A Spitzer/IRAC Census of the Asymptotic Giant Branch Populations in Local Group Dwarfs. I. WLM

摘要

We present Spitzer/IRAC observations at 3.6 and 4.5 microns along withoptical data from the Local Group Galaxies Survey to investigate the evolvedstellar population of the Local Group dwarf irregular galaxy WLM. Theseobservations provide a nearly complete census of the asymptotic giant branch(AGB) stars. We find 39% of the infrared-detected AGB stars are not detected inthe optical data, even though our 50% completeness limit is three magnitudesfainter than the red giant branch tip. An additional 4% of theinfrared-detected AGBs are misidentified in the optical, presumably due toreddening by circumstellar dust. We also compare our results with those of anarrow-band optical carbon star survey of WLM, and find the latter studysensitive to only 18% of the total AGB population. We detect objects withinfrared fluxes consistent with them being mass-losing AGB stars, and derive apresent day total mass-loss rate from the AGB stars of 0.7-2.4 x 10^(-3) solarmasses per year. The distribution of mass-loss rates and bolometricluminosities of AGBs and red supergiants are very similar to those in the LMCand SMC and the empirical maximum mass-loss rate observed in the LMC and SMC isin excellent agreement with our WLM data.