Quantum corrections to microscopic diffusion constants

We review the state of the art regarding the computation of the resistance coefficients in conditions typical of the stellar plasma, and compare the various results studying their effect on the solar model. We introduce and discuss for the first time in an astrophysical context the effect of quantum corrections to the evaluation of the resistance coefficients, and provide simple yet accurate fitting formulae for their computation. Although the inclusion of quantum corrections only weakly modifies the solar model, their effect is growing with density, and thus might be of relevance in case of denser objects like, e.g., white dwarfs.Comment: 8 pages, 5 figures, accepted for publication in A&

Microscopic diffusion of partly ionized metals in the Sun and metal-poor stars

An improved microscopic diffusion in stars is presented considering in detail the partly ionized stages of metals. Besides,the influence of degenerate electron-gas and of the contribution of radiation to the total pressure has been accounted for. The solution of the diffusion equations is then performed following the scheme of Thoul et al. (1994). By defining one mean charged ion per element very few modifications are necessary to solve the improved diffusion scheme. (A portable FORTRAN routine is provided.) The change in the sound-speed profile of a solar model obtained with the new diffusion description is at most about 25% at r=0.6 R(sun). The biggest effect on low-mass stars is expected near the turn-off, where the convective envelope is shallowest. However, only a difference of at most 40 K in the effective temperature could be observed when assuming either fully or partly ionized metals in the diffusion equation. Nevertheless, the surface metal distribution is strongly altered.Comment: 12 pages, 10 figures, A&A accepte

On the helium flash in low-mass Population III Red Giant stars

We investigate the evolution of initially metal-free, low-mass Red Giant stars through the He core flash at the tip of the Red Giant Branch. The low entropy barrier between the helium- and hydrogen-rich layers enables a penetration of the helium flash driven convective zone into the inner tail of the extinguishing H-burning shell. As a consequence, protons are mixed into high-temperature regions triggering a H-burning runaway. The subsequent dredge-up of matter processed by He and H burning enriches the stellar surface with large amounts of helium, carbon and nitrogen. Extending previous results by Hollowell et al. (1990) and Fujimoto et al. (2000), who claimed that the H-burning runaway is an intrinsic property of extremely metal-poor low-mass stars, we found that its occurrence depends on additional parameters like the initial composition and the treatment of various physical processes. We perform some comparisons between predicted surface chemical abundances and observational measurements for extremely metal-deficient stars. As in previous investigations, our results disclose that although the described scenario provides a good qualitative agreement with observations, considerable discrepancies still remain. They may be due to a more complex evolutionary path of `real' stars, and/or some shortcomings in current evolutionary models. In addition, we analyze the evolutionary properties after the He core flash, during both the central and shell He-burning phases, allowing us to deduce some interesting differences between models whose Red Giant Branch progenitor has experienced the H-flash and canonical models. In particular, the Asymptotic Giant Branch evolution of extremely metal-deficient stars and the occurrence of thermal pulses are strongly affected by the previous RGB evolution.Comment: 7 figures, AASTeX, submitted to Ap

New solar opacities, abundances, helioseismology, and neutrino fluxes

We construct solar models with the newly calculated radiative opacities from the Opacity Project (OP) and recently determined (lower) heavy element abundances. We compare results from the new models with predictions of a series of models that use OPAL radiative opacities, older determinations of the surface heavy element abundances, and refinements of nuclear reaction rates. For all the variations we consider, solar models that are constructed with the newer and lower heavy element abundances advocated by Asplund et al. (2005) disagree by much more than the estimated measuring errors with helioseismological determinations of the depth of the solar convective zone, the surface helium composition, the internal sound speeds, and the density profile. Using the new OP radiative opacities, the ratio of the 8B neutrino flux calculated with the older and larger heavy element abundances (or with the newer and lower heavy element abundances) to the total neutrino flux measured by the Sudbury Neutrino Observatory is 1.09 (0.87) with a 9% experimental uncertainty and a 16% theoretical uncertainty, 1 sigma errors.Comment: ApJ Letters (in press), added 3 references, detailed numerical solar models and distributions of neutrino fluxes available at http://www.sns.ias.edu/~jnb (models go back to 1982

Bridging the mass gaps at A=5 and A=8 in nucleosynthesis

In nucleosynthesis three possible paths are known to bridge the mass gaps at A=5 and A=8. The primary path producing the bulk of the carbon in our Universe proceeds via the triple-alpha process He4(2alpha,gamma)C12. This process takes place in helium-burning of red giant stars. We show that outside a narrow window of about 0.5% of the strength or range of the strong force, the stellar production of carbon or oxygen through the triple-alpha process is reduced by factors of 30 to 1000. Outside this small window the creation of carbon or oxygen and therefore also carbon-based life in the universe is strongly disfavored. The anthropically allowed strengths of the strong force also give severe constraints for the sum of the light quark masses as well as the Higgs vacuum expectation value and mass parameter at the 1% level

The surface carbon and nitrogen abundances in models of ultra metal-poor stars

We investigate whether the observed high number of carbon- and nitrogen-enhanced extremely metal-poor stars could be explained by peculiar evolutionary properties during the core He flash at the tip of the red giant branch. For this purpose we compute a series of detailed stellar models expanding upon our previous work; in particular, we investigate if during the major He flash the penetration of the helium convective zone into the overlying hydrogen-rich layers can produce carbon- and nitrogen-rich abundances in agreement with current spectroscopic observations. The dependence of this phenomenon on selected model input parameters, such as initial metallicity and treatment of convection is examined in detail.Comment: 8 pages, 4 figures, submitted to A&