Super-AGB Stars and their role as Electron Capture Supernova progenitors

We review the lives, deaths and nucleosynthetic signatures of intermediate mass stars in the range approximately 6.5-12 Msun, which form super-AGB stars near the end of their lives. We examine the critical mass boundaries both between different types of massive white dwarfs (CO, CO-Ne, ONe) and between white dwarfs and supernovae and discuss the relative fraction of super-AGB stars that end life as either an ONe white dwarf or as a neutron star (or an ONeFe white dwarf), after undergoing an electron capture supernova. We also discuss the contribution of the other potential single-star channels to electron-capture supernovae, that of the failed massive stars. We describe the factors that influence these different final fates and mass limits, such as composition, the efficiency of convection, rotation, nuclear reaction rates, mass loss rates, and third dredge-up efficiency. We stress the importance of the binary evolution channels for producing electron-capture supernovae. We discuss recent nucleosynthesis calculations and elemental yield results and present a new set of s-process heavy element yield predictions. We assess the contribution from super-AGB star nucleosynthesis in a Galactic perspective, and consider the (super-)AGB scenario in the context of the multiple stellar populations seen in globular clusters. A brief summary of recent works on dust production is included. Lastly we conclude with a discussion of the observational constraints and potential future advances for study into these stars on the low mass/high mass star boundary.Comment: 28 pages, 11 figures. Invited review for Publications of the Astronomical Society of Australia, to be published in special issue on "Electron Capture Supernovae". Submitte

The upper mass limit for the formation of TP-SAGB stars and the dredge-out phenomenon

We have computed the evolution of Super-AGB stars from the main sequence and up to a few hundred thermal pulses, with special attention to the low metallicity cases (Z = 1010; 105; 104 and 103). Our computations have been performed using time– dependent mixing and new opacity tables that admit variations in the abundances of carbon and oxygen. By following the evolution along the main central burning stages and the early TP-SAGB, we resolve the upper mass limits for the formation of TP-SAGB stars and determine the mass range at which the dredge-out phenomenon occurs. This phenomenon involves the merger of a convective shell sustained by helium burning at the top of the degenerate core with the hydrogen–rich convective envelope and the occurrence of a hydrogen flash. The dredge–out allows elements synthesised through helium burning to be transported to the stellar surfaces and therefore it can a ect the initial composition of the TP-SAGB stars.Peer ReviewedPostprint (published version

Evolution and nucleosynthesis of helium-rich asymptotic giant branch models

There is now strong evidence that some stars have been born with He mass fractions as high as $Y \approx 0.40$ (e.g., in $\omega$ Centauri). However, the advanced evolution, chemical yields, and final fates of He-rich stars are largely unexplored. We investigate the consequences of He-enhancement on the evolution and nucleosynthesis of intermediate-mass asymptotic giant branch (AGB) models of 3, 4, 5, and 6 M$_\odot$ with a metallicity of $Z = 0.0006$ ([Fe/H] $\approx -1.4$). We compare models with He-enhanced compositions ($Y=0.30, 0.35, 0.40$) to those with primordial He ($Y=0.24$). We find that the minimum initial mass for C burning and super-AGB stars with CO(Ne) or ONe cores decreases from above our highest mass of 6 M$_\odot$ to $\sim$ 4-5 M$_\odot$ with $Y=0.40$. We also model the production of trans-Fe elements via the slow neutron-capture process (s-process). He-enhancement substantially reduces the third dredge-up efficiency and the stellar yields of s-process elements (e.g., 90% less Ba for 6 M$_\odot$, $Y=0.40$). An exception occurs for 3 M$_\odot$, where the near-doubling in the number of thermal pulses with $Y=0.40$ leads to $\sim$ 50% higher yields of Ba-peak elements and Pb if the $^{13}$C neutron source is included. However, the thinner intershell and increased temperatures at the base of the convective envelope with $Y=0.40$ probably inhibit the $^{13}$C neutron source at this mass. Future chemical evolution models with our yields might explain the evolution of s-process elements among He-rich stars in $\omega$ Centauri.Comment: 21 pages, 16 figures, accepted for publication by MNRAS. Stellar yields included as online data table

Hiding in plain sight - red supergiant imposters? Super-AGB stars

Super Asymptotic Giant Branch (Super-AGB) stars reside in the mass range ˜ 6.5-10 M¿ and bridge the divide between low/intermediate-mass and massive stars. They are characterised by off-centre carbon ignition prior to a thermally pulsing phase which can consist of many tens to even thousands of thermal pulses. With their high luminosities and very large, cool, red stellar envelopes, these stars appear seemingly identical to their slightly more massive red supergiant counterparts. Due to their similarities, super-AGB stars may therefore act as stellar imposters and contaminate red supergiant surveys. The final fate of super-AGB stars is also quite uncertain and depends primarily on the competition between the core growth and mass-loss rates. If the stellar envelope is removed prior to the core reaching ˜ 1.375 M¿, an O-Ne white dwarf will remain, otherwise the star will undergo an electron-capture supernova (EC-SN) leaving behind a neutron star. We determine the relative fraction of super-AGB stars that end life as either an O-Ne white dwarf or as a neutron star, and provide a mass limit for the lowest mass supernova over a broad range of metallicities from the Z=0.02 to 0.0001.Peer ReviewedPostprint (published version

The upper-mass limit for the formation of super-agb stars and the dredge-out phenomenon

We have computed the evolution of Super-AGB stars from the main sequence and up to a few hundred thermal pulses, with special attention to the low metallicity cases (Z = 1010; 105; 104 and 103). Our computations have been performed using time– dependent mixing and new opacity tables that admit variations in the abundances of carbon and oxygen. By following the evolution along the main central burning stages and the early TP-SAGB, we resolve the upper mass limits for the formation of TP-SAGB stars and determine the mass range at which the dredge-out phenomenon occurs. This phenomenon involves the merger of a convective shell sustained by helium burning at the top of the degenerate core with the hydrogen–rich convective envelope and the occurrence of a hydrogen flash. The dredge–out allows elements synthesised through helium burning to be transported to the stellar surfaces and therefore it can a ect the initial composition of the TP-SAGB stars.Peer ReviewedPostprint (published version

Transition of the Stellar Initial Mass Function Explored with 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.Comment: 8 pages, 2 figures, accepted by MNRAS Lette

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-AGB and massive AGB star models are in the mass range between 5.0 and 10.0 Msun 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 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 electron-capture supernova (EC-SN) rates for lower metallicities which are significantly lower than existing values from parametric studies in the literature. We conclude 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 approximately 0.2 Msun. This implies that between ~ 2-5 per cent of all gravitational collapse supernova are EC-SNe in the metallicity range Z=0.02 to 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.Comment: 15 pages, 7 figures, accepted for publication in MNRA

Production of short-lived radioactive nuclei in Super Asymptotic Giant Branch stars

Peer ReviewedPostprint (published version

Primordial to extremely metal-poor AGB and Super-AGB stars: White dwarf or supernova progenitors?

Getting a better understanding of the evolution and nucleosynthetic yields of the most metal-poor stars ( Z ¿ 10 ¿5 ) is critical because they are part of the big picture of the history of the primitive universe. Yet many of the remaining unknowns of stellar evolution lie in the birth, life, and death of these objects. We review stellar evolution of intermediate-mass Z = 10 ¿5 models existing in the literature, with a particular focus on the problem of their final fates. We emphasise the importance of the mixing episodes between the stellar envelope and the nuclearly processed core, which occur after stars exhaust their central He (second dredge-up and dredge-out episodes). The depth and efficiency of these episodes are critical to determine the mass limits for the formation of electron-capture SNe. Our knowledge of these phenomena is not complete because they are strongly affected by the choice of input physics. These uncertainties affect stars in all mass and metallicity ranges. However, difficulties in calibration pose additional challenges in the case of the most metal-poor stars. We also consider the alternative SN I1/2 channel to form SNe out of the most metal-poor intermediate-mass objects. In this case, it is critical to understand the thermally pulsing Asymptotic Giant Branch evolution until the late stages. Efficient second dredge-up and, later, third dredge-up episodes could be able to pollute stellar envelopes enough for the stars to undergo thermal pulses in a way very similar to that of higher initial Z objects. Inefficient second and/or third dredge-up may leave an almost pristine envelope, unable to sustain strong stellar winds. This may allow the H-exhausted core to grow to the Chandrasekhar mass before the envelope is completely lost, and thus let the star explode as an SN I1/2. After reviewing the information available on these two possible channels for the formation of SNe, we discuss existing nucleosynthetic yields of stars of metallicity Z = 10 ¿5 and present an example of nucleosynthetic calculations for a thermally pulsing Super-Asymptotic Giant Branch star of Z = 10 ¿5 . We compare theoretical predictions with observations of the lowest [Fe/H] objects detected. The review closes by discussing current open questions as well as possible fruitful avenues for future research.Peer ReviewedPostprint (author's final draft