Nuclear Reaction Rates and Carbon Star Formation

We have studied how the third dredge-up and the carbon star formation inlow-mass Asymptotic Giant Branch stars depends on certain key nuclear reactionrates. We find from a set of complete stellar evolution calculations of a 2Msunmodel with Z=0.01 including mass loss, that varying either the N14(p,g)O15 orthe 3-alpha reaction rate within their uncertainties as given in the NACREcompilation results in dredge-up and yields that differ by a factor of 2. Modeltracks with a higher rate for the 3-alpha rate and a lower rate for theN14(p,g)O15 reaction both show more efficient third dredge-up. New experimentalresults for the N14(p,g)O15 reaction rates are surveyed, yielding a rate whichis about 40% lower than the tabulated NACRE rate, and smaller than NACRE'slower limit. We discuss the possible implications of the revised nuclearreaction stellar evolution calculations that aim to reproduce the observedcarbon star formation at low mass, which requires efficient third dredge-up.Comment: accepted for publication in ApJ Letter