In an attempt to constrain evolutionary models of the asymptotic giant branch ( AGB ) phase at the limit of low masses and low metallicities , we have examined the luminosity functions and number ratio between AGB and red giant branch ( RGB ) stars from a sample of resolved galaxies from the ACS Nearby Galaxy Survey Treasury ( ANGST ) .
This database provides HST optical photometry together with maps of completeness , photometric errors , and star formation histories for dozens of galaxies within 4Â Mpc .
We select 12 galaxies characterized by predominantly metal-poor populations as indicated by a very steep and blue RGB , and which do not present any indication of recent star formation in their color–magnitude diagrams .
Thousands of AGB stars brighter than the tip of the RGB ( TRGB ) are present in the sample ( between 60 and 400 per galaxy ) , hence the Poisson noise has little impact in our measurements of the AGB/RGB ratio .
We model the photometric data with a few sets of thermally pulsing AGB ( TP-AGB ) evolutionary models with different prescriptions for the mass loss .
This technique allows us to set stringent constraints to the TP-AGB models of low-mass metal-poor stars ( with M < 1.5 Â M _ { \odot } , \mbox { [ { Fe } / { H } ] } \lesssim - 1.0 ) .
Indeed , those which satisfactorily reproduce the observed AGB/RGB ratios have TP-AGB lifetimes between 1.2 and 1.8Â Myr , and finish their nuclear burning lives with masses between 0.51 and 0.55Â M _ { \odot } .
This is also in good agreement with recent observations of white dwarf masses in the M 4 old globular cluster .
These constraints can be added to those already derived from Magellanic Cloud star clusters as important mileposts in the arduous process of calibrating AGB evolutionary models .