Comment on "Deep Mixing of 3He: Reconciling Big Bang and Stellar Nucleosynthesis"

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Science  31 Aug 2007:
Vol. 317, Issue 5842, pp. 1170
DOI: 10.1126/science.1140657

Abstract

Eggleton et al. (Reports, 8 December 2006, p. 1580) reported on a deep-mixing mechanism in low-mass stars caused by a Rayleigh-Taylor instability that destroys all of the helium isotope 3He produced during the star's lifetime. Observations of 3He in planetary nebulae, however, indicate that some stars produce prodigious amounts of 3He. This is inconsistent with the claim that all low-mass stars should destroy 3He.

Standard stellar models (14) produce 3He on the Main Sequence outside the main nuclear-burning regions where the proton-proton cycle terminates in 3He. This 3He material is mixed into the convective envelope at the first dredge-up on the lower red giant branch. These models include only mixing produced by thermal convection. Therefore, once in the convective envelope, the 3He survives and is expected to enrich the interstellar medium through stellar winds and planetary nebulae. Observations have long indicated that extra mixing processes must be taking place, for example, overshooting beyond convective radiative boundaries or rotationally induced turbulence (5). Carbon isotopic ratios (12C/13C) in the surfaces of red giant branch stars reveal lower ratios for stars below 2 solar masses (M) than theoretically expected (6). Observations of 3He in the Galaxy have found two planetary nebulae with 3He abundances that are consistent with the standard stellar models (7). In contrast, abundances in fully ionized nebulae (HII regions) are substantially smaller than expected from Galactic chemical evolution, given the standard stellar yields (8).

Eggleton et al. (9) reported an apparent resolution to this conflict of observations with theory. By modeling a red giant star in three dimensions, they found that an extra mixing process naturally arises as a result of a Rayleigh-Taylor instability between the hydrogen-burning shell and the base of the convective envelope. This turbulent region moves outward at speeds of ∼500 m s–1, eventually contacting the convective envelope and reprocessing 3He. The authors predict that all of the 3He produced earlier on the Main Sequence will be destroyed. This implies that all stars or their remnants should have roughly primordial 3He abundances.

This is in direct conflict with measurements of 3He in planetary nebulae (7, 10, 11), where 3He/H abundance ratios have been measured to be at least an order of magnitude larger than the primordial value. There are at least two solutions to this conflict. First, the observations of 3He in planetary nebulae may be wrong. 3He/H abundance ratios have been determined by observing the 3He+ hyperfine transition at 3.46 cm using the Max-Planck-Institut für Radioastronomie (MPIfR) 100-m telescope and the National Radio Astronomy Observatory's (NRAO) 140-foot Radio Telescope and Very Large Array (VLA). The source sample consisted of planetary nebulae located outside the Galactic plane that are associated with an older population of stars with progenitor masses ≤ 2 M. This is consistent with progenitor mass estimates based on stellar evolutionary tracks (12). Because these observations pushed the sensitivity limit of each of these instruments, however, they may suffer from unknown systematic effects. 3He was nevertheless detected in the planetary nebula NGC3242 with both the MPIfR 100-m and NRAO 140-foot telescopes, and in nebula J320 with the VLA. Second, the assumption that all of the 3He produced during the Main Sequence will be destroyed by this deep-mixing mechanism may be wrong. Because the three-dimensional simulation (9) was computationally expensive, only a single simulation was run and only for a fraction of the star's evolution. The speed of the instability could vary as the star evolves on the red giant branch altering the final 3He/H abundance ratios. Moreover, a full exploration of the parameter space with additional simulations could alter how much 3He is typically reprocessed.

Other mixing mechanisms have been proposed that involve a star's rotation. Charbonnel et al. (13) argue for a rotationally induced extra mixing that occurs in low-mass stars above the luminosity function bump produced when the hydrogen-burning shell crosses the chemical discontinuity left by the retreating convective envelope early in the red giant branch phase. This is supported by observed increases in the 13C abundance above the luminosity function bump (13). It is predicted that 96% of all low-mass stars will undergo this mixing process (14). This allows for a few stars to have high 3He abundances. Because such mixing scenarios have existed for some time, the 3He sample of planetary nebulae discussed above was selected based on diagnostics indicating little stellar rotation.

Finally, as discussed in (9), the deep mixing caused by the Rayleigh-Taylor instability occurs regardless of stellar rotation or magnetic fields. Could rotation or some other stellar parameter alter the efficiency of this process, perhaps changing the speed at which the turbulent region moves toward the convective envelope? The planetary nebula observations can be compatible with Rayleigh-Taylor only if mixing can be suppressed in 5 to 10% of the stars.

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