Brown dwarfs and very low mass stars in the Hyades cluster: a dynamically evolved mass function

Aims: We conducted a search for brown dwarfs (BDs) and very low mass (VLM) stars in the 625 Myr-old Hyades cluster in order to derive the cluster's mass function across the stellar-substellar boundary. Methods: We performed a deep (I=23, z=22.5) photometric survey over 16 deg2 around the cluster center and followed up with K-band photometry to measure the proper motion of candidate members and with optical and near-IR spectroscopy of probable BD and VLM members. Results: We report the discovery of the first 2 BDs in the Hyades cluster. The 2 objects have a spectral type early-T and their optical and near-IR photometry as well as their proper motion are consistent with them being cluster members. According to models, their mass is 50 Jupiter masses at an age of 625 Myr. We also report the discovery of 3 new very low mass stellar members of the cluster and confirm the membership of 16 others. We combine these results with a list of previously known cluster members to build the present-day mass function (PDMF) of the Hyades cluster from 50 Jupiter masses to 3 M_⊙. We find the Hyades PDMF to be strongly deficient in very low mass objects and BDs compared to the IMF of younger open clusters such as the Pleiades. We interpret this deficiency as the result of dynamical evolution over the past few 100 Myr, i.e., the preferential evaporation of low mass cluster members due to weak gravitational encounters. Conclusions: We thus estimate that the Hyades cluster currently hosts about 10-15 BDs, while its initial substellar population may have amounted to up to 150-200 members.