Super-AGB Stars and their Role as Electron Capture Supernova Progenitors (original) (raw)
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Supernovae from massive AGB stars
We present new computations of the final fate of massive AGB-stars. These stars form ONeMg cores after a phase of carbon burning and are called Super AGB stars (SAGB). Detailed stellar evolutionary models until the thermally pulsing AGB were computed using three different stellar evolution codes. The subsequent evolution was modeled by a synthetic code with different options for mass loss rate and dredge-up efficiency. We find a range of initial masses between 9.0 M and 9.25 M for which we expect an SAGB star to explode as an electron capture supernova. Our models allow a detailed assessment of the envelope properties of electron-capture supernova progenitors. SAGB stars with lower initial masses are the progenitors of ONeMg white dwarf, while more massive stars ignite (off-center) neon burning and follow the classical core-collapse path.
Super- and massive AGB stars - IV. Final fates - initial-to-final mass relation
Monthly Notices of the Royal Astronomical Society, 2014
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 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 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 ≈ 0.2 M ⊙. 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.
Primordial to extremely metal-poor AGB and Super-AGB stars: White dwarf or supernova progenitors?
Publications of the Astronomical Society of Australia
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. Howev...
Super and Massive AGB Stars: II- Nucleosynthesis and
2013
We have computed detailed evolution and nucleosynthesis models for super and massive 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 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 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.
The end of super AGB and massive AGB stars
Astronomy & Astrophysics, 2012
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 M star 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 M case 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 M for stars with initial masses between 7 and 10 M and, 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.
Observational constraints on the nucleosynthesis in the more massive AGB star
Proceedings of XIII Nuclei in the Cosmos — PoS(NIC XIII), 2015
Most of the stars (M < 8 solar mass, M sun) in the Universe end their lives with a phase of strong mass loss and experience thermal pulses on the Asymptotic Giant Branch (AGB). They * Speaker. † D.A.G.H. acknowledges support provided by the Spanish Ministry of Economy and Competitiveness under grant
Astronomy & Astrophysics, 2013
We investigate the fundamental properties of core-collapse Supernova (SN) progenitors from single stars at solar metallicity. For this purpose, we combine Geneva stellar evolutionary models with initial masses of M ini = 20 − 120 M ⊙ with atmospheric/wind models using the radiative transfer code CMFGEN. We provide synthetic photometry and high-resolution spectra of hot stars at the pre-SN stage. For models with M ini = 9 − 20 M ⊙ , we supplement our analysis using publicly available MARCS model atmospheres of RSGs to estimate their synthetic photometry. We employ well-established observational criteria of spectroscopic classification and find that massive stars, depending on their initial mass and rotation, end their lives as red supergiants (RSG), yellow hypergiants (YHG), luminous blue variables (LBV), and Wolf-Rayet (WR) stars of the WN and WO spectral types. For rotating models, we obtained the following types of SN progenitors:
Super and massive AGB stars - II. Nucleosynthesis and yields - Z = 0.02, 0.008 and 0.004
Monthly Notices of the Royal Astronomical Society, 2014
We have computed detailed evolution and nucleosynthesis models for super and massive 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 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 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.
Advanced Burning Stages and Fate of 8-10M☉STARS
The Astrophysical Journal, 2013
The stellar mass range 8 M/M 12 corresponds to the most massive AGB stars and the most numerous massive stars. It is host to a variety of supernova progenitors and is therefore very important for galactic chemical evolution and stellar population studies. In this paper, we study the transition from super-AGB star to massive star and find that a propagating neon-oxygen burning shell is common to both the most massive electron capture supernova (EC-SN) progenitors and the lowest mass iron-core collapse supernova (FeCCSN) progenitors. Of the models that ignite neon burning off-center, the 9.5 M star would evolve to an FeCCSN after the neon-burning shell propagates to the center, as in previous studies. The neon-burning shell in the 8.8 M model, however, fails to reach the center as the URCA process and an extended (0.6 M) region of low Y e (0.48) in the outer part of the core begin to dominate the late evolution; the model evolves to an EC-SN. This is the first study to follow the most massive EC-SN progenitors to collapse, representing an evolutionary path to EC-SN in addition to that from SAGB stars undergoing thermal pulses. We also present models of an 8.75 M super-AGB star through its entire thermal pulse phase until electron captures on 20 Ne begin at its center and of a 12 M star up to the iron core collapse. We discuss key uncertainties and how the different pathways to collapse affect the pre-supernova structure. Finally, we compare our results to the observed neutron star mass distribution.