  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. We performed a deep (I=23, z=22.5)
photometric survey over 16 sq.deg. around the cluster center, followed up with
K-band photometry to measure the proper motion of candidate members, and
optical and near-IR spectroscopy of probable BD and VLM members. We report the
discovery of the first 2 brown dwarfs 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 3Mo. We find the Hyades PDMF to
be strongly deficient in very low mass objects and brown dwarfs 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. We thus estimate that the Hyades cluster currently hosts about
10-15 brown dwarfs, while its initial substellar population may have amounted
up to 150-200 members.

Astrophysics