Homogeneous photometry for star clusters and resolved galaxies. II. Photometric standard stars

Stars appearing in CCD images obtained over 224 nights during the course of 69 observing runs have been calibrated to the Johnson/Kron-Cousins BVRI ph otometric system defined by the equatorial standards of Landolt (1992, AJ, 104, 340). More than 15,000 stars suitable for use as photometric standards have been identified, where "suitable" means that the star has been observ ed five or more times during photometric conditions and has a standard erro r of the mean magnitude less than 0.02 mag in at least two of the four band passes, and shows no significant evidence of intrinsic variability. Many of these stars are in the same fields as Landolt's equatorial standards or Gr aham's (1982, PASP, 94, 244) southern E-region standards but are considerab ly fainter. This enhances the value of those fields for the calibration of photometry obtained with large telescopes. Other standards have been define d in fields containing popular objects of astrophysical interest, such as s tar clusters and famous galaxies, extending Landolt-system calibrators to d eclinations far from the equator and to stars of subsolar chemical abundanc es. I intend to continue to improve and enlarge this set of photometric sta ndard stars as more observing runs are reduced. The full current database o f photometric indices is being made freely available via a site on the Worl d Wide Web or via direct request to the author. Although the contents of th e database will evolve in detail, at any given time it should represent the largest sample of precise BVRI broadband photometric standards available a nywhere.