Modelling TP-AGB Stars in M31 with PHAT Data

The thermally pulsing asymptotic giant branch (TP-AGB) phase is still the most uncertain phase among the main evolutionary phases of single stars, due to complicated processes involved, such as third dredge-up (3DU) and mass loss. These processes significantly affect the lifetime, luminosity, colors and chemical yields of TPAGB stars. With HST high resolution imaging data from the Panchromatic Hubble Andromeda Treasury (PHAT) survey, TP-AGB stars are resolved in M31. This enables us to study the color-magnitude diagram and luminosity functions of TP-AGBs of different metallicity and mass within different environments (from the bulge to the disk) of M31. Therefore, PHAT data provides strong constraints for TP-AGB models. In this preliminary work, we calibrate TP-AGB evolutionary tracks with PHAT data using stellar population synthesis approach. By adjusting the parameters regulating the third dredge-up occurrence and efficiency as well as the prescription of mass-loss in the model, we are able to satisfactorily reproduce the observed luminosity function (F160W) and color-magnitude diagram (F110W-F160W vs. Fl 60W).