| Summary: | With the goal of studying ML along the RGB, mid-IR observations
of a carefully selected sample of 17 Galactic globular
clusters (GGCs) with different metallicity and horizontal branch
(HB) morphology have been secured with IRAC on board Spitzer:
a global sample counting about 8000 giant has been obtained.
Suitable complementary photometry in the optical and near-IR
has been also secured in order to properly characterize
the stellar counterparts to the Spitzer sources and their photospheric parameters.
Stars with color (i.e. dust) excess have been identified, their likely
circumstellar emission quantified and modelled, and empirical estimates of mass
loss rates and timescales obtained.
We find that mass loss rates increases with increasing
stellar luminosity and decreasing metallicity.
For a given luminosity, we find that ML rates are
systematically higher than the prediction
by extrapolating the Reimers law.
CMDs constructed from ground based near-IR and IRAC bands
show that at a given luminosity some stars have dusty envelopes
and others do not. From this, we deduce that the mass loss
is episodic and is ``on'' for some fraction of the time.
The total mass lost on the RGB can be easily computed by multiplying ML rates
by the ML timescales and integrating over the evolutionary timescale.
The average total mass lost moderately increases with increasing metallicity,
and for a given metallicity is systematically higher in clusters with extended blue HB.
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