No abstract

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.