A Spitzer Study of Asymptotic Giant Branch Stars. III. Dust Production
  and Gas Return in Local Group Dwarf Irregular Galaxies
  We present the third and final part of a census of Asymptotic Giant Branch
(AGB) stars in Local Group dwarf irregular galaxies. Papers I and II presented
the results for WLM and IC 1613. Included here are Phoenix, LGS 3, DDO 210, Leo
A, Pegasus dIrr, and Sextans A. Spitzer photometry at 3.6, 4.5, 5.8, and 8 are
presented, along with a more thorough treatment of background galaxy
contamination than was presented in papers I and II. We find that at least a
small population of completely optically obscured AGB stars exists in each
galaxy, regardless of the galaxy's metallicity, but that higher-metallicity
galaxies tend to harbor more stars with slight IR excesses. The optical
incompleteness increases for the redder AGB stars, in line with the expectation
that some AGB stars are not detected in the optical due to large amounts of
extinction associated with in situ dust production. Overall, there is an
underrepresentation of 30% - 40% in the optical AGB within the 1 sigma errors
for all of the galaxies in our sample. This undetected population is large
enough to affect star formation histories derived from optical color-magnitude
diagrams. As measured from the [3.6] - [4.5] color excesses, we find average
stellar mass-loss rates ranging from 3.1E-7 - 6.6E-6 solar masses per year, and
integrated galaxy mass-loss rates ranging from 4.4E-5 - 1.4E-3 solar masses per
year. The integrated mass-loss rate is sufficient to sustain the current star
formation rate in only LGS 3 and DDO 210, requiring either significant
non-dusty mass loss or gas accretion in Phoenix, Leo A, Pegasus dIrr, Sextans
A, WLM, and IC 1613 if they are to maintain their status as gas-rich galaxies.
