A Survey for Faint Stars of Large Proper Motion Using Extra Poss II Plates

We have conducted a search for new stars of high proper motion ($\geq$0.4 arcsec yr$^{-1}$) using POSS II fields for which an extra IIIaF (red) plate of good quality exists, along with useable IIIaJ (blue) and IV-N (infrared) plates taken at epochs differing by a minimum of 1.5 years. Thirty-five fields at Galactic latitudes $|b| \geq$ 20$^o$ were measured, covering some 1378 deg$^2$, or 3.3% of the sky. Searches with three plate combinations as well as all four plates were also made. Seven new stars were found with $\mu \geq$0.5 arcsec yr$^{-1}$, which were therefore missed in the Luyten Half Second (LHS) Catalog. One of these is a common proper motion binary consisting of two subdwarf M stars; another is a cool white dwarf with probable halo kinematics. As a test of our completeness -- and of our ability to test that of Luyten -- 216 of 230 catalogued high proper motion stars were recovered by the software, or 94%. Reasons for incompleteness of the LHS are discussed, such as the simple fact that POSS II plates have deeper limiting magnitudes and greater overlap than did POSS I. Nonetheless, our results suggest that the LHS is closer to 90% complete than recent estimates in the literature (e.g 60%), and we propose a reason to account for one such lower estimate. The conclusion that the LHS Catalog is more complete has implications for the nature of the halo dark matter. In particular it strengthens the constraint on the local density of halo stars, especially white dwarfs at M$_V\sim$17-18.
Comment: Astronomical Journal, in press, 16 pages, 3 figures