Evolution of massive AGB stars

Abstract

We present new computations of the evolution of solar metallicity stars in the mass range 9−12 M . This first paper of a series focuses on the propagation of the carbon burning flame front and provides a detailed analysis of the structural evolution up to the formation of the neonoxygen core. Our calculations which do not include overshooting indicate that off-center carbon ignition is restricted to a small mass range between 9.0 and 11.3 M . The chemical imprints of the first and second dredge-ups on the surface composition are analyzed and compared to "standard" less massive stars. It results that, aside from being more luminous and slightly bluer in the HR diagram, massive AGB stars are almost indistinguishable from their lower mass counterparts, as far as the chemical composition is concerned. During the second dredge-up, we note however that the envelope penetrates deeper into the He burning shell than lower mass stars. Our simulations indicate that above ∼11.0 M , the depth of the second dredge up is considerably reduced, marking the transition between low and massive stars. We also investigate the effects of the nuclear uncertainties associated with 12 C + 12 C reactions and show that it has a little impact on the core composition. Finally we describe the nucleosynthesis and chemical structure of the newly formed neon-oxygen core.

Evolution of massive AGB stars

“…Computation of SAGB stars requires specific rezoning strategies, during both the flame propagation (Siess 2006) and the interpulse phase where a high spatial resolution is needed at the base of the convective envelope. In this very thin region of a few 10 −5 M (see Sect.…”

“…As a result, the 2DUP is responsible for a very strong envelope enrichment in helium and, to a lower extend, of 23 Na. The dredgeout phenomenon, which occurs in the most massive SAGB stars at the end of carbon burning Siess 2006), also increases the envelope abundances of 4 He and 12 C significantly and at the end if this event the star becomes C-rich (C/O > 1).…”

“…Garcia-Berro et al 1997;Siess 2006), the effects of overshooting (Gil-Pons et al 2007), semiconvection (Poelarends et al 2008), or thermohaline mixing (Siess 2009). The start of electron captures reactions in the oxygen-neon (ONe) core and the URCA process have also been investigated (Ritossa et al 1999), and non-solar models were computed and their main properties analysed (e.g.…”

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

“…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.…”

Helium-rich stars in globular clusters: constraints for self-enrichment by massive stars

“…To assert these implications we have computed a grid of low-mass stellar models from 0.2 to 0.9 M at a metallicity of Z = 0.0005 (similar to the metal-poor globular cluster NGC 6752) for initial He mass fraction between 0.245 and 0.72 with the stellar evolution code STAREVOL V2.92 (see Siess et al 2000;Siess 2006, for more details). These models have been computed without any kind of mixing except for an instantaneous mixing in convection zones.…”