A study of Andromeda to improve our knowledge of the evolution and dust production by AGB stars

Aims. We study the asymptotic giant branch (AGB) population of the galaxy M31, based on available HST and Spitzer data, to characterize the individual sources in terms of mass, metallicity and formation epoch of the progenitors. We dedicated particular attention to the derivation of the dust production rate of the stars, in an attempt to determine the global current dust production rate of the galaxy, divided between the silicate- and the carbonaceous-dust contributions. Methods. The study of the galaxy was addressed by a population synthesis approach, which used results from stellar evolution modelling complemented by the description of the dust formation process in the wind. This step required the knowledge of the star formation history and of the age-metallicity relationship of M31, obtained in previous investigations. We compared the results from synthetic modelling and the data available to characterize AGB stars in M31. Results. We find that the majority of the AGB population of M31 is composed of low-mass stars of different metallicities formed between 6 Gyr and 14 Gyr ago, with an additional, significant contribution from the progeny of 1.7-2.5 M⊙ stars formed during the secondary peak in the star formation history, which occurred between 1 and 2 Gyr ago. The dust production rate of the galaxy is mostly provided by carbon stars, whose contribution is of the order of 4x 10-4 M⊙/yr; the rest of the dust production rate is accounted for by silicate production from massive AGB stars, which occurs at a rate of ∼ 6x 10-5 M⊙/yr. We also comment on the implications of the present results for the reliability of AGB modelling.