Evolution of Thermally Pulsing Asymptotic Giant Branch Stars IV.
  Constraining Mass-Loss & Lifetimes of Low Mass, Low Metallicity AGB Stars
  The evolution and lifetimes of thermally pulsating asymptotic giant branch
(TP-AGB) stars suffer from significant uncertainties. In this work, we analyze
the numbers and luminosity functions of TP-AGB stars in six quiescent, low
metallicity ([Fe/H] $\lesssim -0.86$) galaxies taken from the ANGST sample,
using HST photometry in both optical and near-infrared filters. The galaxies
contain over 1000 TP-AGB stars (at least 60 per field). We compare the observed
TP-AGB luminosity functions and relative numbers of TP-AGB and RGB stars, to
models generated from different suites of TP-AGB evolutionary tracks after
adopting star formation histories (SFH) derived from the HST deep optical
observations. We test various mass-loss prescriptions that differ in their
treatments of mass-loss before the onset of dust-driven winds (pre-dust). These
comparisons confirm that pre-dust mass-loss is important, since models that
neglect pre-dust mass-loss fail to explain the observed TP-AGB/RGB ratio or the
luminosity functions. In contrast, models with more efficient pre-dust
mass-loss produce results consistent with observations. We find that for
[Fe/H]$\lesssim-0.86$, lower mass TP-AGB stars ($M\lesssim1M_\odot$) must have
lifetimes of $\sim0.5$ Myr and higher masses ($M\lesssim 3M_\odot$) must have
lifetimes $\lesssim 1.2$ Myr. In addition, assuming our best-fitting mass-loss
prescription, we show that the third dredge up has no significant effect on
TP-AGB lifetimes in this mass and metallicity range.
