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 photometric 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 observed five or more times during photometric conditions and has a standard error of the mean magnitude less than 0.02 mag in at least two of the four bandpasses, and shows no significant evidence of intrinsic variability. Many of these stars are in the same fields as Landolt's equatorial standards or Graham's (1982, PASP, 94, 244) southern E-region standards, but are considerably fainter. This enhances the value of those fields for the calibration of photometry obtained with large telescopes. Other standards have been defined in fields containing popular objects of astrophysical interest, such as star clusters and famous galaxies, extending Landolt-system calibrators to declinations far from the equator and to stars of sub-Solar chemical abundances. I intend to continue to improve and enlarge this set of photometric standard stars as more observing runs are reduced. The full current database of photometric indices is being made freely available via a site on the World-Wide Web, or by direct request to the author. Although the contents of the database will evolve in detail, at any given time it should represent the largest sample of precise BVRI broad-band photometric standards available anywhere.Comment: Accepted for July 2000 PAS

Color-Magnitude Diagram Constraints on the Metallicities, Ages, and Star Formation History of the Stellar Populations in the Carina Dwarf Spheroidal Galaxy

Victoria-Regina isochrones for $-0.4 \le$ [alpha/Fe] $\le +0.4$ and a wide range in [Fe/H], along with complementary zero-age horizontal branch (ZAHB) loci, have been applied to the color-magnitude diagram (CMD) of Carina. The color transformations that we have used have been "calibrated" so that isochrones provide excellent fits to the $[(B-V)_0,\,M_V]$-diagrams of M3 and M92, when well supported estimates of the globular cluster (GC) reddenings and metallicities are assumed. The adopted distance moduli, for both the GCs and Carina, are based on our ZAHB models, which are able to reproduce the old HB component (as well as the luminosity of the HB clump) of the dwarf spheroidal galaxy quite well --- even if it spans a range in [Fe/H] of ~ 1.5 dex, provided that [alpha/Fe] varies with [Fe/H] in approximately the way that has been derived spectroscopically. Ages derived here agree reasonably well with those found previously for the old and intermediate-age turnoff stars, as well as for the period of negligible star formation (SF) activity (~ 6-10 Gyr ago). CMD simulations have been carried out for the faintest turnoff and subgiant stars. They indicate a clear preference for SF that lasted several Gyr instead of a short burst, with some indication that ages decrease with increasing [Fe/H]. In general, stellar models that assume spectroscopic metallicities provide satisfactory fits to the observations, including the thin giant branch of Carina, though higher oxygen abundances than those implied by the adopted values of [alpha/Fe] would have favorable consequences.Comment: 15 pages, including 12 figures; accepted for publication in the Astrophysical Journa

Fiducial Stellar Population Sequences for the u'g'r'i'z' System

We describe an extensive observational project that has obtained high-quality and homogeneous photometry for a number of different Galactic star clusters (including M 92, M 13, M 3, M 71, and NGC 6791) spanning a wide range in metallicity (-2.3<[Fe/H]<+0.4), as observed in the u'g'r'i'z' passbands with the MegaCam wide-field imager on the Canada-France-Hawaii Telescope. By employing these purest of stellar populations, fiducial sequences have been defined from color-magnitude diagrams that extend from the tip of the red-giant branch down to approximately 4 magnitudes below the turnoff: these sequences have been accurately calibrated to the standard u'g'r'i'z' system via a set of secondary photometric standards located within these same clusters. Consequently, they can serve as a valuable set of empirical fiducials for the interpretation of stellar populations data in the u'g'r'i'z' system.Comment: 16 pages, 7 tables, 13 figures; accepted for publication in A

Neutrino and axion bounds from the globular cluster M5 (NGC 5904)

The red-giant branch (RGB) in globular clusters is extended to larger brightness if the degenerate helium core loses too much energy in "dark channels." Based on a large set of archival observations, we provide high-precision photometry for the Galactic globular cluster M5 (NGC 5904), allowing for a detailed comparison between the observed tip of the RGB with predictions based on contemporary stellar evolution theory. In particular, we derive 95% confidence limits of $g_{ae}<4.3\times10^{-13}$ on the axion-electron coupling and $\mu_\nu<4.5\times10^{-12}\,\mu_{\rm B}$ (Bohr magneton $\mu_{\rm B}=e/2m_e$) on a neutrino dipole moment, based on a detailed analysis of statistical and systematic uncertainties. The cluster distance is the single largest source of uncertainty and can be improved in the future.Comment: 5 pages, 2 figures, accepted for publication in Physical Review Letter

The Distance to Supernova 1998aq in NGC3982

The distance to NGC3982, host galaxy to the Type Ia supernova SN1998aq, is derived using 32 Cepheids discovered in archival multi-epoch Hubble Space Telescope observations. Employing recent Large Magellanic Cloud Cepheid period-luminosity relations and absolute zero point, we find a distance to NGC3982 of 20.5+/-0.8(r)+/-1.7(s) Mpc, including both random (r) and systematic (s) uncertainties, and ignoring any metallicity dependency in the Cepheid period-luminosity relation. Still unpublished light curve photometry promises to make SN1998aq one of the most important calibrators for the Type Ia supernova decline rate-peak luminosity relationship.Comment: 4 pages, 2 figures, 1 table, LaTeX (mn.sty). Accepted for publication in the Monthly Notices of the Royal Astronomical Society. Also available at http://astronomy.swin.edu.au/bgibson/publications.htm