M dwarfs in the Galactic Halo

10.8 Summary

As in the Galactic Disk, low-mass subdwarfs are the majority constituent of the halo. While the properties of these stars are less well understood than their higher-temperature counterparts, recent theoretical work, both on the atmospheres and on the internal structure, combined with the availability of better data, have opened the way to more quantitative analysis. Abundances can be estimated, both from the location of stars in the H-R diagram and from molecular bandstrengths, and the abundance distribution appears to be consistent with that derived for more luminous subdwarfs. The overall luminosity function is well determined, although further work is required to establish both the overall mass function and the extent of any metallicity dependence. Present results suggest that Ψ_halo(M) is somewhat steeper than the disk function over the range 0.8–0.25 M _⊙, but may turn over at low masses and is poorly approximated as a power law. Current results in this area are at best provisional. Despite these uncertainties, however, and even given the current low-accuracy abundance estimates, we can use late-type subdwarfs as calibrators in main sequence fitting. The initial results from that analysis is in good agreement with the more conventional main sequence fitting studies based on F and G subdwarfs.