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 (1,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 years of epoch diαerence. Proper motion errors are typically 1 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 diαerent proper-motion distributions, with the disk displaying the expected reex solar motion. 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; 2:91) ± (0:11; 0:09) mas yr-1, which is in good agreement with previous results. A mean distance of 6:37+0.87-0.77 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 UBV RI photometry for the entire proper-motion sample of ̃ 21; 000 stars. © 2009: Instituto de Astronomía, UNAM.