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Thus we are able to remove the threat that 3 He production in low-mass stars poses to the Big Bang nucleosynthesis of 3 He.

We consider in detail the effect of the emission of "hadronic" invisible axions (which d o not couple to electrons) from the interior of stars on stellar evolution. To this end we calculate plasma emission rates for axions due to the Primakoff process for the full range of conditions encountered in a giant star. Much attention is paid to plasma, degeneracy, and screening effects. We reconsider the solar bound by evolving a 1.0 M g star to solar age and lowering the presolar helium abundance so as to obtain the correct present-day luminosity of the Sun. The previous bound on the axion-photon coupling of G g 5 2.5 (corresponding to m , 5 17 eV R where R is a model-dependent factor of order unity) is confirmed, where G 9 is the coupling constant G in units of l o 9 GeV-'. We then follow the evolution of a 1.3Ma star from zero age to the top of the giant branch. Helium ignites for all values of G consistent with the solar bound; however, the core mass, surface temperature, and luminosity at the helium flash exceed the standard values. The luminosity at the helium flash is larger than about twice the standard value unless G 9 5 0.3 (corresponding to m a 5 2 eV R ) , in conflict with observational data, which are statistically weak, however. We find our most stringent limits from the helium-burning lifetime. In the absence of axion cooling we calculate a lifetime of 1.2X 10' yr which corresponds well with the value 1.5 X lo8 yr derived from the number of red giants in the "clump" of the open cluster M67 and with the value 1.3 X 10' yr derived from the number of such stars in the old galactic disk population. We obtain a conservative limit of Gg i 0 . 3 which, at saturation, results in a helium-burning lifetime an order of magnitude low. We believe that G 9 5 0 . 1 ( m a 5 0.7 eV R ) is a reasonably safe limit which, if saturated, leads to a calculated helium-burning lifetime a factor of 2 below the observed value. Our results exclude the recently suggested possibility of detecting cosmic axions through their 2y decay and probably the possibility of measuring the solar hadronic axion flux which, according to our bounds, must be less than 2x of the solar luminosity. There remains a narrow range of parameters (0.01 5 G 9 5 0 . 1 , m, 5 l o p 4 eV) in which a recently proposed laboratory experiment might still measure axionlike particles.

We use the 3D stellar structure code djehuty to model the ingestion of protons into the intershell convection zone of a 1 M ⊙ asymptotic giant branch star of metallicity Z = 10 −4 . We have run two simulations: a low resolution one of around 300,000 zones, and a high resolution one consisting of 2,000,000 zones. Both simulations have been evolved for about 4 hours of stellar time. We observe the existence of fast, downward flowing plumes that are able to transport hydrogen into close proximity to the helium burning shell before burning takes place. The intershell in the 3D model is richer in protons than the 1D model by several orders of magnitude and so we obtain substantially higher hydrogen-burning luminosities -over 10 8 L ⊙ in the high resolution simulationthan are found in the 1D model. Convective velocities in these simulations are over 10 times greater than the predictions of mixing length theory, though the 3D simulations have greater energy generation due to the enhanced hydrogen burning. We find no evidence of the convective zone splitting into two, though this could be as a result of insufficient spatial resolution or because the models have not been evolved for long enough. We suggest that the 1D mixing length theory and particularly the use of a diffusion algorithm for mixing do not give an accurate picture of these events. An advective mixing scheme may give a better representation of the transport processes seen in the 3D models.

Three-dimensional stellar modeling has enabled us to identify a deep-mixing mechanism that must operate in all low mass giants. This mixing process is not optional, and is driven by a molecular weight inversion created by the 3 He( 3 He,2p) 4 He reaction. In this paper we characterize the behavior of this mixing, and study its impact on the envelope abundances. It not only eliminates the problem of 3 He overproduction, reconciling stellar and big bang nucleosynthesis with observations, but solves the discrepancy between observed and calculated CNO isotope ratios in low mass giants, a problem of more than 3 decades' standing. This mixing mechanism, which we call 'δµ-mixing', operates rapidly (relative to the nuclear timescale of overall evolution, ∼ 10 8 yrs) once the hydrogen burning shell approaches the material homogenized by the surface convection zone. In agreement with observations, Pop I stars between 0.8 and 2.0 M ⊙ develop 12 C/ 13 C ratios of 14.5 ± 1.5, while Pop II stars process the carbon to ratios of 4.0 ± 0.5. In stars less than 1.25 M ⊙ , this mechanism also destroys 90% to 95% of the 3 He produced on the main sequence.Subject headings: stars: red giants; abundance anomalies has the unusual characteristic (among fusion reactions in stars) of lowering the mean molecular weight µ, creating a localised µ-inversion. This has already been noted by Ulrich (1972)

We model the core helium flash in a low-mass red giant using Djehuty, a fully three-dimensional (3D) code. The 3D structures were generated from converged models obtained during the 1D evolutionary calculation of a 1 M ⊙ star. Independently of which starting point we adopted, we found that after some transient relaxation the 3D model settled down with a briskly convecting He-burning shell that was not very different from what the 1D model predicted.

We have constructed a series of non-rotating quasi-hydrostatic evolutionary models for the M2 Iab supergiant Betelgeuse (α Orionis). Our models are constrained by multiple observed values for the temperature, luminosity, surface composition and mass loss for this star, along with the parallax distance and high resolution imagery that determines its radius. We have then applied our best-fit models to analyze the observed variations in surface luminosity and the size of detected surface bright spots as the result of up-flowing convective material from regions of high temperature in the surface convective zone. We also attempt to explain the intermittently observed periodic variability in a simple radial linear adiabatic pulsation model. Based upon the best fit to all observed data, we suggest a best progenitor mass estimate of 20 +5 −3 M ⊙ and a current age from the start of the zero-age main sequence of 8.0 − 8.5 Myr based upon the observed ejected mass while on the giant branch.

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