Mass loss in RGB stars

People involved at OAB: Bragaglia, Cacciari, Carretta, Fusi Pecci, Origlia

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 modelling. 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 pilot survey of GCs performed with ISOCAM in the 10 $\mu$m spectral region (Origlia et al. 2002) has placed the whole problem into a new perspective that we can explore in deeper detail now, thanks to the powerful capabilities of the NASA Spitzer Space Telescope. Follow-up Spitzer observations have been obtained, aimed at studying mass loss along the entire RGB in 17 globular clusters with different metallicities. Data analysis has been completed, mass loss rates and duty cycles have been derived for most of the clusters. First results on 47 Tuc have been published in Origlia et al. (2007; 2010). The results for the other clusters are ready for publication. A relevant fraction of giant stars show an excess of mid-IR light above that expected from their photospheric emission. This is plausibly due to dust formation in mass flowing from these stars. This mass loss extends down to the level of the horizontal branch and increases with luminosity. The mass loss is episodic, occurring in only a fraction of stars at a given luminosity. Using the DUSTY code and our observations we derive mass-loss rates for these stars. Finally, we obtain the first empirical mass-loss formula calibrated with observations of Population II stars. The dependence on luminosity of our mass-loss rate is considerably shallower than the widely used Reimers law.

This work is in collaboration with F. Ferraro, S. Fabbri, E. Dalessandro (Univ. of Bologna), E. Valenti (ESO, Chile), R.T. Rood (Univ. of Virginia, USA), R.M. Rich (UCLA, USA).

Velocity fields in the atmosphere of red giant stars, that may lead to mass loss, can be studied by comparing the CaII K and H$\alpha$ lines from high resolution spectra with accurate chromospheric models. This type of analysis has been performed on several bright red giant stars in the stellar system Omega Cen, selected on the basis of their luminosity and metallicity. Mass loss rates have been estimated, in general agreement with the requirements of stellar evolution. A paper is in press.

This work is in collaboration with P. Mauas and M. Vieytes (Univ. of Buenos Aires, Argentina).