The Large Magellanic Cloud as a laboratory for hot bottom burning in massive asymptotic giant branch stars

We use Spitzer observations of the rich population of asymptotic giant branch (AGB) stars in the Large Magellanic Cloud (LMC) to test models describing the internal structure and nucleosynthesis of the most massive of these stars, i.e. those with initial mass above ~4M⊙. To this aim, we compare Spitzer observations of LMC stars with the theoretical tracks of AGB models, calculated with two of the most popular evolution codes, that are known to differ in particular for the treatment of convection. Although the physical evolution of the two models are significantly different, the properties of dust formed in their winds are surprisingly similar, as is their position in the colour-colour and colour-magnitude diagrams obtained with the Spitzer bands. This model-independent result allows us to select a well-defined region in the ([3.6]-[4.5], [5.8]-[8.0]) plane, populated by AGB stars experiencing hot bottom burning, the progeny of stars with mass M ~ 5.5M⊙. This result opens up an important test of the strength hot bottom burning using detailed near-IR (H and K bands) spectroscopic analysis of the oxygen-rich, high-luminosity candidates found in the well-defined region of the colour- colour plane. This test is possible because the two stellar evolution codes we use predict very different results for the surface chemistry, and the C/O ratio in particular, owing to their treatment of convection in the envelope and of convective boundaries during third dredge-up. The differences in surface chemistry are most apparent when the model stars reach the phase with the largest infrared emission.