ASpitzerIRAC Census of the Asymptotic Giant Branch Populations in Local Group Dwarfs. I. WLM

Abstract

We present Spitzer/IRAC observations at 3.6 and 4.5 µm along with optical data from the Local Group Galaxies Survey to investigate the evolved stellar population of the Local Group dwarf irregular galaxy WLM. These observations provide a nearly complete census of the asymptotic giant branch (AGB) stars. We find 39% of the infrared-detected AGB stars are not detected in the optical data, even though our 50% completeness limit is three magnitudes fainter than the red giant branch tip. An additional 4% of the infrared-detected AGBs are misidentified in the optical, presumably due to reddening by circumstellar dust. We also compare our results with those of a narrow-band optical carbon star survey of WLM, and find the latter study sensitive to only 18% of the total AGB population. We detect objects with infrared fluxes consistent with them being mass-losing AGB stars, and derive a present day total mass-loss rate from the AGB stars of Ṁ = 0.7-2.4×10 −3 M ⊙ yr −1 . The distribution of mass-loss rates and bolometric luminosities of AGBs and red supergiants are very similar to those in the LMC and SMC and the empirical maximum mass-loss rate observed in the LMC and SMC is in excellent agreement with our WLM data.

DUSTiNGS. III. DISTRIBUTION OF INTERMEDIATE-AGE AND OLD STELLAR POPULATIONS IN DISKS AND OUTER EXTREMITIES OF DWARF GALAXIES

“…The difficulty is that the TRGB luminosity is not well-established in the Spitzer infrared filters. At 3.6 μm, the TRGB is estimated to be~-6 mag, but is seen to vary by a few tenths of a magnitude (Jackson et al 2007a(Jackson et al , 2007bBoyer et al 2009). Furthermore, the DUSTiNGS photometry is incomplete in the range of the expected TRGB, which results in false TRGB detections.…”

“…AGB stars exhibit variability, with periods and amplitudes generally increasing as the star evolves. TP-AGB stars are characterized by increasing mass loss that partially or totally obscures up to 30%-40% of the stars at optical wavelengths (Jackson et al 2007a(Jackson et al , 2007bBoyer et al 2009), making it difficult to catalog a complete census of them from optical imaging alone.…”

The AGB population in IC 1613 usingJHKphotometry

“…Furthermore, Jackson et al (2007b) estimated that as Albert et al (2000) applied a blue limit in (R − I) when selecting AGB candidates they actually used only 18% of the AGB population when calculating the global C/M ratio. The poor AGB detection rate reported for Albert et al (2000) was repeated when comparing the IRAC data to other optical data sets in IC 1613 and in other dwarf galaxies (Jackson et al 2007a;Boyer et al 2009). As a result of the difficult of detecting redder AGB stars at optical wavelengths recent studies (e.g Borissova et al 2000;Jung et al 2009) have made use of NIR photometry to detect AGB stars; as these stars emit most of their light at these wavelengths (Frogel et al 1990) they are among the brightest objects in the NIR.…”

The Acs Nearby Galaxy Survey Treasury. Ix. Constraining Asymptotic Giant Branch Evolution With Old Metal-Poor Galaxies

“…The reason for this is still not clear, but is possibly due to the underestimation of the mass-loss rates at low metallicities. As a concurring factor, the circumstellar extinction presently assumed in the models may be somewhat underestimated, as suggested by the Spitzer observations of nearby dwarf irregulars by Jackson et al (2007aJackson et al ( , 2007b and Boyer et al (2009a). Regardless, it is likely that the problem is limited to low metallicities, since the MG07 TP-AGB models have been calibrated in the intermediate-metallicity Magellanic Clouds.…”