A deep large-area search for very low-mass members of the Hyades open cluster

We present the results of a comprehensive I- and Z-band photometric survey of 10.5 deg² of the Hyades to search for low-mass stellar and substellar members. The survey, which is ≳95 per cent photometrically complete to I_c ≈ 20.3, has unearthed a total of 20 candidates, 14 of which are previously uncatalogued. Despite follow-up observations indicating that nine display spectral energy distributions consistent with cluster membership, a detailed astrometric study of all 20 reveals that only one, the previously known stellar member RHy297, displays a proper motion typical of a Hyad. We discuss our failure to detect further low-mass members in terms of the shape of the present-day mass function of the Hyades and find that dynamical evolution has probably led to their preferential evaporation from the cluster.