Predictions of oxygen isotope ratios in stars and of oxygen-rich interstellar grains in meteorites

Abstract

We carried out detailed, self-consistent calculations for stars from 1 to 9 M. over a wide range of metallicities, following the evolution and nucleosynthesis from the pre-main sequence to the asymptotic giant branch (AGB), in order to provide a self-consistent grid for evaluating stellar oxygen isotopic variations. These were calculated for first and second dredge-up, and for some masses also for third dredge-up and ''hot bottom'' convective envelope burning on the AGB. We demonstrate that O-16/O-17 in red giant envelopes is primarily a function of the star's mass, while O-16/O-18 is primarily a function of the initial composition. Uncertainties in the O-17-destruction rate have no effect on the O-16/O-18 ratio for stars from 1 to 2.5 M., but do affect the ratios for higher masses: the stellar O-16/O-17 observations are consistent with the Landre et al. (1990) rates using f = 0.2 for O-17(p, gamma)F-18 and O-17(p, alpha)N-14, and with the Caughlan & Fowler (1988) rates using f almost-equal-to 1. The stellar O-16/O-18 observations require f almost-equal-to 0 in the Caughlan & Fowler O-18(p, alpha)N-15 rate. First dredge-up has the largest effect on the oxygen isotope ratios, decreasing O-16/O-17 significantly from the initial value and increasing O-16/O-18 slightly. Second and third dredge-up have only minor effects for solar metallicity stars. The absence of very low observed O-16/O-18 ratios is consistent with a major increase in the O-18(alpha, gamma)Ne-22 rate over the Caughlan & Fowler (1988) value. Hot bottom burning in stars above about 5 M. can cause a huge increase in O-16/O-18 (to greater than or similar to 10(6)), and possibly a significant decrease in O-16/O-17; these are accompanied by a huge increase in Li-7 and a value of C-12/C-13 almost-equal-to 3. The oxygen isotope ratios in the Al2O3 grains (Orgueil grain B, the Murchison 83-5 grain, and the new Bishunpur B39 grain) can be accounted for if they originated in stars that did NOT have the same initial O-16/O-18 ratio. Thus one cannot assume uniform isotope ratios, even for stars of nearly solar composition. The grains' O-16/O-17 ratios, together with the Mg-26 excesses that indicate grain formation in a Al-26-rich environment, indicate that the Orgueil grain B and Murchison 83-5 grain originated in stars of roughly 1.5 M., during third dredge-up on the AGB. The new Bishunpur B39 grain originated in a star of either 2 or of 4-7 M..