A new L5 brown dwarf member of the Hyades cluster with chromospheric
  activity
  Our aim is to identify brown dwarf members of the nearby Hyades open star
cluster to determine the photometric and spectroscopic properties of brown
dwarfs at moderately old ages and extend the knowledge of the substellar mass
function of the cluster. We cross-matched the 2MASS and AllWISE public
catalogues and measured proper motions to identify low-mass stars and brown
dwarf candidates in an area of radius eight degrees around the central region
of the Hyades cluster. We identified objects with photometry and proper motions
consistent with cluster membership. For the faintest ($J$\,=\,17.2 mag) most
promising astrometric and photometric low-mass candidate
2MASS\,J04183483$+$2131275\@, with a membership probability of 94.5\%, we
obtained low-resolution (R\,=\,300--1000) and intermediate-resolution
(R\,=\,2500) spectroscopy with the 10.4m Gran Telescopio Canarias. From the
low-resolution spectra we determined a L5.0$\pm$0.5 spectral type, consistent
with the available photometry. In the intermediate dispersion spectrum we
detected H$\alpha$ in emission (marginally resolved with a full width half
maximum of $\sim$2.8 \AA{}) and determined a $\log ($L$_{\rm{H}\alpha}$/L$_{\rm
bol})=-6.0$ dex. From H$\alpha$ we obtained a radial velocity of 38.0$\pm$2.9
km$\cdot$s$^{-1}$, which combined with the proper motion leads to space
velocities which are fully consistent with membership in the Hyades cluster.
