Results from simple stellar populations

1. NGC 1851
   1. This is one of the few examples of pre-wide-field era where a GGC has been totally covered with a mosaic. The photometric data set has allowed a vs. color-magnitude diagram for 20500 stars to be constructed.
   2. A detailed study of the peculiar HB morphology has been carried out (the cluster has a clearly bimodal horizontal branch). We have shown that standard evolutionary models are not able to reproduce the observed bimodal distribution of stars along the HB. Synthetic HR diagrams demonstrate that the problem could be solved by assuming that the efficiency of the RGB mass loss actually encompasses values going from 0.25 to 0.48. We have found evidence that the radial distribution of the blue HB stars is different from that of red HB and SGB stars. The BHB stars are significantly more concentrated than the SGB stars for arcsec. Though this distribution cannot be easily interpreted in terms of dynamical evolution, it might be related to the anomalous distribution of the BSs (see below).
   3. With an accurate inspection of the cluster bright-blue objects, and a comparison with the numbers predicted from the background field and the Galactic count models, the presence of seven ``supra-HB'' stars has been confirmed. We have shown that six of the ``supra-HB'' stars could be evolved descendants from HB progenitors (post-HB or planetary nebulae).
   4. All the 27 known variable stars have been identified, and 26 have been measured in both colors (the remaining one being saturated). Twenty-two of them are RR Lyr variables. The positions and photometry for seven new RR Lyr candidates have been given. With these additional variables the ratio of the two types is now , which reduces the current estimate (=0.47; Wehlau et al. 1982).
   5. From a sample of 25 globular clusters a new calibration for as a function of cluster metallicity has been derived. It has been found that NGC 1851 follows this general trend fairly well. From a comparison with the theoretical models, some evidence has also been found of a younger age for metal rich clusters.
   6. 13 blue straggler stars have been identified. These objects do not show any sign of variability. It has been shown that the blue stragglers are less concentrated than the subgiant branch stars with similar magnitudes for .
   7. We have been able to derive a complete LF down to  mag for stars in the region , and down to  mag in the region . The external LF is steeper than the internal one, and we have interpreted this result as a sign of mass segregation. By using the most updated mass-luminosity relations we have obtained MFs which can be well fitted by power laws with distinct exponents . The observed value for the external MF is , which is steeper than the value found for the internal one.
   8. The global MF has been determined correcting the two observed mass functions for the effects of mass segregation, as predicted by the multi-mass King-Michie model which best fits the observed light profile of NGC 1851. The two values for the slope of the MF are compatible with the model if a global MF exponent is adopted. This value for the global MF slope is marginally smaller (MF flatter) than what would be expected from the relation between the slope of the MFs and the position in the Galaxy and the metallicity of the cluster proposed by Djorgovski et al. (1993). This might indicate that NGC 1851 has had a stronger gravitational interaction with the Galactic disc than the average of the Galactic GCs with similar position and metallicity.
   9. In conclusion, the above results indicate that NGC 1851 is a cluster where the dynamical evolution has affected both its evolved and unevolved stellar content. While the single findings are not of high statistical significance (mostly due to the small size of the stellar samples), taken together they give a coherent picture. Stellar encounters have led to mass segregation, as shown by the MF, which is steeper and steeper going from external to internal regions. They have probably contributed to the creation of the observed group of blue straggler stars, and possibly have triggered the formation of a blue tail in the HB. The internal dynamics of NGC 1851 has therefore influenced the evolution of its stars, introducing effects not reproducible by standard models. In turn, the dynamical evolution induced by the external gravitational field of the Galaxy has also very probably contributed to the modification of the present-day stellar population of NGC 1851, as strongly suggested by the anomalously flat global mass function.
2. The red giant branch of Galactic globular clusters
   1. Using the photometric database for Galactic globular clusters, that was assembled for the relative age investigation (Rosenberg et al. 1999, AJ, November issue) a complete set of metallicity indices has been measured, based on the morphology and position of the red-giant branch.
   2. In particular, the first calibration of the , and indices in the plane has been obtained.
   3. The indices have been calibrated in terms of metallicity, both on the Zinn & West (1984) and the Carretta & Gratton (1997) scales. Our new calibrations of the , , and indices are consistent with existing relations.
   4. Using a grid of selected RGB fiducial points, it has been possible to define a function in the space which is able to reproduce the whole set of GGC giant branches in terms of a single parameter (the metallicity).
   5. It has been shown that the function is able to predict the correct trend of the observed indices with metallicity.
   6. The usage of this function will improve the current determinations of metallicity and distances within the Local Group, since it allows to easily map coordinates into ones. To this aim the ``synthetic'' RGB distribution has been generated both for the currently used Lee, Demarque and Zinn (1990) Local Group distance scale, and for the most recent results on the RR Lyr distance scale.