A Hipparcos study of the Hyades open cluster: Improved colour-absolute magnitude and Hertzsprung-Russell diagrams

摘要

Hipparcos parallaxes fix distances to individual stars in the Hyades clusterwith an accuracy of 6%. We use the Hipparcos (and Tycho-2) proper motions,which have a larger relative precision than the trigonometric parallaxes, toderive ~3 times more precise distance estimates, by assuming that all membersshare the same space motion. The improved parallaxes as a set are statisticallyconsistent with the Hipparcos parallaxes. The new parallaxes confirm that theHipparcos measurements are correlated on small angular scales, consistent withthe limits specified in the Hipparcos Catalogue, though with significantlysmaller `amplitudes' than claimed by Narayanan & Gould. The colour-absolute magnitude diagram of the cluster based on the new paral-laxes shows a well-defined main sequence with two gaps/turn-offs. Thesefeatures provide the first direct observational support of Boehm-Vitense'sprediction that (the onset of) surface convection in stars affects their B-Vcolours. We present and discuss the theoretical HRD for an objectively definedset of 88 high-fidelity members of the cluster as well as the delta Scuti startheta^2 Tau, the giants delta^1, theta^1, epsilon, and gamma Tau, and the whitedwarfs V471 Tau and HD 27483 (all of which are also members). The precisionwith which the new parallaxes place individual Hyades members in the Hertz-sprung-Russell diagram is limited by (systematic) uncertainties related to thetransformations from observed colours and absolute magnitudes to effectivetemperatures and luminosities. The new parallaxes provide stringent constraintson the calibration of such transformations when combined with theoreticalstellar evolutionary modelling, tailored to the chemical composition and age ofthe Hyades, over the large stellar mass range probed by Hipparcos.