We present a study of the membership of the Hyades open cluster , derive kinematically-modelled parallaxes of its members , and study the colour-absolute magnitude diagram of the cluster .
We use Gaia DR1 Tycho - Gaia Astrometric Solution ( TGAS ) data complemented by Hipparcos -2 data for bright stars not contained in TGAS .
We supplement the astrometric data with radial velocities collected from a dozen literature sources .
By assuming that all cluster members move with the mean cluster velocity to within the velocity dispersion , we use the observed and the expected motions of the stars to determine individual cluster membership probabilities .
We subsequently derive improved parallaxes through maximum-likelihood kinematic modelling of the cluster .
This method has an iterative component to deal with ’ outliers ’ , caused for instance by double stars or escaping members .
Our method extends an existing method and supports the mixed presence of stars with and without radial velocities .
We find 251 candidate members , 200 of which have a literature radial velocity , and 70 of which are new candidate members with TGAS astrometry .
The cluster is roughly spherical in its centre but significantly flattened at larger radii .
The observed colour-absolute magnitude diagram shows a clear binary sequence .
The kinematically-modelled parallaxes that we derive are a factor \sim 1.7 / 2.9 more precise than the TGAS / Hipparcos -2 values and allow to derive an extremely sharp main sequence .
This sequence shows evidence for fine-detailed structure which is elegantly explained by the full spectrum turbulence model of convection .