Observational tests of theoretical stellar models

People involved at OAB: Bellazzini, Fusi Pecci, Origlia, Pancino, Sollima, Valenti

Stellar evolutionary models are often used to derive relevant properties of globular star clusters and galaxies, such as their age and metal content. The Luminosity Function of the stellar sequences in the CMDs, from the MS Turn Off (TO) up to the termination of the Asymptotic Giant Branch (AGB), has been recognized to be the most powerful tool for testing stellar evolutionary models (in particular the accuracy of the input physics, the reliability of canonical assumptions, etc.).

A fully fruitful test requires that the observations be a) complete, b) statistically significant, and c) accurate and adequate for each specific evolutionary sequence. Point (a) means that virtually all the stars in a given area of the cluster are measured down to a given magnitude level, and that reliable corrections for incompleteness can be applied below that level. Point (b) means that observations should cover most of the cluster extension. Point (c) requires infrared observations to measure the cool Red Giant Branch (RGB) stars and UV observations to properly study the blue sequences as the Horizontal Branch and the Blue Stragglers.

This year a major work on the calibration of the RGB features by using the IR photometric database of Galactic globular clusters (GGCs) collected by our group over the last 5 years has been performed, in collaboration with F. Ferraro (University of Bologna), O. Straniero (INAF-OA Teramo), R.T. Rood (University of Virginia, USA).

By analyzing the color-magnitude diagrams of 24 bulge and halo GGCs a set of photometric indices describing the morphology (i.e. slope) and the location in color and in magnitude of the RGB has been measured and calibrated as a function of the cluster metallicity in all the available IR bands (Valenti et al. 2004). The major RGB evolutionary features, namely the RGB bump and tip, have been also measured and calibrated with varying cluster metallicity (Valenti et al. 2004; Bellazzini et al. 2004). An overall good agreement with the predictions of the most recent models of stellar evolution has been found. The RGB ``bump'' and tip represent powerful tools to obtain independent estimates of the metalicity and distance, respectively, in old stellar systems within the Local Group. The new IR adaptive optics facilities available at ground-based 8m-class telescopes as well as the future imaging capabilities of the James Webb Space Telescope will allow to use the RGB tip as a distance indicator in galaxies well beyond the Local Group, up to several Mpc away. An empirical calibration of the mixing-length theory and a new method to derive reddening and metallicity estimates, based on this database, are still in progress.

The IR spectral range is also particularly suitable to study the mass loss process in giant stars. Mass loss is a crucial parameter in any stellar evolution modeling. The late evolutionary stages of low- and intermediate-mass giant stars are strongly influenced by mass loss processes. Yet, our lack of empirical estimates on mass loss in low-mass RGB and AGB stars remains one of the most serious stumbling blocks for a comprehensive understanding of stellar evolution. A deep survey of the very central regions of six massive globular clusters has been performed using ISOCAM in the 10 $\mu$m spectral region (Origlia et al. 2002). From a combined physical and statistical analysis we derived mass loss rates and frequency. We find that i) significant mass loss (at rates in the range $10^{-7} < dm/dt < 10^{-6} m_{\odot}\,{\rm yr^{-1}}$) occurs only at the very end of the RGB evolutionary stage and is episodic, ii)  the modulation timescales should be shorter than 1 million years, and iii) mass loss occurrence does not show a crucial dependence on the cluster metallicity. A follow-up program using the NASA Spitzer Space Telescope has been proposed for Cycle 1 observations.