A spitzer survey of asymptotic giant branch stars: dust production and mass loss at low metallicity

Abstract

We conducted infrared (IR) surveys of ten Galactic globular clusters (GCs) and eight Local Group dwarf irregular galaxies using the Spitzer Space Telescope. The main objective of these surveys is to further the understanding of dust production in low metallicity environments akin to the early Universe. In GCs, we investigate the stars with IR excesses, attributed to dust, and the intracluster medium (ICM). The GC M15 is the most metal-poor Galactic GC, and is ideal for studying dust production at metallicity less than 1% solar. The most massive Galactic GC, ω Centauri, harbors three distinct populations of differing metallicities, providing the opportunity to study dust production at three metallicities within the same environment. The large population of dusty Asymptotic Giant Branch (AGB) stars present in the eight observed Local Group dwarf galaxies allows a statistically significant study of dusty stellar mass loss at a broad range of metallicities (2% -- 19% solar). In all observed systems, we find large populations of dust enshrouded stars and, in some cases, dusty interstellar medium. The surplus of both interstellar dust and the dust producing stars in M15 is surprising, given its extremely low metal-content. No significant amount of ICM dust is detected in any other GC observed, suggesting that ICM dust does not survive long compared to its production rate and is thus a part of a stochastic process. In ω Cen, we see no difference in dust production between the three populations, and overall, we see that dust is not formed in larger quantities than seen in M15. In dwarf galaxies, we see that circumstellar dust is prolific enough to create at least a small population of completely optically obscured AGB stars in each galaxy, regardless of the galaxy's metallicity, but higher metallicity galaxies tend to harbor more stars with slight IR excesses. These results suggest that dust production is not prohibited at very low metallicity, although it may be produced in smaller quantities.