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arXiv · 0903.1567

Optical and Near-Infrared Spectroscopy of the L Subdwarf SDSS J125637.13-022452.4

Adam J. Burgasser·Soeren Witte·Christiane Helling·Robyn E. Sanderson·John J. Bochanski·Peter H. Hauschildt
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Abstract

Red optical and near-infrared spectroscopy are presented for SDSS J125637.13-022452.4, one of only four L subdwarfs reported to date. These data confirm the low-temperature, metal-poor nature of this source, as indicated by prominent metal-hydride bands, alkali lines, and collision-induced H2 absorption. The optical and near-infrared spectra of SDSS J1256-0224 are similar to those of the sdL4 2MASS J16262034+3925190, and we derive a classification of sdL3.5 based on the preliminary scheme of Burgasser, Cruz, & Kirkpatrick. The kinematics of SDSS J1256-0224 are consistent with membership in the Galactic inner halo, with estimated $UVW$ space velocities indicating a slightly prograde, eccentric and inclined Galactic orbit (3.5 <~ R <~ 11 kpc; |Zmax| = 7.5 kpc). Comparison to synthetic spectra computed with the Phoenix code, including the recent implementation of kinetic condensate formation (Drift-Phoenix), indicate Teff ~ 2100-2500 K and [M/H] ~ -1.5 to -1.0 for logg ~ 5.0-5.5 (cgs), although there are clear discrepancies between model and observed spectra particularly in the red optical region. The stronger metal-oxide bands present in the Drift-Phoenix model spectra, a result of phase-non-equilibrium abundances of grain species, appears to contradict prior suggestions that grain formation is inhibited in metal-poor atmospheres; conclusive statements on the metallicity dependence of grain formation efficiency are as yet premature. In addition, an apparent shift in the temperature scale of L subdwarfs relative to L dwarfs may obviate the need for modified grain chemistry to explain some of the former's unique spectral characteristics.

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Adam J. Burgasser, Soeren Witte, Christiane Helling, Robyn E. Sanderson, John J. Bochanski, Peter H. Hauschildt. 2009-03-09. Optical and Near-Infrared Spectroscopy of the L Subdwarf SDSS J125637.13-022452.4. https://doi.org/10.1088/0004-637x%2F697%2F1%2F148

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