We use Spitzer observations of the rich population of Asymptotic Giant Branch 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 \sim 4 M _ { \odot } .
To this aim , we compare Spitzer observations of LMC stars with the theoretical tracks of Asymptotic Giant Branch 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 ( CCD ) and colour–magnitude ( CMD ) 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 \sim 5.5 M _ { \odot } .
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 emmission .