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

TL;DR

This study models AGB stars in the SMC, comparing theoretical predictions with Spitzer observations to understand dust production, stellar evolution, and population characteristics, revealing distinct properties of oxygen-rich and carbon-rich AGB stars.

Contribution

It provides a comprehensive evolutionary and dust production model for SMC AGB stars, linking infrared colours with stellar properties and dust content, and compares these with observations.

Findings

75% of AGB stars are scarcely obscured, mainly low-mass, oxygen-rich stars formed 700 Myr to 5 Gyr ago.

25% of AGB stars are heavily obscured, mostly carbon stars with significant dust emission.

Differences between SMC and LMC AGB populations are discussed.

Abstract

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 1Msun < M < 8Msun, 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% of the AGB population of the SMC is composed by scarcely obscured objects, mainly stars of mass M < 2.5Msun at various metallicity, formed between 700 Myr and 5 Gyr ago; ~ 70% of these sources are oxygen--rich stars, while ~ 30% are C-stars. The sample of the most obscured AGB stars, accounting for ~ 25% 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 1.5-2Msun stars of metallicity Z=0.004, formed between 700 Myr and 2 Gyr ago, and from lower metallicity objects, of mass below 1.5Msun, 2-5 Gyr old. We also identify obscured oxygen-rich stars (M ~ 4-6Msun) experiencing hot bottom burning. The differences between the AGB populations of the SMC and LMC are also commented.