We describe the Garching Stellar Evolution Code. General features, treatment of the microphysics, details of the numerical solution, handling and particularities are discussed. The standard solar model serves as the most basic benchmark to test the accurateness of the code and is presented, too.

The core helium-flash in low-mass stars with extreme mass loss occurs after the tip of the RGB, when the H-rich envelope is very thin. The low efficiency of the H-shell source enables the He-flash driven convective zone to penetrate H-rich layers and trigger a thermonuclear runaway, resulting in a subsequent surface enrichment with He and C. In this work we present the first full computations of Population II low-mass stellar models through this phase. Models experiencing this dredge-up event are significantly hotter than their counterparts with H-rich envelopes, which makes them promising candidates for explaining the existence of stars observed beyond the canonical blue end of the horizontal branch ("blue hook stars"). Moreover, this temperature difference could explain the observed gap in M V between extreme blue horizontal-branch and blue hook stars. A first comparison with spectroscopic observations of blue hook stars in the globular cluster ω Cen is also presented.

Helioseismological sound-speed profiles severely constrain possible deviations from standard solar models, allowing us to derive new limits on anomalous solar energy losses by the Primakoff emission of axions. For an axion-photon coupling gaγ < ∼ 5 × 10 −10 GeV −1 the solar model is almost indistinguishable from the standard case, while gaγ > ∼ 10 × 10 −10 GeV −1 is probably excluded, corresponding to an axion luminosity of about 0.20 L⊙. This constraint on gaγ is much weaker than the well-known globular-cluster limit, but about a factor of 3 more restrictive than previous solar limits. Our result is primarily of interest to the large number of current or proposed search experiments for solar axions because our limit defines the maximum gaγ for which it is self-consistent to use a standard solar model to calculate the axion luminosity.

Abstract:The bulk of the carbon in our universe is produced in the triple-alpha process in helium-burning red giant stars. We calculated the change of the triple-alpha reaction rate in a microscopic 12-nucleon model of the 12 C nucleus and looked for the effects of minimal variations of the strengths of the underlying interactions. Stellar model calculations were performed with the alternative reaction rates. Here, we show that outside a narrow window of 0.5 and 4 % of the values of the strong and Coulomb forces, respectively, the stellar production of carbon or oxygen is reduced by factors of 30 to 1000. The formation of12 C through the triple-alpha process takes place in two sequential steps in the He-burning phase of red giants. In the first step, the unstable 8 Be with a lifetime of only about 10 −16 s is formed in a reaction equilibrium with the two alpha particles, α + α ⇀ ↽ 8 Be. In the second step, an additional alpha particle is captured, 8 Be(α, γ) 12 C. Without a suitable resonance in 12 C, the triple-alpha rate would be much too small to account for the 12 C abundance in our universe. Hoyle (1) suggested that a resonance level in 12 C, at about 300-400 keV above the three-alpha threshold, would enhance the triple-alpha reaction rate and would explain the abundance of 12 C in our universe. Such a level was subsequently found experimentally when a resonance that possessed the required properties was discovered (2, 3). It is the second 0 + state in 12 C, denoted by 0 + 2 . Its modern parameters (4) are ε = (379.47 ± 0.18) keV, Γ = (8.3 ± 1) eV, and Γ γ = (3.7 ± 0.5) meV, where ε is the resonance energy in the center-of-mass frame relative to the three-alpha threshold, and Γ and Γ γ are the total width and radiative width, respectively.The isotope 12 C is synthesized further in the He burning in red giants by alpha capture to the O isotope 16 O, leading to an abundance ratio in the universe of 12 C : 16 O ≈ 1 : 2 (5). If the carbon abundance in the universe were suppressed by orders of magnitude, no carbon-based life could have developed in the universe. But the production of O is also necessary because no spontaneous development of carbon-based life is possible without the existence of water.Here, we investigated the abundance ratios of C and O by starting from slight variations of the strength of the nucleon-nucleon (N-N) interaction with a microscopic 12-nucleon model. In previous studies, only hypothetical ad hoc shifts of the resonance energy of the 0 + 2 state were considered (6). Some preliminary results of our cal-1

Abstract. We show that the inclusion of special relativistic corrections in the revised OPAL and MHD equations of state has a significant impact on the helioseismic determination of the solar age. Models with relativistic corrections included lead to a reduction of about 0.05−0.08 Gyr with respect to those obtained with the old OPAL or MHD EOS. Our best-fit value is t seis = (4.57 ± 0.11) Gyr which is in remarkably good agreement with the meteoritic value for the solar age. We argue that the inclusion of relativistic corrections is important for probing the evolutionary state of a star by means of the small frequency separations δν ,n = ν ,n − ν +2,n−1 , for spherical harmonic degrees = 0, 1 and radial order n .

Conversion Coefficients for Radiological Protection Quantities for External Radiation Exposures * If the monitoring devices are not designed to measure H 0 (3, X) or H p (3), H 0 (0.07, X) and H p (0.07) may be applied.

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 heliumand 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.

A new series of organ equivalent dose conversion coefficients for whole body external photon exposure is presented for a standardized couple of human voxel models, called Rex and Regina. Irradiations from broad parallel beams in antero-posterior, postero-anterior, left- and right-side lateral directions as well as from a 360 degrees rotational source have been performed numerically by the Monte Carlo transport code EGSnrc. Dose conversion coefficients from an isotropically distributed source were computed, too. The voxel models Rex and Regina originating from real patient CT data comply in body and organ dimensions with the currently valid reference values given by the International Commission on Radiological Protection (ICRP) for the average Caucasian man and woman, respectively. While the equivalent dose conversion coefficients of many organs are in quite good agreement with the reference values of ICRP Publication 74, for some organs and certain geometries the discrepancies amount to 30% or more. Differences between the sexes are of the same order with mostly higher dose conversion coefficients in the smaller female model. However, much smaller deviations from the ICRP values are observed for the resulting effective dose conversion coefficients. With the still valid definition for the effective dose (ICRP Publication 60), the greatest change appears in lateral exposures with a decrease in the new models of at most 9%. However, when the modified definition of the effective dose as suggested by an ICRP draft is applied, the largest deviation from the current reference values is obtained in postero-anterior geometry with a reduction of the effective dose conversion coefficient by at most 12%.