M. Zoccali

The Initial Mass Function of the Galactic Bulge down to ~0.15 M☉*

We present a luminosity function (LF) for lower main sequence stars in the Galactic bulge near (1, b) _ (0°, —6°) to M,, — 9.3. This LF is derived from HST+NICMOS observations of a region of 221.’5 x 221’5, with the F11OW and F160W filters. Our derived mass function extends to 0.15M ⊙with a power law slope of a = —1.33 ± 0.07. Although shallower than the Salpeter one, this IMF is steeper than that recently found for the Galactic disk (a = —0.8 and a = —0.54 from the data of Reid & Gizis,1997,and Gould et al. 1997, respectively, in the same mass interval), but is virtually identical to the disk IMF derived by Kroupa et al. (1993). The bulge IMF is also quite similar to the mass functions derived for those globular clusters which are believed to have experienced little or no dynamical evolution

New Evidence for the Complex Structure of the Red Giant Branch in ω Centauri

We report on the complex structure of the red giant branch (RGB) of ω Centauri, based on a new wide-field and wide-color baseline B and I photometry. Our color-magnitude diagram (CMD) shows the presence of multiple populations along this branch; in particular, we discovered an anomalous branch (RGB-a), which appears to be well separated from the bulk of the RGB stars. On the basis of our CMD and from the previous literature, we conclude that (1) these stars, clearly identified as a separate population in our CMD, represent the extreme metal-rich extension ([Ca/H] > -0.3) of the stellar content of ω Cen and show anomalous abundances of s-process elements (as Ba and Zr) as well; (2) they are physical members of the ω Cen system; (3) they comprise ~5% of the stars of the whole system; and (4) this component and the metal intermediate one (-0.4 > [Ca/H] > -1) have been found to share the same spatial distribution, both of them differing significantly from the most metal-poor one ([Ca/H] < -1). This last piece evidence supports the hypothesis that metal-rich components could belong to an independent (proto?) stellar system captured in the past by ω Cen.

A New Spin on the Problem of Horizontal-Branch Gaps: Stellar Rotation along the Blue Horizontal Branch of Globular Cluster M13* **

We have determined the projected rotational velocities of 13 blue horizontal-branch (BHB) stars in the globular cluster M13 via rotational broadening of metal absorption lines. Our sample spans the photometric gap observed in the horizontal-branch distribution at Teff 11,000 K and reveals a pronounced difference in stellar rotation on either side of this feature—blueward of the gap, all the stars show modest rotations, v sin i < 10 km s-1, while to the red side of the gap, we confirm the more rapidly rotating population (v sin i 40 km s-1) previously observed by R. C. Peterson and coworkers. Taken together with these prior results, our measurements indicate that a stars rotation is indeed related to its location along the HB, although the mechanism behind this correlation remains unknown. We explore possible connections between stellar rotation and mass-loss mechanisms which influence the photometric morphology of globular cluster HBs.

On the Helium Content of Galactic Globular Clusters via the R-Parameter*

We estimate the empirical R-parameter in 26 Galactic globular clusters covering a wide metallicity range, imaged by Wide Field Planetary Camera 2 on board the Hubble Space Telescope. The improved spatial resolution permits a large fraction of evolved stars to be measured and permits accurate assessment of radial population gradients and completeness corrections. In order to evaluate both the He abundance and the He-to-metal enrichment ratio, we construct a large set of evolutionary models by adopting similar metallicities and different He contents. We find an absolute He abundance that is lower than that estimated from spectroscopic measurements in H II regions and from primordial nucleosynthesis models. This discrepancy could be removed by adopting a 12C(α,γ)16O nuclear cross section about a factor of 2 smaller than the canonical value, although different assumptions for mixing processes also can introduce systematic effects. The trend in the R-parameter toward solar metallicity is consistent with an upper limit to the He-to-metal enrichment ratio of the order of 2.5. Detailed calculations of central He burning times as a function of the horizontal-branch (HB) morphology suggest that He lifetimes for hot HB stars are on average ≈20% longer than for RR Lyrae and red HB stars. Therefore, the increase in the empirical R-values of metal-poor clusters characterized by blue HB morphologies is due to an increase in the HB lifetime and not due to an increase in the He abundance.

Comparison between Predicted and Empirical ΔVHBbump in Galactic Globular Clusters

We present observational estimates of ΔV in a sample of 28 galactic globular clusters (GGCs) observed by the Hubble Space Telescope. The photometric accuracy and the sizable number of stars measured in each cluster allowed us to single out the red giant branch bump both in metal-poor and in metal-rich GGCs. Empirical values are compared with homogeneous theoretical predictions that account for both H- and He-burning phases over a wide range of metal abundances (0.0001<Z<0.02). We found that, within current observational uncertainties on both iron and α-element abundances, theory and observations are in very good agreement, provided that the metallicity scale by Carretta & Gratton as extended by Cohen et al. is adopted. Interestingly enough, we also found that both theoretical and observed values show a change in the slope of the ΔV-[M/H] relation toward higher metal contents.

Star Counts across the Red Giant Branch Bump and Below

We present a new observable?Rbump?which is the ratio between the star counts across the red giant branch (RGB) bump and fainter RGB stars to investigate the occurrence of a deep-mixing phenomenon during these evolutionary phases. The comparison between predicted and empirical Rbump-values, based on a large and homogeneous set of Hubble Space Telescope data, brings out that evolutionary lifetimes predicted by canonical RGB models do account for the bulk of Galactic globular clusters included in our sample (29). This evidence suggests that bump and fainter RGB stars do not show the occurrence of deep mixing, which significantly changes their chemical stratification. A few possible exceptions to this general rule are briefly discussed.

Galactic Globular Cluster Relative Ages: Clues on the Milky Way Early Evolution

Based on two new large, homogeneous photometric databases of 35 (from V and I groundbased data) and 15 (fromBand V HST data) Galactic globular clusters, a set of distance and reddening independent relative age indicators has been measured. The observed(V —i)02 5,8(B —V)@2.5, and AVM vs. metallicity relations have been compared to the relations predicted by two recent updated libraries of isochrones. Using these models and two independent methods, we have found that self-consistent relative ages can be estimated for our GGC samples. The main results are: (a) there is no age-metallicity trend; (b) there is no evidence of an age spread for clusters with [Fe/H]< —1.2, all the clusters of our sample in this range being old and coeval; (c) for the more metal rich clusters there are indications of a larger age dispersion, of the order of 10-15%; (d) there is no trend of the age with the distance from the Galactic center out to 25 Kpc.