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

^* To whom correspondence should be addressed. E-mail: dbalser{at}nrao.edu

Standard stellar models (1–4) 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.

References and Notes

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