THE DWARF IRREGULAR GALAXY SEXTANS-A .1. HST PHOTOMETRY OF THE RESOLVED STARS

We have used the Hubble Space Telescope to observe the resolved stars of the nearby (D = 1.4 Mpc) dwarf irregular galaxy Sextans A (DDO 75, A 1008-04). The data consist of dithered WFPC2 images in 3 bands: F439 W (1 hour), F555W (30 minutes), and F814W (30 minutes). The stellar ph otometry was extracted using the DoPHOT program developed by Schechter et al, (1993, PASP, 105, 1342) with modifications for WFPC2 data by S aha et al. (1996, ApJ, 466, 550). The. dithering improved the photomet ric accuracy of faint stars since it produced stellar profiles that we re more uniform than in undithered images. Also, the PSF fitting metho d used by DoPHOT was insensitive to the intra-pixel sensitivity variat ion of the WFPC2 chips, while simple aperture photometry was slightly sensitive to this variation. The photometry was calibrated to V, B, an d I with the equations presented in Holtzman et al. (1995, PASP, 107, 1065), Comparisons with previously published ground-based data showed excellent agreement with stars brighter than V approximate to 21. Fain ter stars are severely hampered by crowding from the ground, At the di stance of Sextans A, the superior resolution of the HST overcomes this confusion, and enables very accurate and deep photometry. Artificial star tests showed the data to be 50% complete to V=25.6, B=25.4, and I =25.3. This high quality data produced color-magnitude diagrams (CMDs) with excellent detail. The main sequence (MS), red supergiants, and r ed giant branch (RGB) are all clearly distinct populations, Most strik ing is a population just redward of the MS, which had not been photome trically resolved from the MS in ground-based observations. These star s are massive core Helium burning stars (HeB) in the bluest extent of the so-called ''blue-loop'' phase. This population has never before be en clearly identifiable in a low metallicity system. (C) 1997 American Astronomical Society. [S0004-6256(97)00612-2].