2015MNRAS.454.4235D

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 1M_☉ ≤ M ≤ 8M_☉, 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 percent of the AGB population of the SMC is composed by scarcely obscured objects, mainly stars of mass M ≤ 2M_☉ at various metallicity, formed between 700 Myr and 5 Gyr ago; ∼ 70 percent of these sources are oxygen-rich stars, while ∼ 30 percent are C-stars. The sample of the most obscured AGB stars, accounting for ∼ 25 percent 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 1.5-2M_☉ stars of metallicity Z = 4x10^–3, formed between 700 Myr and 2 Gyr ago, and from lower metallicity objects, of mass below 1.5M_☉, 2-5 Gyr old. We also identify obscured oxygen-rich stars (M ∼ 4-6M_☉) experiencing hot bottom burning. The differences between the AGB populations of the SMC and LMC are also commented.