E. Nasi

Scaled solar tracks and isochrones in a large region of the Z-Y plane I. From the ZAMS to the TP-AGB end for 0.15-2.5 M stars

Context. In many astrophysical contexts, the helium content of stars may differ significantly from those usually assumed in evolution- ary calculations. Aims. In order to improve upon this situation, we have computed tracks and isochrones in the range of initial masses 0.15−20 Mfor a grid of 39 chemical compositions with the metal content Z between 0.0001 and 0.070 and helium content Y between 0.23 and 0.46. Methods. The Padova stellar evolution code has been implemented with updated physics. New synthetic TP-AGB models allow the extension of stellar models and isochrones until the end of the thermal pulses along the AGB. Software tools for the bidimensional interpolation (in Y and Z) of the tracks have been tuned. Results. This first paper presents tracks for low mass stars (from 0.15 to 2.5 M� ) with scaled-solar abundances and the corresponding isochrones from very old ages down to about 1 Gyr. Conclusions. Tracks and isochrones are made available in tabular form for the adopted grid of chemical compositions in the plane Z-Y. As soon as possible an interactive web interface will allow users to obtain isochrones of any chemical composition and also simulated stellar populations with different Y(Z) helium-to-metal enrichment laws.

Testing Intermediate-Age Stellar Evolution Models with VLT Photometry of Large Magellanic Cloud Clusters. III. Padova Results*

The color-magnitude diagrams (CMDs) of three intermediate-age Large Megallanic Cloud clusters, NGC 2173, SL 556, and NGC 2155, are analyzed to determine their age and metallicity based on Padova stellar models. Synthetic CMDs are compared with cluster data. The best match is obtained using two fitting functions based on star counts in the different bins of the cluster CMD. Two different criteria are used. One of them takes into account the uncertainties in the color of the red clump stars. Given the uncertainties on the experimental values of the clusters metallicity, we provide a set of acceptable solutions. They define the corresponding values of metallicity, age, reddening and distance modulus (for the assumed initial mass function). The comparison with Padova models suggests for NGC 2173 a prolonged star formation (spanning a period of about 0.3 Gyr), beginning 1.7 Gyr and ending 1.4 Gyr ago. The metallicity Z is in the range 0.0016–0.003. For SL 556 an age of 2.0 Gyr is obtained. The metallicity value is in the range 0.002–0.004, depending on the adopted comparison criterion. The derived age for NGC 2155 is 2.8 Gyr, and its metallicity Z is in the range 0.002–0.003. The CMD features of this cluster suggest that a more efficient overshoot should be adopted in the evolutionary models. A period of extended star formation is not required to fit the SL 556 and NGC 2155 observations.

Testing Intermediate-Age Stellar Evolution Models with VLT Photometry of Large Magellanic Cloud Clusters. I. The Data*

This is the first of a series of three papers devoted to the calibration of a few parameters of crucial importance in the modeling of the evolution of intermediate-mass stars, with special attention to the amount of convective core overshoot. To this end we acquired deep V and R photometry for three globular clusters of the Large Magellanic Cloud (LMC), namely NGC 2173, SL 556 and NGC 2155, in the age interval 1-3 Gyr. In this first paper, we describe the aim of the project, the VLT observations and data reduction, and we make preliminary comparisons of the color-magnitude diagrams with both Padova and Yonsei-Yale isochrones. Two following papers in this series present the results of a detailed analysis of these data, independently carried out by members of the Yale and Padova stellar evolution groups. This allows us to compare both sets of models and discuss their main differences, as well as the systematic effects that they would have to the determination of the ages and metallicities of intermediate-age single stellar populations.