AGB Stars as Tracers of Stellar Populations

Abstract

The AGB stars that we see now were formed some time ago, and their presence in a particular galaxy informs us about the history of that galaxy. The brightest AGB stars (30000 L⊙) are perhaps 100 Myr old, whereas 3000 L⊙ AGB stars are found in globular clusters with an age exceeding 10 Gyr. Other types of evolved stars, e.g., RR Lyrae variables, may also be used as tracers of galactic history, but AGB stars have two important advantages: (i) In any galaxy they are the most luminous red stars (except for the even rarer and much younger red supergiants), and (ii) they are at their brightest in the near-infrared, where the effect of interstellar extinction is often unimportant. Unfortunately, AGB stars are short-lived and therefore rare, which is clearly a disadvantage if their progenitor population is small, as it is in dwarf galaxies. In this chapter we will discuss only the most luminous AGB stars, a still rarer species: If, for example, we restrict ourselves to AGB stars more luminous than stars at the tip of the red giant branch (and that is close to what we do), only one luminous AGB star is predicted per 12 000 main-sequence stars of 1 M⊙ and 1 per 240 stars of 5 M⊙ [190]; the metallicity Z has a minor influence on these values.