Pilar Gil-Pons

Super- and massive AGB stars - IV. Final fates initial-to-final mass relation

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 circle dot 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 approximate to 0.2 M circle dot. 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.

Super and massive AGB stars – II. Nucleosynthesis and yields – Z = 0.02, 0.008 and 0.004

We have computed detailed evolution and nucleosynthesis models for super and massive AGB stars over the mass range 6.5-9.0 Msun in divisions of 0.5 Msun 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 alter 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 He4, Li7 (depending on the mass-loss formulation) C13, N14, O17, Na23, Mg25, as well the radioactive isotope Al26 in agreement with previous investigation. In addition our results show enrichment of Ne22, Mg26 and Fe60, 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 Galaxys inventory of the heavier Mg isotopes, N14, Li7 and Al27.

Super and massive AGB stars - III. nucleosynthesis in metal-poor and very metal-poor stars - Z = 0.001 and 0.0001

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 and 14 N, 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, 27 Al 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 analyze 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.

The frequency of occurrence of novae hosting an ONe white dwarf

In this paper, we revisit the problem of the determination of the frequency of occurrence of galactic nova outbursts which involve an oxygen-neon (ONe) white dwarf. The improvement with respect to previous work on the subject derives from the fact that we use the results that our evolutionary calculations provide for the final mass and for the chemical profiles of intermediate-to-massive primary components of close binary systems. In particular, the final evolutionary stages, such as the carbon burning phase, have been carefully followed for the whole range of masses of interest. The chemical profiles obtained with our evolutionary code are of interest in determining the chemical composition of the ejecta after being processed through the thermonuclear runaway, although such other factors as the eciency of the mixing between the accreted material and that of the underlying white dwarf must also be considered. In our calculations of the frequency of occurrence of nova outbursts in- volving an ONe white dwarf, we also take into account the observational selection eects introduced by the dierent recurrence times of the outbursts and by the spatial distribution of novae. In spite of the very dierent evolutionary sequences, we find that approximately 1/3 of the observed nova outbursts should involve an oxygen-neon white dwarf, in agreement with previous theoretical estimates.

Transition of the stellar initial mass function explored using binary population synthesis

The stellar initial mass function (IMF) plays a crucial role in determining the number of surviving stars in galaxies, the chemical composition of the interstellar medium, and the distribution of light in galaxies. A key unsolved question is whether the IMF is universal in time and space. Here we use state-of-the-art results of stellar evolution to show that the IMF of our Galaxy made a transition from an IMF dominated by massive stars to the present-day IMF at an early phase of the Galaxy formation. Updated results from stellar evolution in a wide range of metallicities have been implemented in a binary population synthesis code, and compared with the observations of carbon-enhanced metal-poor (CEMP) stars in our Galaxy. We find that applying the present-day IMF to Galactic halo stars causes serious contradictions with four observable quantities connected with the evolution of AGB stars. Furthermore, a comparison between our calculations and the observations of CEMP stars may help us to constrain the transition metallicity for the IMF which we tentatively set at [Fe/H] = -2. A novelty of the current study is the inclusion of mass loss suppression in intermediate-mass AGB stars at low-metallicity. This significantly reduces the overproduction of nitrogen-enhanced stars that was a major problem in using the high-mass star dominated IMF in previous studies. Our results also demonstrate that the use of the present day IMF for all time in chemical evolution models results in the overproduction of Type I.5 supernovae. More data on stellar abundances will help to understand how the IMF has changed and what caused such a transition.

The end of super AGB and massive AGB stars - I. The instabilities that determine the final mass of AGB stars

Context. The literature is rich in analysis and results related to thermally pulsing-asymptotic giant branch (TP-AGB) stars, but the problem of the instabilities that arise and cause the divergence of models during the late stages of their evolution is rarely addressed. Aims. We investigate the physical conditions, causes and consequences of the interruption in the calculations of massive AGB stars in the late thermally-pulsing AGB phase. Methods. We have thoroughly analysed the physical structure of a solar metallicity 8.5 Mstar and described the physical conditions at the base of the convective envelope (BCE) just prior to divergence. Results. We find that the local opacity maximum caused by M-shell electrons of Fe-group elements lead to the accumulation of an energy excess, to the departure of thermal equilibrium conditions at the base of the convective envelope and, eventually, to the divergence of the computed models. For the 8.5 Mcase we present in this work the divergence occurs when the envelope mass is about 2 M� . The remaining envelope masses range between somewhat less than 1 and more than 2 Mfor stars with initial masses between 7 and 10 Mand, therefore, our results are relevant for the evolution and yields of super-AGB stars. If the envelope is ejected as a consequence of the instability we are considering, the occurrence of electron-capture supernovae would be avoided at solar metallicity.

Carbon burning in intermediate-mass primordial stars

The evolution of a zero metallicity 9 Mstar is computed, analyzed and compared with that of a solar metallicity star of identical ZAMS mass. Our computations range from the main sequence until the formation of a massive oxygen-neon white dwarf. Special attention has been payed to carbon burning in conditions of partial degeneracy as well as to the subsequent thermally pulsing Super-AGB phase. The latter develops in a fashion very similar to that of a solar metallicity 9 Mstar, as a consequence of the significant enrichment in metals of the stellar envelope that ensues due to the so-called third dredge-up episode. The abundances in mass of the main isotopes in the final ONe core resulting from the evolution are X( 16 O) ≈ 0.59, X( 20 Ne) ≈ 0.28 and X( 24 Mg) ≈ 0.05. This core is surrounded by a 0.05 Mbuffer mainly composed of carbon and oxygen, and on top of it a He envelope of mass ∼10 −4 M� .

The first nova explosions

Classical novae are stellar explosions with an energy release that is only overcome by supernovae and γ-ray bursts. Theoretical and observational evidence suggests that these cataclysmic events are major sources of the Galactic 15N, 17O, and 13C, and contribute to the abundances of 7Li, and 26Al, with a likely nucleosynthetic endpoint around Ca. However, there are reasons to believe that these nuclear signatures have changed during the overall Galactic history. In this Letter, the first that addresses the nature of nova explosions in the most primitive, low-metallicity binaries, we show that primordial novae eject more massive envelopes and display a larger nuclear activity than classical novae, leading to a new type of explosion, halfway between a supernova and a nova. The ejected shells from the most violent, massive primordial novae yield large excesses of Ti and a likely nucleosynthetic endpoint around Cu-Zn, signatures never before associated with a nova-like explosion. We conclude that the chemical abundance pattern of the nova ejecta is strongly modified at low metallicities and suggest that primordial novae are a possible birthplace of a rare variety of presolar grains whose origin remains uncertain.

Evolution and CNO yields of Z = 10-5 stars and possible effects on carbon-enhanced metal-poor production

Aims. Our main goals are to get a deeper insight into the evolution and final fates of intermediate-mass, extremely metal-poor (EMP) stars. We also aim to investigate the C, N, and O yields of these stars. Methods. Using the Monash University Stellar Evolution code MONSTAR we computed and analysed the evolution of stars of metallicity Z = 10-5 and masses between 4 and 9 M?, from their main sequence until the late thermally pulsing (super) asymptotic giant branch, TP-(S)AGB phase. Results. Our model stars experience a strong C, N, and O envelope enrichment either due to the second dredge-up process, the dredge-out phenomenon, or the third dredge-up early during the TP-(S)AGB phase. Their late evolution is therefore similar to that of higher metallicity objects. When using a standard prescription for the mass loss rates during the TP-(S)AGB phase, the computed stars are able to lose most of their envelopes before their cores reach the Chandrasekhar mass (mCh), so our standard models do not predict the occurrence of SNI1/2 for Z = 10-5 stars. However, we find that the reduction of only one order of magnitude in the mass-loss rates, which are particularly uncertain at this metallicity, would prevent the complete ejection of the envelope, allowing the stars to either explode as an SNI1/2 or become an electron-capture SN. Our calculations stop due to an instability near the base of the convective envelope that hampers further convergence and leaves remnant envelope masses between 0.25 M? for our 4 M? model and 1.5 M? for our 9 M? model. We present two sets of C, N, and O yields derived from our full calculations and computed under two different assumptions, namely, that the instability causes a practically instant loss of the remnant envelope or that the stars recover and proceed with further thermal pulses. Conclusions. Our results have implications for the early chemical evolution of the Universe and might provide another piece for the puzzle of the carbon-enhanced EMP star problem.

The Impact of the Chemical Stratification of White Dwarfs on the Classification of Classical Novae

We analyze the impact of the initial abundances of the underlying white dwarf core on the nucleosynthesis accompanying classical nova outbursts, in the framework of hydrodynamic models of the explosion. Specifically, we take into account the chemical stratification of the white dwarf. It turns out that the presence of a thick CO buffer on top of the ONe-rich core, as results from detailed calculations of previous evolution of the progenitor star, may lead to significant amounts of both 7Li and 26Al, after an outburst that, because of the lack of neon isotopes in the ejecta, would be misclassified as a non-neon nova (i.e., CO nova).