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We study the post-main-sequence evolution of products of collisions between main-sequence stars (blue stragglers), with particular interest paid to the horizontal branch and asymptotic giant branch phases. We found that the blue straggler progeny populate the color-magnitude diagram slightly blueward of the red giant branch and between 0.2 and 1 magnitudes brighter than the horizontal branch. We also found that the lifetimes of collision products on the horizontal branch is consistent with the numbers of so-called "evolved blue straggler stars" (E-BSS) identified by various authors in a number of globular clusters, and is almost independent of mass or initial composition profile. The observed ratio of the number of E-BSS to blue stragglers points to a main-sequence lifetime for blue stragglers of approximately 1-2 Gyr on average.

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 3He(3He,2p)4He 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 3He 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 three decades standing. This mixing mechanism, which we call " d m mixing," operates rapidly (relative to the nuclear timescale of overall evolution, ~108 yr) once the hydrogen-burning shell approaches the material homogenized by the surface convection zone. In agreement with observations, Population I stars between 0.8 and 2.0 M develop 12C/13C ratios of 14.5 +- 1.5, while Population II stars process the carbon to ratios of 4.0 +- 0.5. In stars less than 1.25 M, this mechanism also destroys 90%-95% of the 3He produced on the main sequence.

Presolar SiC grains of the mainstream, Y, and Z type are believed to come from carbon stars. We compared their C and Si isotopic ratios with theoretical models for the envelope compositions of AGB stars. Two sets of models (FRANEC and Monash) use a range of stellar masses (1.5-5 M) and metallicities, different prescriptions for mass loss, and two sets of neutron-capture cross sections for the Si isotopes. They predict that the shifts in Si isotopic ratios and the increase of 12C/13C in the envelope during third dredge-up are higher for higher stellar mass, lower metallicity, and lower mass-loss rate. Because the 22Ne neutron source dominates Si nucleosynthesis, the effect of the 13C source is negligible. Comparison of the model predictions with grain data confirms an AGB origin for these grains, with Y and Z grains having originated in stars with lower than solar metallicity. The Si isotopic ratios of the Z grains favor the Si cross sections by Guber et al. over those by Bao et al. The 12C/13C ratios of low-metallicity models are much higher than those found in Z grains, and cool bottom processing must be invoked to explain the grains' C isotopic ratios. By combining Z grain Si data with the models, we determined the evolution of the 29Si/28Si ratio in the Galaxy as function of metallicity Z. At Z < 0.01 this ratio rises much faster than current Galactic evolution models predict and suggests an early source of the heavy Si isotopes not considered in these models.

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 one-dimensional (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 present fluorine abundances in seven red giant members of the globular cluster M4 (NGC 6121). These abundances were derived from the HF (1-0) R9 line at 2.3357 mm in high-resolution infrared spectra obtained with the Phoenix spectrograph on the Gemini South telescope. Many abundances in the target stars have been studied previously, so their overall abundance distributions are well mapped. The abundance of fluorine is found to vary by more than a factor of 6, with the 19F variations being correlated with the already established oxygen variations and anticorrelated with the sodium and aluminium variations. In this paper we thus add fluorine to the list of elements known to vary in globular cluster stars, and this provides further evidence that H burning is the root cause of the chemical inhomogeneities. The fact that 19F is found to decrease in the M4 stars, as the signature of H burning appears, indicates that the polluting stars must have masses greater than about 3.5 M, as less massive stars than this should produce, not destroy, fluorine.

We present nucleosynthesis calculations and the resulting 19F stellar yields for a large set of models with different masses and metallicity. During the asymptotic giant branch (AGB) phase, 19F is produced as a consequence of nucleosynthesis occurring during the convective thermal pulses and also during the interpulse periods if protons from the envelope are partially mixed in the top layers of the He intershell (partial mixing zone). We find that the production of fluorine depends on the temperature of the convective pulses, the amount of primary 12C mixed into the envelope by third dredge-up, and the extent of the partial mixing zone. Then we perform a detailed analysis of the reaction rates involved in the production of 19F and the effects of their uncertainties. We find that the major uncertainties are associated with the 14C(a, g)18O and 19F(a, p)22Ne reaction rates. For these two reactions we present new estimates of the rates and their uncertainties. In both cases the revised rates are lower than previous estimates. The effect of the inclusion of the partial mixing zone on the production of fluorine strongly depends on the very uncertain 14C(a, g)18O reaction rate. The importance of the partial mixing zone is reduced when using our estimate for this rate. Overall, rate uncertainties result in uncertainties in the fluorine production of about 50% in stellar models with mass [?]3 M and of about a factor of 7 in stellar models of mass [?]5 M. This larger effect at high masses is due to the high uncertainties of the 19F(a, p)22Ne reaction rate. Taking into account both the uncertainties related to the partial mixing zone and those related to nuclear reactions, the highest values of 19F enhancements observed in AGB stars are not matched by the models. This is a problem that will have to be revised by providing a better understanding of the formation and nucleosynthesis in the partial mixing zone, as well as in relation to reducing the uncertainties of the 14C(a, g)18O reaction rate. At the same time, the possible effect of cool bottom processing at the base of the convective envelope should be included in the computation of AGB nucleosynthesis. This process could, in principle, help to match the highest 19F abundances observed by decreasing the C/O ratio at the surface of the star, while leaving the 19F abundance unchanged.

We study the slow neutron capture process (s-process) in asymptotic giant branch (AGB) stars using three different stellar evolutionary models computed for a 3 M, solar metallicity star. First we investigate the formation and the efficiency of the main neutron source: the 13C(a, n)16O reaction that occurs in radiative conditions. A tiny region rich in 13C (the 13C pocket) is created by proton captures on the abundant 12C in the top layers of the He intershell, the zone between the H shell and the He shell. We parametrically vary the number of protons mixed from the envelope. For high local proton-to-12C number ratios, p/12C [?] 0.3, most of the 13C nuclei produced are further converted by proton capture to 14N. Besides, 14N nuclei represent a major neutron poison. We find that a linear relationship exists between the amount of 12C in the He intershell and the maximum value of the time-integrated neutron flux. Then we generate detailed s-process calculations on the basis of stellar evolutionary models constructed with three different codes, all of them self-consistently finding the third dredge-up, although with different efficiency. One of the codes includes a mechanism at each convective boundary that simulates time-dependent hydrodynamic overshoot. This mechanism depends on a free parameter f and results in partial mixing beyond convective boundaries, the most efficient third dredge-up, and the formation of the 13C pocket. For the other two codes, an identical 13C pocket is introduced in the postprocessing nucleosynthesis calculations. The models typically produce enhancements of heavy elements of about 2 orders of magnitude in the He intershell and of up to 1 order of magnitude at the stellar surface, after dilution with the convective envelope, thus generally reproducing spectroscopic observations. The results of the cases without overshoot are remarkably similar, pointing out that the important uncertainty in s-process predictions is the 13C pocket and not the intrinsic differences among different codes when no overshoot mechanism is included. The code including hydrodynamic overshoot at each convective boundary finds that the He intershell convective zone driven by the recurrent thermal instabilities of the He shell (thermal pulses) penetrates the C-O core, producing a He intershell composition near that observed in H-deficient central stars of planetary nebulae. As a result of this intershell dredge-up, the neutron fluxes have a higher efficiency, both during the interpulse periods and within thermal pulses. The s-element distribution is pushed toward the heavier s-process elements, and large abundances of neutron-rich isotopes fed by branching points in the s-process path are produced. Several observational constraints are better matched by the models without overshoot. Our study needs to be extended to different masses and metallicities and in the space of the free overshoot parameter f.

The evolution of Population III stars (Z = 0) is followed from the pre-main-sequence phase up to the asymptotic giant branch (AGB) phase for intermediate-mass stars and up to C ignition in more massive stars. Our grid includes 11 stars, covering the mass range 0.8-20 M. During the H and He core burning phases an overshooting characterized by d = 0.20HP was applied, and during the AGB phase a small extension of the convective envelope was also allowed, characterized by d = 0.05HP. We find that at the beginning of the AGB phase, following the development of a convective instability in the He-burning shell, a secondary convective zone forms at the He-H discontinuity. This unusual convective zone then expands and overlaps with the region previously occupied by the receding He-driven instability. Carbon is engulfed and an H flash takes place as a result of the activation of the CNO cycle. Following these successive (H + He) flashes, the convective envelope penetrates deeper into the star, reaches the secondary H convective shell, and allows CNO catalysts to be dredged up to the surface. These mixing episodes, which have been found to occur in our 1, 1.5, 2, 3, 4, and 5 M models, increase the carbon abundance in the envelope and allow low- and intermediate-mass stars to achieve a "standard" thermally pulsing AGB phase, confirming the recent results by Chieffi and collaborators. We also find that at the beginning of the double shell burning evolution, our 4 and 5 M models experience so-called degenerate thermal pulses, which are very similar to those found by Frost et al. but absent from the Chieffi et al. simulations. Finally, in the 7 M model, the CNO envelope abundance following the second dredge-up is so large that the star does not experience the carbon injection episode and follows a standard thermally pulsing AGB evolution. Our computations also indicate that, thanks to the small overshooting at the base of the convective envelope, the third dredge-up is already operating in stars with M [?] 1.5 M after a few pulses, and that by the end of our modeling, hot bottom burning is activated in stars more massive than ~2 M. This evolutionary behavior suggests that primordial low- and intermediate-mass stars could have been significant contributors to the production of primary 12C and 14N and may have contributed to some extent to the production of Mg and Al and possibly s-process elements (despite the lack of iron seeds) in the early universe.

In this paper we study the evolution of 7Li in the Galaxy considering the contributions of various stellar sources: Type II supernovae, novae, red giant stars, and asymptotic giant branch (AGB) stars. We present new results for the production of 7Li in AGB stars via the hot bottom burning process, based on stellar evolutionary models. In the light of recent observations of dense circumstellar shells around evolved stars in the Galaxy and in the Magellanic Clouds, we also consider the impact of a very high mass-loss rate episode (superwind) before the evolution off the AGB phase on the 7Li enrichment in the interstellar medium. We compare the Galactic evolution of 7Li obtained with these new 7Li yields (complemented with a critical reanalysis of the role of supernovae, novae and giant stars) with a selected compilation of spectroscopic observations including halo and disk field stars as well as young stellar clusters. We conclude that even allowing for the large uncertainties in the theoretical calculation of mass-loss rates at the end of the AGB phase, the superwind phase has a significant effect on the 7Li enrichment of the Galaxy.

AbstractIn this article I review our understanding of s-process nucleosynthesis in Asymptotic Giant Branch (AGB) stars. The discussion begins with the nuclear back~ ground and then shows how the required conditions are found in thermally pulsing AGB stars. The two competing neutron sources (Ne22and C13) are discussed, as is the observational and theoretical evidence for and against each of them. We conclude by discussing the recent evidence for hot bottom burning, including the observed enhancements of Li7and Al26in some AGB stars.

AbstractAlthough stars can be approximated very satisfactorily by spherical models, it is clear from observations of star forming regions that the break-up of molecular gas to form proto-stars is a highly complex process that cannot be approximated simply. As a consequence, the theoretical picture of fragmentation, which is based on assuming that fragmentation occurs in uniform, static, isothermal clouds, is too crude to provide a guide to the star formation process. The alternative is to simulate the fragmentation process numerically with the ultimate aim of evolving the gas to proto-stars. The difficulties involved with this procedure are discussed in this paper. We also discuss our proposed solution to this problem, together with a very preliminary calculation.

AbstractThe astrophysical literature contains many discussions on exactly which changes in a star’s structure are responsible for the extremely large radii found after core hydrogen exhaustion. Different authors favour different mechanisms. Unfortunately none of the proposals are easily verified nor are they easily disproven. In this paper we examine the suggestion that it is the increased central mass concentration (i.e., the growth of the hydrogen exhausted core) which is the primary agent responsible for envelope growth.

AbstractThis paper presents recent stellar evolutionary calculations of the Asymptotic Giant Branch evolution of a 5M?model. It is found that temperatures at the base of the convective envelope reach up to almost 100 million degrees, high enough for much nucleo-synthesis to occur. Some implications are discussed briefly.

We review the observational evidence for extra mixing in stars on the red giant branch (RGB) and discuss why thermohaline mixing is a strong candidate mechanism. We recall the simple phenomenological description of thermohaline mixing and aspects of mixing in stars in general. We use observations of M3 to constrain the form of the thermohaline diffusion coefficient and any associated free parameters. This is done by matching [C/Fe] and [N/Fe] along the RGB of M3. After taking into account a presumed initial primordial bimodality of [C/Fe] in the CN-weak and CN-strong stars, our thermohaline mixing models can explain the full spread of [C/Fe]. Thermohaline mixing can produce a significant change in [N/Fe] as a function of absolute magnitude on the RGB for initially CN-weak stars, but not for initially CN-strong stars, which have so much nitrogen to begin with that any extra mixing does not significantly affect the surface nitrogen composition.