Structure of the Galactic Stellar Halo Prior to Disk Formation
arXiv:astro-ph/0011113v1 6 Nov 2000
Masashi Chiba National Astronomical Observatory, Mitaka, Tokyo 181-8588, Japan email:
[email protected] Timothy C. Beers Department of Physics & Astronomy, Michigan State University, E. Lansing, MI 48824 email:
[email protected] ABSTRACT We develop a method for recovering the global density distribution of the ancient Galactic stellar halo prior to disk formation, based on the present orbits of metal-poor stars observed in the solar neighborhood. The method relies on the adiabatic invariance of the action integrals of motion for the halo population during the slow accumulation of a disk component, subsequent to earlier halo formation. The method is then applied to a sample of local stars with [Fe/H]≤ −1.5, likely to be dominated by the halo component, taken from Beers et al.’s recently revised and supplemented catalog of metal-poor stars selected without kinematic bias. We find that even if the Galactic potential is made spherical by removing the disk component in an adiabatic manner, the halo density distribution in the inner halo region (R ≤ 15 kpc) remains moderately flattened, with axial ratio of about 0.8 for stars in the abundance range [Fe/H]≤ −1.8 and about 0.7 for the more metal-rich interval −1.8
