The First Empirical Mass-Loss Law for Population II Giants

Abstract

Using the Spitzer IRAC camera we have obtained mid-IR photometry ofthe red giant branch stars in the Galactic globular cluster 47 Tuc.About 100 stars show an excess of mid-IR light above that expectedfrom their photospheric emission. This is plausibly due to dust formationin mass flowing from these stars. This mass loss extends down tothe level of the horizontal branch and increases with luminosity.The mass loss is episodic, occurring in only a fraction of starsat a given luminosity. Using a simple model and our observationswe derive mass-loss rates for these stars. Finally, we obtain thefirst empirical mass-loss formula calibrated with observations ofPopulation II stars. The dependence on luminosity of our mass-lossrate is considerably shallower than the widely used Reimers law.The results presented here are the first from our Spitzer surveyof a carefully chosen sample of 17 Galactic globular clusters, spanningthe entire metallicity range from about one hundredth up to almostsolar. This work is based on observations made with the Spitzer SpaceTelescope, which is operated by the Jet Propulsion Laboratory, CaliforniaInstitute of Technology, under a contract with NASA. Support forthis work was provided by NASA through an award issued by JPL/Caltech.