Variable stars in M31 - clues on the AGB evolution

Goal of the project is the characterisation of the population of Asymptotic Giant Branch (AGB) stars by their pulsating members in the central part of M31. For this purpose the pulsation and related properties of AGB stars showing radial pulsations, so-called Long Period Variables (LPVs) will be studied. Numerous LPVs were identified by the Wendelstein Calar Alto Pixellensing Project (WeCAPP), a microlensing experiment to search for compact halo dark matter in M31. Its extensive database, which will be open to Vienna scientists, comprises the published catalogue of more than 20 000 variable stars, the difference imaging data, and the light curves covering a time span of up to 9 years. This unique dataset will be combined with public domain Hubble Space Telescope (HST) data to obtain information on the excited periods, the pulsation modes, the pulsation masses, and the amount of lost mass in the late stages of stellar evolution. For this purpose we will derive, for the first time, the various LPV period-luminosity (PL) relations in another galaxy than the Milky Way or the Magellanic Clouds. This will allow to investigate the presence of population effects like possible age and metalicity dependences of the relations. The comparison of the empirical data with model predictions will constrain the amount of lost mass along the Red Giant Branch (RGB) and the AGB. The study will be complemented using public domain Spitzer Space Telescope data obtained in near- and mid-infrared wavebands yielding insight into the mass loss at higher intensity levels, and, together with the HST data, into the presence of an intermediate age population in the M31 bulge field. Furthermore, the investigation of possible systematic changes of the LPV light curve shape will offer an improved understanding of the pulsation mechanisms acting in these stars. Although the focus of the proposed work at the Vienna institute lies on the red variables being located on the upper RGB and the AGB, also the WeCAPP variables which occupy the classical instability strip (especially Cepheids II and RV Tauri stars) will be investigated, both classes being closely connected to AGB and post-AGB stellar evolution. Their PL relations, for the first time, will allow to investigate the influence of possible population effects on the pulsation properties of these variables.