The stars in the Magellanic Clouds with the largest degree of obscuration are used to probe the highly uncertain physics of stars in the asymptotic giant branch ( AGB ) phase of evolution .
Carbon stars in particular , provide key information on the amount of third dredge-up ( TDU ) and mass loss .
We use two independent stellar evolution codes to test how a different treatment of the physics affects the evolution on the AGB .
The output from the two codes are used to determine the rates of dust formation in the circumstellar envelope , where the method used to determine the dust is the same for each case .
The stars with the largest degree of obscuration in the LMC and SMC are identified as the progeny of objects of initial mass 2.5 - 3 ~ { } M _ { \odot } and \sim 1.5 ~ { } M _ { \odot } , respectively .
This difference in mass is motivated by the difference in the star formation histories of the two galaxies , and offers a simple explanation of the redder infrared colours of C-stars in the LMC compared to their counterparts in the SMC .
The comparison with the Spitzer colours of C-rich AGB stars in the SMC shows that a minimum surface carbon mass fraction X ( C ) \sim 5 \times 10 ^ { -3 } must have been reached by stars of initial mass around 1.5 ~ { } M _ { \odot } .
Our results confirm the necessity of adopting low-temperature opacities in stellar evolutionary models of AGB stars .
These opacities allow the stars to obtain mass-loss rates high enough ( \gtrsim 10 ^ { -4 } M _ { \odot } / yr ) to produce the amount of dust needed to reproduce the Spitzer colours .