Proper Motions in the Galactic Bulge: Plaut's Window

Abstract

A proper-motion study of a field of 20' × 20' inside Plaut's low-extinction window (l,b) = (0°, - 8°) has been completed. Relative proper motions and photographic BV photometry have been derived for ~21,000 stars reaching to V ~ 20.5 mag, based on the astrometric reduction of 43 photographic plates, spanning over 21 yr of epoch difference. Proper-motion errors are typically 1 mas yr-1, and field-dependent systematics are below 0.2 mas yr-1. Cross-referencing with the 2MASS catalog yielded a sample of ~8700 stars, from which predominantly disk and bulge subsamples were selected photometrically from the JH color-magnitude diagram. The two samples exhibited different proper-motion distributions, with the disk displaying the expected reflex solar motion as a function of magnitude. Galactic rotation was also detected for stars between ~2 and ~3 kpc from us. The bulge sample, represented by red giants, has an intrinsic proper-motion dispersion of (σl,σb) = (3.39 ± 0.11,2.91 ± 0.09) mas yr-1, which is in good agreement with previous results and indicates a velocity anisotropy consistent with either rotational broadening or triaxiality. A mean distance of 6.37 kpc has been estimated for the bulge sample, based on the observed K magnitude of the horizontal branch red clump. The metallicity [M/H] distribution was also obtained for a subsample of 60 bulge giants stars, based on calibrated photometric indices. The observed [M/H] shows a peak value at [M/H] ~ -0.1 with an extended metal-poor tail and around 30% of the stars with supersolar metallicity. No change in proper-motion dispersion was observed as a function of [M/H]. We are currently in the process of obtaining CCD UBVRI photometry for the entire proper-motion sample of ~21,000 stars.