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S. K. Leggett (JAC Hilo Hawaii), F. A. Allard (CRAL, ENS, Lyon France), C. C. Dahn (USNO Flagstaff Station), P. H. Hauschildt (U. Georgia), J. Rayner (NASA IRTF, Honolulu Hawaii)
We have obtained infrared (1-2.5 microns) spectroscopy for 54 halo and disk stars around the stellar/substellar boundary. These data are combined with optical data where available and compared to synthetic spectra generated by the latest model atmospheres of Allard & Hauschildt. We find good agreement between observation and theory except for known problems in the V and H passbands probably due to incomplete molecular data for TiO, metal hydrides and H2O. The metal-poor M subdwarfs are well matched by the models as oxide opacity sources are less important in this case. A version of the NextGen-Dusty models which includes homogeneous formation and extinction by dust grains produces a much improved match to the coolest objects in the sample, compared to the grainless models. Photospheric parameters - metallicity, effective temperature, radius - are determined for the sample. We present relationships between bolometric correction and color, and effective temperature and color, as a function of metallicity, and compare the radii and effective temperatures to recent structural models of low mass stars and brown dwarfs.