Heavy elements in Globular Clusters: the role of AGB stars

Recent observations of heavy elements in Globular Clusters reveal intriguing deviations from the standard paradigm of the early galactic nucleosynthesis. If the r-process contamination is a common feature of halo stars, s-process enhancements are found in a few Globular Clusters only. We show that the combined pollution of AGB stars with mass ranging between 3 to 6 M$_\odot$ may account for most of the features of the s-process overabundance in M4 and M22. In these stars, the s process is a mixture of two different neutron-capture nucleosynthesis episodes. The first is due to the 13C(a,n)16O reaction and takes place during the interpulse periods. The second is due to the 22Ne(a,n)25Mg reaction and takes place in the convective zones generated by thermal pulses. The production of the heaviest s elements (from Ba to Pb) requires the first neutron burst, while the second produces large overabundances of light s (Sr, Y, Zr). The first mainly operates in the less-massive AGB stars, while the second dominates in the more-massive. From the heavy-s/light-s ratio, we derive that the pollution phase should last for $150\pm 50$ Myr, a period short enough compared to the formation timescale of the Globular Cluster system, but long enough to explain why the s-process pollution is observed in a few cases only. With few exceptions, our theoretical prediction provides a reasonable reproduction of the observed s-process abundances, from Sr to Hf. However, Ce is probably underproduced by our models, while Rb and Pb are overproduced. Possible solutions are discussed.Comment: Accepted by the Ap

On the need of the Light Elements Primary Process (LEPP)

Extant chemical evolution models underestimate the Galactic production of Sr, Y and Zr as well as the Solar System abundances of s-only isotopes with 90<A<130. To solve this problem, an additional (unknown) process has been invoked, the so-called LEPP (Light Element Primary Process). In this paper we investigate possible alternative solutions. Basing on Full Network Stellar evolutionary calculations, we investigate the effects on the Solar System s-only distribution induced by the inclusion of some commonly ignored physical processes (e.g. rotation) or by the variation of the treatment of convective overshoot, mass-loss and the efficiency of nuclear processes. Our main findings are: 1) at the epoch of the formation of the Solar System, our reference model produces super-solar abundances for the whole s-only distribution, even in the range 90<A<130; 2) within errors, the s-only distribution relative to 150Sm is flat; 3) the s-process contribution of the less massive AGB stars (M<1.5 M_SUN) as well as of the more massive ones (M>4.0 M_SUN) are negligible; 4) the inclusion of rotation implies a downward shift of the whole distribution with an higher efficiency for the heavy s-only isotopes, leading to a flatter s-only distribution; 5) different prescriptions on convection or mass-loss produce nearly rigid shifts of the whole distribution. In summary, a variation of the standard paradigm of AGB nucleosynthesis would allow to reconcile models predictions with Solar System s-only abundances. Nonetheless, the LEPP cannot be definitely ruled out, because of the uncertainties still affecting stellar and Galactic chemical evolution models.Comment: Accepted for publication on Ap

Observational Properties of SNe Ia Progenitors Close to the Explosion

We determine the expected signal in various observational bands of Supernovae Ia progenitors just before the explosion by assuming the rotating Double Degenerate scenario. Our results are valid also for all the evolutionary scenarios invoking rotation as the driving mechanism of the accretion process as well as the evolution up to the explosion. We find that the observational properties depend mainly on the mass of the exploding object, even if the angular momentum evolution after the end of the mass accretion phase and before the onset of C-burning plays a non-negligible role. Just before the explosion the magnitude M_V ranges between 9 and 11 mag, while the colour (F225W-F555W) is about -1.64 mag. The photometric properties remain constant for a few decades before the explosion. During the last few months the luminosity decreases very rapidly. The corresponding decline in the optical bands varies from few hundredths up to one magnitude, the exact value depending on both the WD total mass and the braking efficiency at the end of the mass transfer. This feature is related to the exponentially increasing energy production which drives the formation of a convective core rapidly extending over a large part of the exploding object. Also a drop in the angular velocity occurs. We find that observations in the soft X band (0.5 -2 keV) may be used to check if the SNe Ia progenitors evolution up to explosion is driven by rotation and, hence, to discriminate among different progenitor scenarios.Comment: 8 pages, 6 figures, 2 tables. Accepted for the publication on MNRA

Nucleation of small silicon carbide dust clusters in AGB stars

Silicon carbide (SiC) grains are a major dust component in carbon-rich AGB stars. The formation pathways of these grains are, however, not fully understood.\ We calculate ground states and energetically low-lying structures of (SiC)$_n$, $n=1,16$ clusters by means of simulated annealing (SA) and Monte Carlo simulations of seed structures and subsequent quantum-mechanical calculations on the density functional level of theory. We derive the infrared (IR) spectra of these clusters and compare the IR signatures to observational and laboratory data.\ According to energetic considerations, we evaluate the viability of SiC cluster growth at several densities and temperatures, characterising various locations and evolutionary states in circumstellar envelopes.\ We discover new, energetically low-lying structures for Si$_{4}$C$_{4}$, Si$_{5}$C$_{5}$, Si$_{15}$C$_{15}$ and Si$_{16}$C$_{16}$, and new ground states for Si$_{10}$C$_{10}$ and Si$_{15}$C$_{15}$. The clusters with carbon-segregated substructures tend to be more stable by 4-9 eV than their bulk-like isomers with alternating Si-C bonds. However, we find ground states with cage ("bucky"-like) geometries for Si$_{12}$C$_{12}$ and Si$_{16}$C$_{16}$ and low-lying, stable cage structures for n $\ge$ 12. The latter findings indicate thus a regime of clusters sizes that differs from small clusters as well as from large-scale crystals. Thus, and owing to their stability and geometry, the latter clusters may mark a transition from a quantum-confined cluster regime to crystalline, solid bulk-material. The calculated vibrational IR spectra of the ground-state SiC clusters shows significant emission. They include the 10-13 $\mu$m wavelength range and the 11.3 $\mu$m feature inferred from laboratory measurements and observations, respectively, though the overall intensities are rather low.Comment: 16 pages, 25 figures, 3 tables, accepted for publication in Ap

Pre-explosive Accretion and Simmering Phases of SNe Ia

This publication is part of the project I + D + I PGC2018-095317-B-C21 funded by MICIN/AEI/10.13039/501100011033 and FEDER "A way of doing Europe" (E.B. and I.D.); L.P. and O.S. acknowledge financial support from the INAF-mainstream project "Type Ia Supernovae and their Parent Galaxies: Expected Results from LSST." O.S. and L.P. acknowledge their participation to the V:ANS project (Vanvitelli program on standard candles in astrophysics: Atomic and Nuclear physics in SNIa) supported by the Vanvitelli University.In accreting white dwarfs (WDs) approaching the Chandrasekhar limit, hydrostatic carbon burning precedes the dynamical breakout. During this simmering phase, e-captures are energetically favored in the central region of the star, while beta-decay are favored more outside, and the two zones are connected by a growing convective instability. We analyze the interplay between weak interactions and convection, the so-called convective URCA process, during the simmering phase of Type Ia supernovae (SNe Ia) progenitors and its effects on the physical and chemical properties at the explosion epoch. At variance with previous studies, we find that the convective core powered by the carbon burning remains confined within the (21)(Ne,F) URCA shell. As a result, a much larger amount of carbon has to be consumed before the explosion that eventually occurs at larger density than previously estimated. In addition, we find that the extension of the convective core and its average neutronization depend on the the WD progenitor's initial metallicity. For the average neutronization in the convective core at the explosion epoch, we obtain (eta) over bar (exp) = (1.094 +/- 0.143) x 10(-3) + (9.168 +/- 0.677) x 10(-2) x Z. Outside the convective core, the neutronization is instead determined by the initial amount of C + N + O in the progenitor star. Since S, Ca, Cr, and Mn, the elements usually exploited to evaluate the pre-explosive neutronization, are mainly produced outside the heavily neutronized core, the problem of too high metallicity estimated for the progenitors of the historical Tycho and Kepler SNe Ia remains unsolved.FEDER "A way of doing Europe"INAF-mainstream project "Type Ia Supernovae and their Parent Galaxies: Expected Results from LSST"Vanvitelli University PGC2018-095317-B-C21MICIN/AEI/10.13039/50110001103

Extremely Metal-Poor Asymptotic Giant Branch Stars

Little is known about the first stars, but hints on this stellar population can be derived from the peculiar chemical composition of the most metal-poor objects in the Milky Way and in resolved stellar populations of nearby galaxies. In this paper, we review the evolution and nucleosynthesis of metal-poor and extremely metal-poor (EMP) stars with low and intermediate masses. In particular, new models of 6 M⊙ with three different levels of metallicity, namely Z=10−4, 10−6 and 10−10, are presented. In addition, we illustrate the results obtained for a 2 M⊙, Z=10−5 model. All these models have been computed by means of the latest version of the FuNS code. We adopted a fully coupled scheme of solutions for the complete set of differential equations describing the evolution of the physical structure and the chemical abundances, as modified by nuclear processes and convective mixing. The scarcity of CNO in the material from which these stars formed significantly affects their evolution, their final fate and their contribution to the chemical pollution of the ISM in primordial galaxies. We show the potential of these models for the interpretation of the composition of EMP stars, with particular emphasis on CEMP stars

On the very long term evolutionary behavior of hydrogen-accreting Low-Mass CO white dwarfs

Hydrogen-rich matter has been added to a CO white dwarf of initial mass 0.516 \msun at the rates $10^{-8}$ and $2\times 10^{-8}$ \msun \yrm1, and results are compared with those for a white dwarf of the same initial mass which accretes pure helium at the same rates. For the chosen accretion rates, hydrogen burns in a series of recurrent mild flashes and the ashes of hydrogen burning build up a helium layer at the base of which a He flash eventually occurs. In previous studies involving accretion at higher rates and including initially more massive WDs, the diffusion of energy inward from the H shell-flashing region contributes to the increase in the temperature at the base of the helium layer, and the mass of the helium layer when the He flash begins is significantly smaller than in a comparison model accreting pure helium; the He shell flash is not strong enough to develop into a supernova explosion. In contrast, for the conditions adopted here, the temperature at the base of the He layer becomes gradually independent of the deposition of energy by H shell flashes, and the mass of the He layer when the He flash occurs is a function only of the accretion rate, independent of the hydrogen content of the accreted matter. When the He flash takes place, due to the high degeneracy at the base of the He layer, temperatures in the flashing zone will rise without a corresponding increase in pressure, nuclear burning will continue until nuclear statistical equilibrium is achieved; the model will become a supernova, but not of the classical type Ia variety.Comment: 14 pages and 3 Postscript figures, Accepted for publication on ApJ Letter

The Influence of N14(e-,nu)C14(alpha,gamma)O18 reaction on the He-Ignition in Degenerate Physical Conditions

The importance of NCO chain on the onset of the He-flash in degenerate physical conditions has been reevaluated. We find that low-mass, metal-rich (Z $\ge$ 0.001) structures climbing the Red Giant Branch do never attain the physical conditions suitable for the onset of this chain, while at lower metallicities the energy contribution provided by NCO reaction is too low to affect the onset of the central He-flash. At the same time, our evolutionary models suggest that for a Carbon-Oxygen White Dwarf of mass M_{WD}=0.6 M_sun accreting He-rich matter, directly or as a by-product of an overlying H-burning shell, at rates suitable for a dynamical He-flash, the NCO energy contribution is not able to keep hot enough the He-shell and in turn to avoid the occurrence of a strong electron degeneracy and the ensuing final explosion.Comment: 15 pages, 3 tables, 10 figure, to appear in Ap

Carbon-Oxygen White Dwarf Accreting CO-Rich Matter. II. Self-Regulating Accretion Process up to the Explosive Stage

We investigate the effect of rotation on the evolution of double-degenerate white dwarf systems, which are possible progenitors of Type Ia supernovae. We assume that prior to merging, the two white dwarfs rotate synchronously at the orbital frequency and that in the merger process, the lighter white dwarf is transformed into a thick disk from which the more massive white dwarf initially accretes at a very high rate (~10-5 M☉ yr-1). Because of the lifting effect of rotation, the accreting white dwarf expands until the gravitational acceleration and centripetal acceleration required for binding at the surface become equal, initiating a Roche instability. The white dwarf continues to accrete matter from the disk, but at a rate that is determined by the balance between two competing processes operating in outer layers: (1) heating, expansion, and spin-up due to accretion and (2) cooling and contraction due to thermal diffusion. The balance produces an accretion rate such that the angular velocity of the white dwarf ωWD and the break-up angular velocity ωcr remain equal. Because of the deposition of angular momentum by accreted matter and the contraction of the accreting star, ωWD increases continuously until the rotational energy reaches about 14% of the gravitational binding energy; then, another instability sets in: the structure is forced to adopt an elliptical shape and emit gravitational waves. Thereafter, a balance between the rate of deposition of angular momentum by accreted matter and the rate of loss of angular momentum by gravitational waves produces a nearly constant or "plateau" accretion rate of ~4 × 10-7 M☉ yr-1. The mass of the accreting white dwarf can increase up to and beyond the Chandresekhar mass limit for nonrotating white dwarfs before carbon ignition occurs. Independent of the initial value of the accretion rate, the physical conditions suitable for carbon ignition are achieved at the center of the accreting white dwarf and, because of the high electron degeneracy, the final outcome is an event of SN Ia proportions. Our results apply to merged binary white dwarf systems which, at the onset of explosive carbon ignition, have a total mass in the range 1.4-1.5 M☉