AAS 205th Meeting, 9-13 January 2005
Session 11 Brown Dwarfs and Exoplanets
Poster, Monday, January 10, 2005, 9:20am-6:30pm, Exhibit Hall

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[11.06] Metallicity Diagnostics in the Spectra of Ultracool M and L Subdwarfs

A.J. Burgasser (AMNH), J.D. Kirkpatrick (IPAC/Caltech), S. Lepine (AMNH), J.E. Gizis (U Delaware), I.N. Reid (STScI), M. Cushing (NASA Ames)

Recent red optical proper motion and near-infrared photometric surveys have uncovered a substantial population of metal-poor, ultracool subdwarfs extending down to the end of the M spectral sequence and into the L dwarf regime. Identifying differences in the spectra of these objects as compared to local field dwarfs can provide important metallicity diagnostics across the stellar/substellar boundary. To pursue this goal, we have observed a suite of ultracool late-type M and L subdwarfs in the optical using Keck LRIS and in the near-infrared using the NASA IRTF Spex spectrograph. Low-resolution, broad-band spectral energy distribution (SED) measurements highlight the strong effects of collision-induced H2 absorption in these spectra, resulting in increasingly blue near-infrared colors for cooler subdwarfs, contrary to trends observed in the dwarf sequence. Metal hydride bands also play an increasingly important role at red optical and J-band wavelengths, with traditionally stong CaH bands joined by enhanced CrH and FeH absorption features. Significant pressure broadening is also present in the line shapes of the 1.25 \mum K I lines, reflecting the high pressure, high density photospheres of these objects. The subdwarf spectra exhibit natural gradations in band strength and spectral slope, with strong similarities to dwarf spectra shortward of 1.3 \mum. These trends suggest means of updating existing optically-based subdwarf classification schemes and implementing near-infrared schemes for the cooler L subdwarfs. We point out a number of theoretical studies needed to advance research on these ultracool subdwarfs, including the influence of metallicity on chemical equilibrium abundances and dust formation, and improved spectral models incorporating subsolar metallicity.

Support for this work was provided by NASA through the Spitzer Fellowship Program.

The author(s) of this abstract have provided an email address for comments about the abstract: adam@amnh.org

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