ALBERT

Description: We conduct a series of comparisons between spectroscopic and photometric observations of globular clusters and stellar models to examine their predictive power. Data from medium-to-high resolution spectroscopic surveys of lithium allow us to investigate first dredge-up and extra mixing in two clusters well separated in metallicity. Abundances at first dredge-up are satisfactorily reproduced but there is preliminary evidence to suggest that the models overestimate the luminosity at which the surface composition first changes in the lowest metallicity system. Our models also begin extra mixing at luminosities that are too high, demonstrating a significant discrepancy with observations at low metallicity. We model the abundance changes during extra mixing as a thermohaline process and determine that the usual diffusive form of this mechanism cannot simultaneously reproduce both the carbon and lithium observations. Hubble Space Telescope photometry provides turn-off and bump magnitudes in a large number of globular clusters and offers the opportunity to better test stellar modelling as function of metallicity. We directly compare the predicted main-sequence turn-off and bump magnitudes as well as the distance-independent parameter $\Delta M_V \,^{\rm {MSTO}}_{\rm {bump}}$ . We require 15 Gyr isochrones to match the main-sequence turn-off magnitude in some clusters and cannot match the bump in low-metallicity systems. Changes to the distance modulus, metallicity scale and bolometric corrections may impact on the direct comparisons but $\Delta M_V \,^{\rm {MSTO}}_{\rm {bump}}$ , which is also underestimated from the models, can only be improved through changes to the input physics. Overshooting at the base of the convective envelope with an efficiency that is metallicity dependent is required to reproduce the empirically determined value of $\Delta M_V \,^{\rm {MSTO}}_{\rm {bump}}$ .

Description: The presence of multiple populations in globular clusters has been well established thanks to high-resolution spectroscopy. It is widely accepted that distinct populations are a consequence of different stellar generations: intracluster pollution episodes are required to produce the peculiar chemistry observed in almost all clusters. Unfortunately, the progenitors responsible have left an ambiguous signature and their nature remains unresolved. To constrain the candidate polluters, we have measured lithium and aluminium abundances in more than 180 giants across three systems: NGC 1904, NGC 2808, and NGC 362. The present investigation along with our previous analysis of M12 and M5 affords us the largest data base of simultaneous determinations of Li and Al abundances. Our results indicate that Li production has occurred in each of the three clusters. In NGC 362, we detected an M12-like behaviour, with first- and second-generation stars sharing very similar Li abundances favouring a progenitor that is able to produce Li, such as asymptotic giant branches stars. Multiple progenitor types are possible in NGC 1904 and NGC 2808, as they possess both an intermediate population comparable in lithium to the first generation stars and also an extreme population, that is enriched in Al but depleted in Li. A simple dilution model fails in reproducing this complex pattern. Finally, the internal Li variation seems to suggest that the production efficiency of this element is a function of the cluster's mass and metallicity – low-mass or relatively metal-rich clusters are more adept at producing Li.

Description: The detection of mixed oscillation modes offers a unique insight into the internal structure of core helium burning (CHeB) stars. The stellar structure during CHeB is very uncertain because the growth of the convective core, and/or the development of a semiconvection zone, is critically dependent on the treatment of convective boundaries. In this study we calculate a suite of stellar structure models and their non-radial pulsations to investigate why the predicted asymptotic g-mode  = 1 period spacing 1 is systematically lower than is inferred from Kepler field stars. We find that only models with large convective cores, such as those calculated with our newly proposed ‘maximal-overshoot’ scheme, can match the average 1 reported. However, we also find another possible solution that is related to the method used to determine 1 : mode trapping can raise the observationally inferred 1 well above its true value. Even after accounting for these two proposed resolutions to the discrepancy in average 1 , models still predict more CHeB stars with low 1 ( 270 s) than are observed. We establish two possible remedies for this: (i) there may be a difficulty in determining 1 for early CHeB stars (when 1 is lowest) because of the effect that the sharp composition profile at the hydrogen burning shell has on the pulsations, or (ii) the mass of the helium core at the flash is higher than predicted. Our conclusions highlight the need for the reporting of selection effects in asteroseismic population studies in order to safely use this information to constrain stellar evolution theory.

Description: The treatment of convective boundaries during core helium burning is a fundamental problem in stellar evolution calculations. In the first paper of this series, we showed that new asteroseismic observations of these stars imply they have either very large convective cores or semiconvection/partially mixed zones that trap g modes. We probe this mixing by inferring the relative lifetimes of asymptotic giant branch (AGB) and horizontal branch (HB) from R 2 , the observed ratio of these stars in recent HST photometry of 48 Galactic globular clusters. Our new determinations of R 2 are more self-consistent than those of previous studies and our overall calculation of R 2  = 0.117 ± 0.005 is the most statistically robust now available. We also establish that the luminosity difference between the HB and the AGB clump is $\Delta \log {L}_{\rm HB}^{\rm AGB} = 0.455 \pm 0.012$ . Our results accord with earlier findings that standard models predict a lower R 2 than is observed. We demonstrate that the dominant sources of uncertainty in models are the prescription for mixing and the stochastic effects that can result from its numerical treatment. The luminosity probability density functions that we derive from observations feature a sharp peak near the AGB clump. This constitutes a strong new argument against core breathing pulses, which broaden the predicted width of the peak. We conclude that the two mixing schemes that can match the asteroseismology are capable of matching globular cluster observations, but only if (i) core breathing pulses are avoided in models with a semiconvection/partially mixed zone, or (ii) that models with large convective cores have a particular depth of mixing beneath the Schwarzschild boundary during subsequent early-AGB ‘gravonuclear’ convection.

Description: We have computed detailed evolution and nucleosynthesis models for super and massive asymptotic giant branch (AGB) stars over the mass range 6.5–9.0 M in divisions of 0.5 M with metallicities Z  = 0.02, 0.008 and 0.004. These calculations, in which we find third dredge-up and hot bottom burning, fill the gap between existing low- and intermediate-mass AGB star models and high-mass star models that become supernovae. For the considered metallicities, the composition of the yields is largely dominated by the thermodynamic conditions at the base of the convective envelope rather than by the pollution arising from third dredge-up. We investigate the effects of various uncertainties, related to the mass-loss rate, mixing length parameter, and the treatment of evolution after the envelope instability that develops near the end of the (super)AGB phase. Varying these parameters alters the yields mainly because of their impact on the amount of third dredge-up enrichment, and to a lesser extent on the hot bottom burning conditions. Our models produce significant amounts of 4 He, 7 Li (depending on the mass-loss formulation) 13 C, 14 N, 17 O, 23 Na, 25 Mg, as well the radioactive isotope 26 Al in agreement with previous investigation. In addition, our results show enrichment of 22 Ne, 26 Mg and 60 Fe, as well as a substantial increase in our proxy neutron capture species representing all species heavier than iron. These stars may provide important contributions to the Galaxy's inventory of the heavier Mg isotopes, 14 N, 7 Li and 27 Al.

Description: We present a new grid of stellar models and nucleosynthetic yields for super-AGB stars with metallicities Z = 0.001 and 0.0001, applicable for use within galactic chemical evolution models. Contrary to more metal-rich stars where hot bottom burning is the main driver of the surface composition, in these lower metallicity models the effect of third dredge-up and corrosive second dredge-up also have a strong impact on the yields. These metal-poor and very metal-poor super-AGB stars create large amounts of 4 He, 13 C, 14 N and 27 Al as well as the heavy magnesium isotopes 25 Mg and 26 Mg. There is a transition in yield trends at metallicity Z 0.001, below which we find positive yields of 12 C, 16 O, 15 N and 28 Si, which is not the case for higher metallicities. We explore the large uncertainties derived from wind prescriptions in super-AGB stars, finding 2 orders of magnitude difference in yields of 22 Ne, 23 Na, 24, 25, 26 Mg, 27 Al and our s-process proxy isotope g . We find inclusion of variable composition low-temperature molecular opacities is only critical for super-AGB stars of metallicities below Z 0.001. We analyse our results, and those in the literature, to address the question: Are super-AGB stars the polluters responsible for extreme population in the globular cluster NGC 2808? Our results, as well as those from previous studies, seem unable to satisfactorily match the extreme population in this globular cluster.

Description: We explore the final fates of massive intermediate-mass stars by computing detailed stellar models from the zero-age main sequence until near the end of the thermally pulsing phase. These super-asymptotic giant branch (super-AGB) and massive AGB star models are in the mass range between 5.0 and 10.0 M for metallicities spanning the range Z  = 0.02–0.0001. We probe the mass limits M up , M n and M mass , the minimum masses for the onset of carbon burning, the formation of a neutron star and the iron core-collapse supernovae, respectively, to constrain the white dwarf/electron-capture supernova (EC-SN) boundary. We provide a theoretical initial-to-final mass relation for the massive and ultra-massive white dwarfs and specify the mass range for the occurrence of hybrid CO(Ne) white dwarfs. We predict EC-SN rates for lower metallicities which are significantly lower than existing values from parametric studies in the literature. We conclude that the EC-SN channel (for single stars and with the critical assumption being the choice of mass-loss rate) is very narrow in initial mass, at most 0.2 M . This implies that between ~2 and 5 per cent of all gravitational collapse supernova are EC-SNe in the metallicity range Z  = 0.02–0.0001. With our choice for mass-loss prescription and computed core growth rates, we find, within our metallicity range, that CO cores cannot grow sufficiently massive to undergo a Type 1.5 SN explosion.

Description: In recent years much interest has been shown in the process of thermohaline mixing in red giants. In low- and intermediate-mass stars this mechanism first activates at the position of the bump in the luminosity function, and has been identified as a likely candidate for driving the slow mixing inferred to occur in these stars. One particularly important consequence of this process, which is driven by a molecular weight inversion, is the destruction of lithium. We show that the degree of lithium destruction, or in some cases production, is extremely sensitive to the numerical details of the stellar models. Within the standard 1D diffusion approximation to thermohaline mixing, we find that different evolution codes, with their default numerical schemes, can produce lithium abundances that differ from one another by many orders of magnitude. This disagreement is worse for faster mixing. We perform experiments with four independent stellar evolution codes, and derive conditions for the spatial and temporal resolution required for a converged numerical solution. The results are extremely sensitive to the time-steps used. We find that predicted lithium abundances published in the literature until now should be treated with caution.

Description: Globular clusters display significant variations in their light-element content, pointing to the existence of a second stellar generation formed from the ejecta of an earlier generation. The nature of these internal polluters is still a matter of debate: the two most popular scenarios indicate intermediate-mass asymptotic giant branch (IM-AGB) stars and fast rotating massive stars. Abundances determination for some key elements can help distinguish between these competitor candidates. We present in this paper Y abundances for a sample of 103 red giant branch stars in NGC 6121. Within measurement errors, we find that the [Y/Fe] is constant in this cluster contrary to a recent suggestion. For a subsample of six stars we also find [Rb/Fe] to be constant, consistent with previous studies showing no variation in other s-process elements. We also present a new set of stellar yields for IM-AGB stellar models of 5 and 6 M , including heavy element s-process abundances. The uncertainties on the mass-loss rate, the mixing length parameter and the nuclear reaction rates have a major impact on the stellar abundances. Within the IM-AGB pollution scenario, the constant abundance of heavy elements inside the cluster requires a marginal s-process efficiency in IM-AGB stars. Such a constrain could still be satisfied by the present models assuming a stronger mass-loss rate. The uncertainties mentioned above are limiting the predictive power of IM-AGB models. For these reasons, at the moment we are not able to clearly rule out their role as main polluters of the second population stars in globular clusters.