Spitzer Studies of Ultracool Subdwarfs : Metal - poor Late - type M , L and T Dwarfs

Recent discoveries from red optical proper motion and wide-field near-infrared surveys have uncovered a new population of ultracool subdwarfs, metal-poor stars and brown dwarfs extending into the late-type M, L and possibly T spectral classes. These objects are among the first low-mass stars and brown dwarfs formed in the Galaxy, and are valuable tracers of metallicity effects in low-temperature atmospheres. Like solar metallicity late-type dwarfs, ultracool subdwarfs emit the majority of their emergent flux at infrared wavelengths. Here I discuss how Spitzer observations will contribute to the study of these objects, enabling measurement of the temperature scale, tests for dust formation and the identification of surface gravity and metallicity diagnostics in the coldest brown dwarfs. Subdwarfs are metal-deficient stars, classically defined as lying below the stellar main sequence in optical color-magnitude diagrams (Kuiper 1939). These objects are not subluminous but rather hotter (i.e., bluer optical colors) than equivalent mass main sequence dwarfs, a consequence of their reduced metal opacity (e.g., Chamberlin & Aller 1951). Cool subdwarfs (spectral types sdK and sdM) are typically found to have halo kinematics, and are presumably relics of the early Galaxy. They are therefore important tracers of Galactic chemical history and are representatives of the first generations of star formation. The high space velocities of halo subdwarfs allow them to stand out in proper motion surveys (Reid 1984). Early surveys using photographic plates (e.g., LHS catalog, Luyten 1979a; APM Proper Motion Survey, Scholz et al. 2000) identified several cool subdwarfs with spectral types as late as sdM7/esdM7 (Gizis 1997) and effective temperatures (T ef f) down to 3000 K (Leggett et al. 2000). More recent surveys, such as the SUPERBLINK catalog (Lépine et al. 2003b,c) and the SuperCosmos Sky Survey (Hambly et al. 2001a; Porkorny et al. 2004, SSS), have pushed our compendium of cool subdwarfs to even cooler temperatures. With spectral types extending beyond the end of the M dwarf regime (Scholz et al. 2003), these objects are ultracool subdwarfs. The first L subdwarf, 2MASS 0532+82 (Burgasser et al. 2003a), was serendipi-tously identified in the 2MASS catalog as a faint source invisible in optical survey 1