Spitzer Space Telescope - Directors Discretionary Time Proposal #251


Old and Cold:  Identifying and Characterizing the Coldest Stellar and
Substellar Halo Subdwarfs

Principal Investigator: Adam Burgasser
           Institution: Massachusetts Institute of Technology

Science Category: brown dwarfs/very low mass stars 
 Observing Modes: IracMap IrsStare 
  Hours Approved: 19.8 

Abstract:
Wide-field near-infrared surveys such as 2MASS, DENIS, and SDSS have
uncovered a vast population of ultracool stars and brown dwarfs in the
Solar Neighborhood, the majority of which are Solar metallicity
members of the disk population.  In the past year, a number of
ultracool dwarfs with spectral features indicating subsolar
metallicity, as well as halo kinematics, have been identified.  These
ultracool subdwarfs extend into the late-type M and L spectral
classes, a regime in which condensate dust plays an increasingly
important role in the emergent spectral energy distributions of disk
dwarfs.  Because dust grains, and indeed many of the dominant
absorption features in ultracool dwarfs, are comprised of metals,
differences in overall atmospheric metallicity will have a substantial
effect on the spectral morphology, cloud formation processes, and
evolution of M, L, and T-type stars and brown dwarfs.  As part of a
comprehensive investigation of metal-poor ultracool dwarfs, I will
undertake Spitzer IRAC and IRS observations of several of the coolest
currently known subdwarfs and, through comparison to observations of
disk dwarfs, search for diagnostics of metallicity.  Further
comparison to spectral models will enable the determination of the
physical properties of these low-mass stars and brown dwarfs while
also providing important feedback into our theoretical understanding
of cool atmospheres.  These observations are part of the PIs Spitzer
Fellowship.