The new cool subdwarfs: metal-poor stars and brown dwarfs extending into the L and T dwarf regimes
Adam Burgasser (UCLA, Dept. Physics & Astronomy)
J. Davy Kirkpatrick (Caltech/IPAC)
Sebastien Lepine (American Museum of Natural History)
M-type subdwarfs (sdMs) are the most common component of the metal-poor stellar
halo, probing early stellar masses down to ≈ 0.1 Mˆ. Most of the
coolest sdMs have been previously identified in photographic plate proper motion
surveys, notably Luyten's LHS and NLTT surveys amongst others. New red optical
and near-infrared (NIR) proper motion and photometric surveys are now turning up
the next generation of cool subdwarfs that straddle the stellar/substellar
boundary, including metal-poor analogs of the L and T spectral classes. In this
presentation, I will review some of these discoveries, including many of the
coolest brown dwarfs currently known (Teff 900 K). I will show how
metallicity strongly affects the emergent spectral energy distributions and
atmospheric properties (e.g., cloud formation) of very cool subdwarfs, as well as
their thermal evolution. Finally, I will examine the prospects for a NIR proper
motion survey, including some early results, and show how measurement of the
metal-poor LF below 2000 K could provide strong constraints on the star
formation history of this population as a whole.
http://www.astro.ucla.edu/~adam