HALO COMMON PROPER MOTION STARS - SUBDWARF DISTANCE SCALE, HALO BINARY FRACTION, AND UBVRI COLORS

UBVRI photometry was obtained of common proper motion stars in the NLTT catalog, and genuine halo pairs were identified. Photometric parallaxes were computed using UBV and VRI colors, and differences in the distance moduli for stars in a given pair were investigated. About 20% of the stars with [Fe/H] < - 1.0 were found to have random errors >several X 0.1 mag in distance modulus, in addition to the photometric errors, thought to be due to the existence of close binary companions to some of the common proper motion (CPM) stars. The implications of these errors for kinematic studies' of field Population Il stars are considered. Systematic errors of order 16% were also detected in distances derived from the U - B, B - V deblanketing technique. The close binary fraction in these stars, approximately 20% of the apparently single stars, may be the best record of the primordial close binary fraction of Population II stars. This value is very similar to that for Population I stars. It is suggested that Population II field stars did not originate in globular clusters which subsequently disintegrated (unless they dissolved within the first approximately 10(8) yr) but formed in less dense environments. Several K subdwarfs were found with very high ultraviolet excesses, comparable with those predicted by some synthetic colors, but spectroscopic data are required to clarify their nature. The halo CPM pairs may be useful for later inclusion in lists of standard stars, providing pairs of metal deficient stars spanning a range of colors.