Evolution of massive AGB stars

Abstract

Context. Massive AGB (hereafter super-AGB or SAGB) stars ignite carbon off-center and have initial masses ranging between M up , the minimum initial mass for carbon ignition, and M mas the minimum mass for the formation of an iron core collapse supernova. In this mass interval, stars more massive than M n will undergo an electron capture supernova (EC-SN). Aims. We study the fate and selected evolutionary properties of SAGB stars up to the end of the carbon burning phase as a function of metallicity and core overshooting. Methods. The method is based on the analysis of a large set of stellar models covering the mass range 5−13 M and calculated for 7 different metallicities between Z = 10 −5 and twice solar. Core overshooting was considered in two subsets for Z = 10 −4 and 0.02. The models are available online at http://www-astro.ulb.ac.be/∼siess/database.html. The fate of SAGB stars is investigated through a parametric model which allows us to assess the role of mass loss and of the third dredge-up. Results. Our main results can be summarized as follows: a) prior to C-burning, the evolution of SAGB stars is very similar to that of intermediate-mass stars, being more luminous, b) SAGB stars suffer a large He enrichment at the end of the second dredge-up, c) the limiting masses M up , M n and M mas present a nonlinear behavior with Z, characterized by a minimum around Z = 10 −4 , d) the values of M up , M n and M mas are decreased by ∼2 M when core overshooting is considered, e) our models predict a minimum oxygenneon white dwarf mass of ∼1.05 M , f) the determination of M n is highly dependent on the mass loss and core growth rates, g) the evolutionary channel for EC-SN is limited to a very narrow mass range of < ∼ 1−1.5 M width and this mass window can be further decreased if some metallicity scaling factor is applied to the mass loss rate, h) the final fate of SAGB stars is connected to the second dredge-up and this property allowed us to refine the initial mass range for the formation of EC-SN. We find that if the ratio of the mass loss rate to the core growth rate averaged over the post carbon-burning evolution ζ = Ṁ loss /Ṁ core is greater than about 70−90, the evolutionary path to EC-SN is not accessible.

Asymptotic Giant Branch Evolution and the Initial-Final Mass Relation of Single CO White Dwarfs

“…They will proceed through the so-called super-AGB phase, ending as either ONe white dwarfs or electron-capture supernovae, depending on the core mass. Both critical masses M up and M mas heavily depend on uncertain aspects of stellar models; in particular present predictions locate M up in the range ∼ 6 − 8 M (Siess 2007). …”

Tracing the Evolution of High-Redshift Galaxies Using Stellar Abundances

“…Intermediate-mass stars are presumed to be the main producers of heavy s-process nuclidesand also contribute substantially to the yields of several other nuclides, most notably carbon and nitrogen, during their AGB phase (Siess 2007). Karakas & Lattanzio (2007) and Karakas (2010) calculated detailed stellar models and post-processed nucleosynthetic data to produce AGB yields.…”

“…Super-AGB stars are defined by a specific mass range between the minimum mass for carbon ignition and the mass limit above which the star ignites neon at its centerand evolves through all nuclear burning stages up to an iron-core-collapse SN (Siess 2007). The mass range of super-AGB stars varies with metallicity, with a lower limit between 7.5 and 9 M e and an upper limit of approximately 11 M e (Siess 2007(Siess , 2010.…”

The Formation and Evolution of ONe White Dwarfs: Prospects for Accretion Induced Collapse

“…The rate at which the H-He discontinuity moves outward and the degree of dredge-up determine the chemical enrichment of the envelope, and therefore the ultimate fate of SAGB stars. This is because the mass-loss rate depends on the metallicity of the envelope, which depends on the prescription adopted for determining the position of the inner edge of the convective envelope (Gil-Pons et al 2005, 2007. However, in contrast to what occurs for normal AGB stars, the number of thermal pulses necessary to remove the massive envelope is in this case very large, ∼10 3 (Poelarends et al 2008).…”

“…In the following I summarize the main results of self-consistent fully evolutionary studies of these stars. However, for the sake of conciseness only a brief overview of the very interesting physical phenomena occurring during the carbon burning phase in this mass interval will be given, and I refer 54 E. García-Berro the interested reader to the series of papers by Ritossa et al (1996), García-Berro et al (1997), Iben et al (1997), Ritossa et al (1999), the more recent studies of Siess (2006Siess ( , 2007Siess ( , 2009Siess ( , 2010 which confirm the main findings of the previous papers, and Poelarends et al (2008). Also, the possible outcomes of the evolution of SAGB stars will be analyzed, but I will not discuss here other interesting aspects, like the possible contribution of this range of masses to r-process nucleosynthesis (Ning et al 2007, Wanajo et al 2006.…”