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-subs tellar boundary. Methods. We performed a deep (I = 23, z = 22.5) photometric survey over 16 deg 2 around the cluster center, followed up with K-band photometry to measure the proper motion of candidate member s, and optical and near-IR spectroscopy of probable BD and VL M members. Results. We report the discovery of the first 2 brown dwarfs in the Hyade s 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. Acco rding 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 membe rs of the cluster, and confirm the membership of 16 others. We com bine these results with a list of previously known cluster me mbers to build the present-day mass function (PDMF) of the Hyades clu ster from 50 Jupiter masses to 3 M ⊙ . We find the Hyades PDMF to be strongly deficient in very low mass objects and brown dwarfs c ompared to the IMF of younger open clusters such as the Pleiad es. We interpret this deficiency as the result of dynamical evoluti on over the past few 100 Myr, i.e., the preferential evaporat ion of low mass cluster members due to weak gravitational encounters. Conclusions. We thus estimate that the Hyades cluster currently hosts abo ut 10-15 brown dwarfs, while its initial substellar populat ion may have amounted up to 150-200 member