard errors of about 1.5 milliarcsec (mas). Tests which provide confidence in the quality of the results are described, together suggesting that the corresponding 'external' errors are unlikely to be underestimated by more than about 10-20 per cent. Parallaxes are absolute, with present estimates of the limit on any global zero-point offset, i.e. common to all stars, of less than 0.1 mas. The Hertzsprung-Russell diagram derived from the satellite data provides powerful confirmation of the astrometric data quality, with a well-defined broad main sequence extending to M = -5 mag, a well-defined giant branch, and a distinct degenerate sequence. Solar neighbourhood 'clump giants', stars in a post-helium flash stage of evolution, are located in the HR diagram on the basis of trigonometric parallax distance estimates for the first time, and the first direct luminosity calibration of M giants is reported. A comparison with stars previously considered to lie within 25 pc demonstrates significant discrepancies for those distances less reliably determined from ground-based observations, and indicates that a corresponding re-evaluation of stellar masses and luminosities within the solar neighbourhood is implied.