Faint star counts in the near-infrared

We discuss near-infrared star counts at the Galactic pole with a view to guiding the NGST and ground-based NIR cameras. Star counts from deep K-band images from the CFHT are presented, and compared with results from the 2MASS survey and some Galaxy models. With appropriate corrections for detector artifacts and galaxies, the data agree with the models down to K~18, but indicate a larger population of fainter red stars. There is also a significant population of compact galaxies that extend to the observational faint limit of K=20.5. Recent Galaxy models agree well down to K$\sim$19, but diverge at fainter magnitudes.Comment: 14 pages and 4 diagrams; to appear in PAS

Galactic Globular Cluster Metallicity Scale from the Calcium Triplet. II. Rankings, Comparisons and Puzzles

We compare our compilation of the W' calcium index for 71 Galactic globular clusters to the widely used Zinn and West (1984 ApJS, 55, 45) [Fe/H] scale and to Carretta and Gratton's (1997 A&A Supplement 121, 95) scale from high-dispersion spectra analyzed with Kurucz (1992, private communication) model atmospheres. We find our calcium ranking to be tightly correlated with each comparison set, in a non-linear and a linear fashion, respectively. By combining our calcium index information with the Zinn and West ranking, we are able to rank the globular clusters in our sample with a typical precision of +/- 0.05 dex for [Fe/H] < -0.5 on the Zinn and West scale; for clusters more metal rich than this, the ranking is less precise. The significant differences between these metallicity scales raise important questions about our understanding of Galactic formation and chemical enrichment processes. Furthermore, in spite of the apparent improvement in metallicity ranking for the Galactic globular clusters that results from our addition of information from the Ca II triplet lines to the potpourri of other metallicity indicators, caution -- perhaps considerable -- may be advisable when using W' as a surrogate for metallicity, especially for systems where ranges in age and metallicity are likely.Comment: To appear in the August 1997 issue of PASP Also available at http://www.hia.nrc.ca/eprints.htm

Time-Series BVI Photometry for the Globular Cluster NGC 6981 (M72)

We present new BVI photometry of the globular cluster NGC 6981 (M72), based mostly on ground-based CCD archive images. We present a new color-magnitude diagram (CMD) that reaches almost four magnitudes below the turn-off level. We performed new derivations of metallicity and morphological parameters of the evolved sequences, in good agreement with previous authors, obtaining a value of [Fe/H] ~ -1.50 in the new UVES scale. We also identify the cluster's blue straggler population. Comparing the radial distribution of these stars with the red giant branch population, we find that the blue stragglers are more centrally concentrated, as found in previous studies of blue stragglers in globular clusters. Taking advantage of the large field of view covered by our study, we analyzed the surface density profile of the cluster, finding extratidal main sequence stars out to r ~ 14.1 arcmin or about twice the tidal radius. We speculate that this may be due to tidal disruption in the course of M72's orbit, in which case tidal tails associated with the cluster may exist. We also take a fresh look at the variable stars in the cluster, recovering all previous known variables, including three SX Phoenicis stars, and adding three previously unknown RR Lyrae (1 c-type and 2 ab-type) to the total census. Finally, comparing our CMD with unpublished data for M3 (NGC 5272), a cluster with similar metallicity and horizontal branch morphology, we found that both objects are essentially coeval.Comment: Accepted for publication in A

Surface Brightness and Stellar Populations at the Outer Edge of the Large Magellanic Cloud: No Stellar Halo Yet

We present a high quality CMD for a 36'x 36' field located 8 degrees (7 kpc) from the LMC center, as well as a precise determination of the LMC surface brightness derived from the resolved stellar population out to this large galactocentric radius. This deep CMD shows for the first time the detailed age distribution at this position, where the surface brightness is V=26.5 mag/sq". At a radius R=474' the main sequence is well populated from the oldest turnoff at I=21.5 to the 2.5 Gyr turnoff at I=19.5. Beyond this radius, a relatively strong gradient in the density of stars with ages in the 2.5-4 Gyr range is apparent. There are some stars brighter and bluer than the main population, quite uniformly distributed over the whole area surveyed, which are well matched by a 1.5 Gyr isochrone and may be indicative of a relatively recent star formation, or merger, event. The surface brightness profile of the LMC remains exponential to this large galactocentric radius and shows no evidence of disk truncation. Combining the information on surface brightness and stellar population we conclude that the LMC disk extends (and dominates over a possible stellar halo) out to a distance of at least 7 kpc. These results confirm that the absence of blue stars in the relatively shallow off-center CMDs of dIrr galaxies is not necessarily evidence for an exclusively old stellar population resembling the halo of the Milky Way.Comment: ApJLett, in press 13 pages including 3 color figure

The origin of the LMC stellar bar: clues from the SFH of the bar and inner disk

We discuss the origin of the LMC stellar bar by comparing the star formation histories (SFH) obtained from deep color-magnitude diagrams (CMDs) in the bar and in a number of fields in different directions within the inner disk. The CMDs, reaching the oldest main sequence turnoffs in these very crowded fields, have been obtained with VIMOS on the VLT in service mode, under very good seeing conditions. We show that the SFHs of all fields share the same patterns, with consistent variations of the star formation rate as a function of time in all of them. We therefore conclude that no specific event of star formation can be identified with the formation of the LMC bar, which instead likely formed from a redistribution of disk material that occurred when the LMC disk became bar unstable, and shared a common SFH with the inner disk thereafter. The strong similarity between the SFH of the center and edge of the bar rules out significant spatial variations of the SFH across the bar, which are predicted by scenarios of classic bar formation through buckling mechanisms.Comment: MNRAS Letters, accepte

The Stellar Populations of the Carina Dwarf Spheroidal Galaxy: I. a New Color-Magnitude Diagram for the Giant and Horizontal Branches

We report on the first in a series of studies of the Carina dwarf spheroidal galaxy, a nearby satellite of our Galaxy. Our two major results are: 1) precise BI photometry (\sigma_{B-I} \simlt 0.05 for V \simlt 22) for 11,489 stars in the Carina field, and 2) the detection of two, morphologically distinct, horizontal branches, which confirms that star formation in Carina occurred in two well-separated episodes. The old horizontal branch and RR Lyrae instability strip belong to a > 10 Gyr stellar population, while the populous red-clump horizontal branch belongs to an approximately 6 Gyr stellar population. We derive a distance modulus $(m-M)_0=20.09 \pm 0.06$ for Carina from the apparent magnitudes of the old horizontal branch and the tip of the red giant branch, and discuss modifications to the previously estimated distance, total magnitude, and stellar ages. Using the color of the red giant branch, we estimate the metallicities of the younger and older populations to be [Fe/H] = -2.0 and -2.2, respectively.Comment: 18 pages, 9 figures, uses AAS LaTex macros, PostScript figures available through anonymous ftp, accepted for publication in the Astronomical Journal, DAO-tsh94-

On the central helium-burning variable stars of the LeoI dwarf spheroidal galaxy

We present a study of short period, central helium-burning variable stars in the Local Group dwarf spheroidal galaxy LeoI, including 106 RR Lyrae stars and 51 Cepheids. So far, this is the largest sample of Cepheids and the largest Cepheids to RR Lyrae ratio found in such a kind of galaxy. The comparison with other Local Group dwarf spheroidals, Carina and Fornax, shows that the period distribution of RR Lyrae stars is quite similar, suggesting similar properties of the parent populations, whereas the Cepheid period distribution in LeoI peaks at longer periods (P \sim 1.26d instead of ~0.5d) and spans over a broader range, from 0.5 to 1.78d. Evolutionary and pulsation predictions indicate, assuming a mean metallicity peaked within -1.5<= [Fe/H]<=-1.3, that the current sample of LeoI Cepheids traces a unique mix of Anomalous Cepheids (blue extent of the red--clump, partially electron degenerate central helium-burning stars) and short-period classical Cepheids (blue-loop, quiescent central helium-burning stars). Current evolutionary prescriptions also indicate that the transition mass between the two different groups of stars is MHeF \sim 2.1 Mo, and it is constant for stars metal-poorer than [Fe/H]\sim-0.7. Finally, we briefly outline the different implications of the current findings on the star formation history of LeoI.Comment: 5 Pages, 4 Figures, ApJ letter, accepte