Is Mixing or Mass Loss the Cause of the Decline in ^12C/^13C in Evolved Halo Stars?

Details Is Mixing or Mass Loss the Cause of the Decline in ^12C/^13C in Evolved Halo Stars? Is Mixing or Mass Loss the Cause of the Decline in ^12C/^13C in Evolved Halo Stars?

Jul 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995hst..prop.5857d&link_type=abstract

HST Proposal ID #5857

Astronomy and Astrophysics

Astronomy

Hst Proposal Id #5857 Cool Stars

Scientific paper

We propose to make Boron observations which can definitively distinguish between mass loss and mixing as the cause of the low ^12C/^13C ratios seen in evolved halo stars. Rarely can a few key observations in astronomy definitively distinguish between alternate scenarios for an interesting, well defined, and instructive problem. This long-standing problem involves the low ^12C/^13C isotope ratios observed in evolved halo stars, which signal a fundamental failure of the standard stellar evolution theory. As standard models evolve off the main sequence and their convection zones deepen, CN- synthesized ^13C is dredged up, lowering slightly the ^12C/^13C ratio from ~ 90 to a final minimum of ~ 30 -- 60, in agreement with observation. However, the observed carbon isotope ratio in more evolved stars declines precipitously, contradicting the models, and reaches the CN-cycle equilibrium value of 3 -- 4. Several mechanisms have been proposed in the past to explain the low ratios. Mass loss and rotational (or other) mixing remain viable today. Lithium abundances offer some evidence in favor of mixing, but are inconclusive. Boron abundances will remain constant in the case of mass loss, decline below their maximal dilution value in the case of mixing, allowing the proposed observations to clearly discriminate between the models. This result may have important implications for cosmology as well (globular cluster ages, or big bang nucleosynthesis and dark matter).

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