AGB Variables in the Fornax Dwarf Galaxy and the Large Magellanic Cloud

The Dwarf Spheroidal Galaxies, of which Fornax is one of the largest, are the least populous galaxies known and are generally thought to have extremely large mass-to-light ratios. Coleman & de Jong (2008) recently discussed the star formation history of Fornax showing that although many of the stars were formed more than 10 Gyr ago, there is a clear intermediate age population with a strong burst of formation around 3 to 4 Gyr ago. The dominant population has a mean metallicity of [Fe/H]~-1.4, while stars younger than 5 Gyr reach [Fe/H]~-0.5. The asymptotic giant branch (AGB) variables are the most luminous evolutionary phase that low and intermediate mass stars pass through. As such, AGB stars are of particular interest as they allow us to probe star formation to signifi cant distances. In particular we know, from the LMC, that the largest amplitude and most luminous of these variables, the Miras, obey a period luminosity relation. The work described here is the outcome of a programme monitoring Fornax over a period of several years using the 1.4-m InfraRed Survey Facility (IRSF) at Sutherland with the Sirius three-channel camera (Nagayama et al. 2003) to obtain near-infrared (JHK) photometry. We have discovered 18 red variables of which 5 have the large amplitudes (∆K>0.4 mag) that identify them as Miras. We assume these to be carbon-rich, although spectroscopic confi rmation is required. They have periods between 258 and 495 days indicating a range of initial masses, although obviously not as large as in the LMC. As the illustrated period-colour diagram shows some of these stars are much redder than Miras of comparable period in the Galaxy or the LMC. They appear to have high mass-loss rates and therefore high circumstellar extinction. A further two variables have erratically variable mass-loss rates, similar to several well monitored stars in the LMC. It is particularly interesting that this relatively low metallicity system shows AGB stars with thick dust shells and high mass-loss rates