Spectral energy distributions for disk and halo M dwarfs

We have obtained infrared (1-2.5 mu m) spectroscopy for 42 halo and disk dw arfs with spectral types M1-M6.5. These data are compared to synthetic spec tra generated by the latest model atmospheres of Allard & Hauschildt. Photo spheric parameters metallicity, effective temperature, and radius are deter mined for the sample. We find good agreement between observation and theory except for known problems due to incomplete molecular data for metal hydri des and H2O. The metal-poor M subdwarfs are well matched by the models, as oxide opacity sources are less important in this case. The derived effectiv e temperatures for the sample range from 3600 to 2600 K; at these temperatu res grain formation and extinction are not significant in the photosphere. The derived metallicities range from solar to 1/10 solar. The radii and eff ective temperatures derived agree well with recent models of low-mass stars . The spectra are available in electronic form upon request.