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The main aim of the present work is to derive an empirical mass-loss (ML) lawfor Population II stars in first and second ascent red giant branches. We usedthe Spitzer InfraRed Array Camera (IRAC) photometry obtained in the 3.6-8micron range of a carefully chosen sample of 15 Galactic globular clustersspanning the entire metallicity range and sampling the vast zoology ofhorizontal branch (HB) morphologies. We complemented the IRAC photometry withnear-infrared data to build suitable color-magnitude and color-color diagramsand identify mass-losing giant stars. We find that while the majority of starsshow colors typical of cool giants, some stars show an excess of mid-infraredlight that is larger than expected from their photospheric emission and that isplausibly due to dust formation in mass flowing from them. For these stars, weestimate dust and total (gas + dust) ML rates and timescales. We finallycalibrate an empirical ML law for Population II red and asymptotic giant branchstars with varying metallicity. We find that at a given red giant branchluminosity only a fraction of the stars are losing mass. From this, we concludethat ML is episodic and is active only a fraction of the time, which we defineas the duty cycle. The fraction of mass-losing stars increases by increasingthe stellar luminosity and metallicity. The ML rate, as estimated fromreasonable assumptions for the gas-to-dust ratio and expansion velocity,depends on metallicity and slowly increases with decreasing metallicity. Incontrast, the duty cycle increases with increasing metallicity, with the netresult that total ML increases moderately with increasing metallicity, about0.1 Msun every dex in [Fe/H]. For Population II asymptotic giant branch stars,we estimate a total ML of <0.1 Msun, nearly constant with varying metallicity.

An empirical mass-loss law for Population II giants from the Spitzer-IRAC survey of Galactic globular clusters