Super asymptotic giant branch stars. I - Evolution code comparison

Abstract

We present an extensive set of detailed stellar models in the mass range 7.7–10.5 M⊙ over the metallicity range Z= 10−5–0.02. These models were produced using the Monash University version of the Mount Stromlo Stellar Structure Program (monstar) and follow the evolution from the pre‐main sequence to the first thermal pulse of these super asymptotic giant branch stars. A quantitative comparison is made to the study of Siess. Prior to this study, only qualitative comparisons and code validations existed in this critical mass range, and the large variations in the literature were largely unexplained. The comparison presented here is particularly detailed due to the standardization of the input physics, where possible. The minimum initial mass of star which ignites carbon, Mup, was found to agree within 0.2 M⊙ between the codes over the entire metallicity range. We find exceptional agreement in the model results between these two codes for all stages of evolution up to and including carbon burning. For additional comparison, we also present results from the evolve code, a modified version of the iben code as described in Gil‐Pons, Gutiérrez & García‐Berro for some important variables during the carbon burning phase. Several numerical tests showed that the carbon burning phase is weakly dependent on the spatial resolution but that inadequate temporal resolution alters the behaviour of the convective zones. We also discovered that stars just below Mup may experience a carbon flash that is not followed by the development of the flame. Such aborted carbon burning models thus preserve a CO core surrounding by a 0.2–0.3 M⊙ shell of partially burnt carbon material. We present a simplified algorithm for calculating carbon burning that only relies on tracking two species, 12C and 16O, but which tests show works quite accurately for the a wide range of initial masses and compositions.

HV2112, a Thorne–Żytkow object or a super asymptotic giant branch star

“…By a simple integration of the Kroupa, Tout & Gilmore (1993) mass function this implies that one SAGB progenitor should form per 453 M⊙ of total star formation. The SAGB stage lasts for a few 10 5 yr while the total lifetimes of the progenitors are a few 10 8 yr (Doherty et al 2010(Doherty et al , 2014. Hence roughly one out of every thousand stars of the correct mass should be a SAGB star at any given time.…”

The end of super AGB and massive AGB stars

“…) we know that the stellar evolution code STAREVOL (Siess 2010), presents the same convergence problems during late massive AGB and super-AGB evolution. Both codes and their respective super-AGB models have been described by Doherty et al (2010). It is worth noting that such convergence problems are common.…”

Explosion of white dwarfs harboring hybrid CONe cores

“…Recent analysis (Denissenkov et al 2013) has shown that such a stratified chemical structure of the core might occur because of the effect of convective boundary mixing, which is able to prevent the C-burning flame from reaching the central regions of the star. In fact, similar structures that are composed of a central carbon-rich region surrounded by an ONe (i.e., carbon-poor) zone have been found using different evolutionary codes, such as MONSTAR (Doherty et al 2010) or the code used by Garcia-Berro & Iben (1994). The question is nevertheless far from closed (see, e.g., Waldman & Barkat 2007), and it is interesting to analyze the outcome of different configurations of such CONe cores.…”

“…They are the result of carbon burning starting very far off-center in IMS with core masses of ∼1.04-1.06 M , and switching off before reaching the center because of cooling by neutrinos at the innermost region of the core. In the cases that concern us, i.e., the lightest objects able to ignite carbon, suitable conditions for carbon burning are not recovered after the first off-center burning episode (for instance, Ritossa et al 1999;Gil-Pons & García-Berro 2001;Siess 2006;Doherty et al 2010). …”

“…The size of the initial mass range of IMS that are able to end as a hybrid CONe WD is ∼0.1-0.2 M in standard calculations (e.g., Doherty et al 2010;Ventura & D'Antona 2011;Doherty et al 2015), but it can be as large as ∼0.4-1.0 M , depending on the strength of convective boundary mixing (Denissenkov et al 2013) and uncertainties in C-burning rates (Chen et al 2014). We note that CONe cores are found over a large variety of metallicities and also for a range of helium enrichments (Shingles et al 2015).…”