Distances and absolute magnitudes from trigonometric parallaxes

Abstract

We first review the current knowledge of Hipparcos systematic and random errors, in particular small-scale correlations. Then, assuming Gaussian parallax errors and using examples from the recent Hipparcos literature, we show how random errors may be misinterpreted as systematic errors, or transformed into systematic errors. Finally we summarise how to get unbiased estimates of absolute magnitudes and distances, using either Bayesian or non-parametrical methods. These methods may be applied to get either mean quantities or individual estimates. In particular, we underline the notion of astrometry-based luminosity, which avoids the truncation biases and allows a full use of Hipparcos samples.

20 pages, 8 figures, Invited paper in Haguenau Colloquium "Harmonizing Cosmic Distance Scales in a Post-Hipparcos Era", 14-16/09/98, to appear in ASP Conf. Series, D. Egret and A. Heck eds