AGB stars in the SMC: evolution and dust properties based on Spitzer observations

We study the population of asymptotic giant branch (AGB) stars in the Small Magellanic Cloud (SMC) by means of full evolutionary models of stars of mass 1 M⊙ ≤ M ≤ 8 M⊙, evolved through the thermally pulsing phase. The models also account for dust production in the circumstellar envelope. We compare Spitzer infrared colours with results from theoretical modelling. We show that ∼75 per cent of the AGB population of the SMC is composed by scarcely obscured objects, mainly stars of mass M ≤ 2 M⊙ at various metallicity, formed between 700 Myr and 5 Gyr ago; ∼70 per cent of these sources are oxygen-rich stars, while ∼30 per cent are C-stars. The sample of the most obscured AGB stars, accounting for ∼25 per cent of the total sample, is composed almost entirely by carbon stars. The distribution in the colour–colour ([3.6] − [4.5], [5.8] − [8.0]) and colour–magnitude ([3.6] − [8.0], [8.0]) diagrams of these C-rich objects, with a large infrared emission, traces an obscuration sequence, according to the amount of carbonaceous dust in their surroundings. The overall population of C-rich AGB stars descends from 1.5–2 M⊙ stars of metallicity Z = 4 × 10−3, formed between 700 Myr and 2 Gyr ago, and from lower metallicity objects, of mass below 1.5 M⊙, 2–5 Gyr old. We also identify obscured oxygen-rich stars (M ∼ 4–6 M⊙) experiencing hot bottom burning. The differences between the AGB populations of the SMC and LMC are also commented.