Search for the sub-stellar lithium depletion boundary in the open star
  cluster Coma Berenices
  We mainly aim to search for the lithium depletion boundary (LDB) among the
sub-stellar population of the open star cluster Coma Berenices. We carried out
a search for brown dwarf (BD) candidates using colour-magnitude diagrams
combining optical and infrared photometry from the latest public releases of
the following large-scale surveys: UKIRT/UKIDSS, Pan-STARRS, SDSS, and AllWISE.
We checked astrometric consistency with cluster membership using $Gaia$ DR2. A
couple dozen new candidate BDs located inside the tidal radius of Coma Ber are
reported, but none of these are significantly fainter and cooler than
previously known members. A search for Li in three new and five previously
known BD candidate cluster members was performed via spectroscopic observations
using the OSIRIS instrument at the 10.4-m GTC. No LiI resonance doublet at
6707.8 A was detected in any of eight Coma Ber targets in the magnitude range
J=15--19 and G=20--23 observed with the GTC. Spectral types and radial
velocities were derived from the GTC spectra. These values confirm the cluster
membership of four L2--L2.5 dwarfs, two of which are new in the literature. The
large Li depletion factors found among the four bona fide BD members in Coma
Ber implies that the LDB must be located at spectral type later than L2.5 in
this cluster. Using the latest evolutionary models for BDs, a lower limit of
550 Myr on the cluster age is set. This constraint has been combined with other
dating methods to obtain an updated age estimate of 780$\pm$230 Myr for the
Coma Ber open cluster. Identification of significantly cooler sub-stellar
cluster members in Coma Ber awaits the advent of the Euclid wide survey, which
should reach a depth of about J=23; this superb sensitivity will make it
possible to determine the precise location of the sub-stellar LDB in this
cluster and to carry out a complete census of its sub-stellar population.
