In a recent survey for faint red stars
from a digital stack of Schmidt plates a number of
candidate objects were identified. Parallax’s for
three of these objects have been reported showing
them to have luminosities which interpreted within
the available evolutionary models indicate them to
be good brown dwarf candidates. Here we examine
spectra of these objects and others from the plate
stack. Using standard spectral indices we find
that for a given spectral type their spectra are
more consistent with the Pleiades brown dwarfs
(PPL 15, Teide 1 and Calar 3) than with standard
late-type M dwarfs. Our interpretation is that
this is due to their selection by
RF IN
colours which at values > 3 preferentially
selects objects with relatively low gravities. For
late-type M dwarfs and brown dwarfs low gravities
are expected to be a reliable indication of youth.
We also notice that the stack objects generally
have strong FeH absorption for their spectral
type. Current model atmospheres suggest that FeH
strongly increases in strength toward lower
metallicities and lower temperatures. We believe
that this is not consistent with the available
observational evidence from late-type M dwarfs. It
is possible that solid Fe is forming inthe low
temperature atmospheres relatively depleting FeH
strengths toward lower temperatures. We find some
evidence that for dwarfs at low temperatures dust
formation is less prevalent in lower gravity
objects suggesting that dwarfs at low temperatures
stronger FeH may be an indication of youth. In
addition to the spectral evidence the three stack
objects whose parallax’s have been measured show
small tangential velocities which is a further
indication of youth.
TESS Observations of the Luhman 16 AB Brown Dwarf System: Rotational Periods, Lightcurve Evolution, and Zonal Circulation
