Optical Spectroscopy of 2MASS Color‐selected Ultracool Subdwarfs

We present Gemini GMOS and Magellan LDSS-3 optical spectroscopy for sevenultracool subdwarf candidates color-selected from the Two Micron All SkySurvey. Five are identified as late-type subdwarfs, including the previouslyreported sdM9.5 SSSPM 1013-1356 and L subdwarf 2MASS 1626+3925, and a newsdM8.5 2MASS 0142+0523. 2MASS 1640+1231 exhibits spectral features intermediatebetween a late-type M dwarf and subdwarf, similar to the previously identifiedhigh proper motion star SSSPM 1444-2019, and we classify both sources as mildsubdwarfs, d/sdM9. 2MASS 1227-0447 is a new ultracool extreme subdwarf,spectral type esdM7.5. Spectral model fits yield metallicities that areconsistent with these metallicity classifications. Effective temperatures trackwith numerical subtype within a metallicity class, although they are notequivalent across metallicity classes. As a first attempt to delineate subtypesin the L subdwarf regime we classify 2MASS 1626+3925 and the previouslyidentified 2MASS 0532+8246 as sdL4 and sdL7, respectively, to reflect theirsimilarity to equivalently classified, solar metallicity L-type field dwarfsover the 7300-9000 A region. We also detail preliminary criteria fordistinguishing L subdwarf optical spectra as a roadmap for defining this newspectral class. The strong TiO bands and Ca I and Ti I lines in the spectrum of2MASS 1626+3925 provide further evidence that condensate formation may beinhibited in metal-deficient L subdwarfs. We conclude with a compendium ofcurrently known, optically classified ultracool subdwarfs.Comment: 27 pages, 14 figures, 6 tables; accepted for publication to ApJ. Figures 2a-2e are included in PDF format in the source file