No abstract

We present a new calibration of the Stroemgren metallicity index m 1 using red giant (RG) stars in a sample of Galactic globular clusters (GGCs: M92, M13, NGC 1851, 47 Tuc) that cover a broad range in metallicity (−2.2 ≤ [Fe/H] ≤ −0.7), are marginally affected by reddening uncertainties (E(B-V ) ≤ 0.04) and for which accurate u, v, b, y Stroemgren photometry is available to well below 1 Based in part on observations collected with the 1.54m Danish telescope operated at ESO (La Silla) and with the Nordic Optical Telescope (NOT) operated at La Palma (Spain).-2the turnoff region. The main difference between the new empirical metallicityindex-color (MIC) relations and similar relations available in the literature is that we have adopted the u-y and v-y colors instead of the b-y color. These colors present a stronger sensitivity to effective temperature, and the MIC relations show a linear and well-defined slope. The net difference between photometric estimates and spectroscopic measurements, for RG stars in five GGCs: M71, NGC 288, NGC 362, NGC 6397, NGC 6752, is 0.04 ± 0.03 dex with σ = 0.11 dex. We also apply the new MIC relations to a sample of field stars for which spectroscopic metallicity (−2.4 ≤ [Fe/H] ≤ −0.5), accurate Strömgren photometry, and reddening estimates (Anthony-Twarog & Twarog 1994 are all available. We find that the difference between photometric estimates and spectroscopic measurements is on average −0.14 ± 0.01 dex, with σ = 0.17 dex. We also provide two independent sets of MIC relations based on evolutionary models that have been transformed into the observational plane by adopting either semi-empirical or theoretical color-temperature relations (CTRs). We apply the semi-empirical α−enhanced MIC relations to the nine GCs and find that the difference between photometric estimates and spectroscopic measurements is 0.04 ± 0.03 dex, with σ = 0.10 dex. A similar agreement is also found for the sample of field stars, and indeed the difference is −0.09 ± 0.03 dex, with σ = 0.19 dex. The difference between metallicity estimates based on theoretical scaled-solar and spectroscopic measurements −0.11±0.03 dex, with σ = 0.14 dex for the nine GGCs and −0.24 ± 0.03 dex, with σ = 0.15 dex for the field stars. On the whole, current findings support the evidence that new Strömgren MIC relations provide metallicity estimates with an intrinsic accuracy better than 0.2 dex.

We report new photometry for 10 globular clusters in M31, observed to a uniform depth of four orbits in F5555W(V) and F814W(I) using WFPC2 on board HST. Additionally we have reanalyzed HST archival data of comparable quality, for 2 more clusters. A special feature of our analysis is the extraordinary care taken to account for the effects of blended stellar images and required subtraction of contamination from the field stellar populations in M31 in which the clusters are embedded. We thus reach 1 mag fainter than the horizontal branch (HB) even in unfavorable cases. We also show that an apparent peculiar steep slope of the HB for those clusters with blue HB stars is actually due to blends between blue HB stars and red giants.We present the color-magnitude diagrams (CMDs) and discuss their main features also in comparison with the properties of the Galactic globular clusters. This analysis is augmented with CMDs previously obtained and discussed by Fusi Pecci et al. (1996) on 8 other M31 clusters. We report the following significant results:1. The locus of the red giant branches give reliable photometric metallicity determinations which compare generally very well with ground-based integrated spectroscopic and photometric measures, as well as giving good reddening estimates.2. The HB morphologies follow the same behavior with metallicity as the Galactic globular clusters, with indications that the 2nd-parameter effect can be present in some clusters of our sample. However, at [Fe/H]=∼ −1.7 we observe a number of clusters with red HB morphology such that the HB type versus [Fe/H] relationship is offset from the Milky Way and resembles that of the Fornax dwarf spheroidal galaxy. One explanation for the offset is that the most metal poor M31 globular clusters are younger than their Milky Way counterparts by 1-2 Gyr; further study is required.3. The M V (HB) versus [Fe/H] relationship has been re-determined and the slope (∼0.20) is very similar to the values derived from RR Lyrae stars in the MW and the LMC. The zero-point of this relation (M V = 0.51 at [Fe/H]=-1.5) is based on the assumed distance modulus (m-M) 0 (M31)=24.47±0.03, and is consistent with the distance scale that places the LMC at (m-M) 0 (LMC)=18.55.

We present a new (V, BÀV ) color-magnitude diagram of the Carina dwarf spheroidal galaxy (dSph) that extends from the tip of the red giant branch (RGB) down to V $ 25 mag. Data were collected with the Wide Field Imager available at the 2.2 m ESO/MPI telescope and cover an area of %0.3 deg 2 around the center of the galaxy. We confirm the occurrence of a substantial number of old stars with ages around 11 Gyr, together with an intermediate-age population around 5 Gyr. Moreover, we also detected a new, well-defined blue plume of young main-sequence stars with an age, at most, on the order of 1 Gyr. This finding is further supported by the detection of a sizable sample of anomalous Cepheids, whose occurrence can be understood in terms of stars with ages %0.6 Gyr. The evidence for such a young population appears at odds with current cosmological models, which predict that the most recent star formation episodes in dSph's should have taken place 2-3 Gyr ago. At odds with previous results available in the literature, we found that stars along the RGB of old and intermediate-age stellar populations indicate a mean metallicity roughly equal to Z ¼ 0:0004 (½Fe=H % À1:7) and a small dispersion around this value. This finding is further strengthened by the reduced spread in luminosity of RR Lyrae and horizontal-branch stars in the old stellar population and of the red clump in the intermediate-age group. We find evidence of a smooth spatial distribution of the intermediate-age stellar population (%5 Gyr), which appears more centrally concentrated than the oldest one (%11 Gyr). The radial distribution of the old population appears more clumpy, with a peak off-center by %2 0 when compared with the Carina center. Star counts show a well-defined '' shoulder '' in the northeast direction along both the minor and major axes. Current data do not allow us to assess whether this feature is the break in the slope of star-count profiles predicted by Johnston, Sigurdsson, & Hernquist.