Title:  Brown Dwarfs and Planetary-Mass Candidates in Upper Scorpius: A
        Test of Evolutionary Models through High-Resolution Spectral
        Analysis 

Subu Mohanty
Harvard-Smithsonian Center for Astrophysics 


Abstract:
 
We present an analysis of high resolution optical spectra for a sample
of very young, mid- to late M, low-mass stellar and substellar objects
in Upper  Scorpius.  Effective temperatures and  gravities are derived
from a  multi-feature spectral  analysis, through comparison  with the
latest synthetic  spectra.  We  also calculate extinctions,  radii and
masses  for  our  sample  based  on our  temperatures  and  gravities,
observed  photometry,  surface   fluxes  predicted  by  the  synthetic
spectra, and  the known  distance to Upper  Sco.  These  estimates are
INDEPENDENT  of theoretical  evolutionary models  and  isochrones.  We
also  preform  the same  analysis  for GG  Tau  B,  for which  stellar
parameters have previously  been estimated from isochrone comparisons.
We  compare  all  our  results   to  the  predictions  of  the  BCAH98
evolutionary tracks.

We  show  that  for  the  coolest, lowest  mass  objects,  significant
discrepancies exist between the BCAH98  tracks and our results.  For a
given  mass and  age,  the  tracks predict  smaller  radii and  larger
temperatures than  our values.   Recent studies of  low-mass eclipsing
binary   systems  with   very  well   determined  masses,   radii  and
luminosities  show that the  theoretical tracks  indeed systematically
underestimate  radius and overestimate  temperature.  We  propose that
the  same  effect is  responsible  for  the  disagreement between  our
results and BCAH98's theoretical  predictions.  If so, it also implies
that the uncertainties in the evolutionary tracks are enhanced for the
coolest, lowest mass pre-Main Sequence (PMS) objects.

In  Upper Sco,  we  find two  of our  objects  to be  well within  the
planetary mass  regime (~ 6 M_J).   If we are correct,  then these are
the  first  planetary-mass objects  to  be  confirmed cluster  members
through Lithium  detection, and with masses  determined independent of
theoretical   evolutionary  tracks.   Our   result  also   raises  the
possibility that the  some of latest spectral type  PMS objects may be
significantly  lower  in mass  than  a  simple  comparison to  current
isochrones might  suggest.  At  the very least,  both our  objects are
ultra-low mass  ones, at  the brown dwarf  / planetary  mass boundary.
Their presence, combined with  that of similar ultra-low mass isolated
objects being found in other clusters, by different groups, raises the
possibility  that  such objects  are  ubiquitous.   How  to form  them
remains a serious challenge to theory.