Subdwarf studies. I. UBVRI photometry of NLTT stars

UBVRI photometry is presented for a sample of 1656 southern stars, including 1211 that were previously unmeasured, drown from the NLTT proper-motion catalog. The catalog is shown to be a rich source of subdwarfs. The normalized ultraviolet excess delta (U - B)0.6, photometric parallax, and interstellar reddening are calculated for each star when possible. Photometric parallaxes are compared with trigonometric parallaxes from the literature. It is found that the former do not have systematic errors greater than about 25 percent. In agreement with other studies, the bluest subdwarfs are found at B - V = 0.35. The selection of the program stars on the basis of large reduced proper motions restricted subgiant contamination of the sample to about 5 percent and increased the discovery fraction of halo stars relative to disk stars. The claim is made here that the sample can be used to investigate the abundance distribution of the halo. The sample includes stars with ultraviolet excesses characteristic of disk abundances but with velocities up to 150 km/s. These are believed to be stars that, quite expectedly, reside in the high-velocity tail of the disk velocity distribution. 63 refs