The first spectroscopically confirmed brown dwarfs in NGC 2264
  We present spectroscopic follow-up observations of 68 red, faint candidates
from our multi-epoch, multi-wavelength, previously published survey of NGC
2264. Using near-infrared spectra from VLT/KMOS, we measure spectral types and
extinction for 32 young low-mass sources. We confirm 13 as brown dwarfs in NGC
2264, with spectral types between M6 and M8, corresponding to masses between
0.02 and 0.08$M_{\odot}$. These are the first spectroscopically confirmed brown
dwarfs in this benchmark cluster. 19 more objects are found to be young M-type
stars of NGC 2264 with masses of 0.08 to 0.3$\,M_{\odot}$. 7 of the confirmed
brown dwarfs as well as 15 of the M-stars have IR excess caused by a disc.
Comparing with isochrones, the typical age of the confirmed brown dwarfs is
$<$0.5 to 5Myr. More than half of the newly identified brown dwarfs and very
low mass stars have ages $<$0.5Myr, significantly younger than the bulk of the
known cluster population. Based on the success rate of our spectroscopic
follow-up, we estimate that NGC 2264 hosts 200-600 brown dwarfs in total (in
the given mass range). This would correspond to a star-to-brown dwarf ratio
between 2.5:1 and 7.5:1. We determine the slope of the substellar mass function
as $\alpha = 0.43^{+0.41}_{-0.56}$, these values are consistent with those
measured for other young clusters. This points to a uniform substellar mass
function across all star forming environments.
