Early in their lives, stars with masses a few times that of the sun burn the hydrogen in their cores to make helium, become red giant stars, and then burn that helium to make carbon and oxygen. What happens subsequently, in the so-called Asymptotic Giant Branch (AGB) phase, is much less certain. We know that AGB stars continue to burn hydrogen and helium, now in shells that surround the carbon-oxygen core. And we know that flashes occur in the helium shell and that the carbon they produce is eventually dredged up into the envelop of the star and then to the stellar surface. These flashes also lead eventually to the ejection of much of the star’s envelope to form a planetary nebula, leaving behind a white dwarf.
We also know that this picture cannot be entirely correct. The standard model of the process leads to stars with a dominantly oxygen envelope. Yet light AGB stars observed in nature appear to have carbon rich envelopes. The standard models involve certain approximations whose detailed effect is not certain and which might explain the discrepancy. There have, however, been no evaluations of the sensitivity of the AGB process to uncertainties in the important nuclear reaction rates that govern the process. We have undertaken such sensitivity studies using detailed calculations based first on the standard nuclear reaction rates and then on rates changed within their uncertainties.
We find that production of carbon is insensitive to the Carbon-12 + α → Oxygen-16 + γ reaction, but that changes in the Nitrogen-14 + p → Oxygen-15 + γ or the 3α → Carbon-12 (triple α) reaction can increase the carbon abundance by a factor of two, leading to a carbon-rich surface.
This does not imply that changing the reaction rates is the solution to the problem. But it may be, and it tells us that better measurements of the reaction rates are imperative. New measurements of these reactions are in various stages and will in the next few years tell us whether we have truly found the solution to the problem.
F. Herwig and Sam M. Austin, Astrophysical Journal Letters 613, L73 (2004)
F. Herwig, Sam M. Austin, and John C. Lattanzio, astro-ph/0511386
S. M. Austin
austin at nscl.msu.edu, 517-353-6311