Thermohaline mixing in super-AGB stars

¹ Institut d'Astronomie et d'Astrophysique, Université Libre de Bruxelles (ULB), CP 226, 1050 Brussels, Belgium e-mail: siess@astro.ulb.ac.be
² Centre for Stellar and Planetary Astrophysics, School of Mathematical Sciences, Monash University, Victoria 3800, Australia

Received: 17 November 2008
Accepted: 26 January 2009

Abstract

Aims. We present the first study of the effects of thermohaline mixing on the structure and evolution of solar-composition super-AGB (SAGB) stars in the mass range  .

Methods. We developed and analyzed stellar models taking into account thermohaline mixing and varying mixing efficiencies.

Results. In SAGB stars, thermohaline mixing becomes important after carbon has been ignited off-center and it affects significantly the propagation of the flame. In the radiative layers located below the convective carbon-burning zone, a molecular weight inversion is created which allows the efficient transport of chemicals. The outward diffusion of ¹²C from the CO-rich core into the flame, depletes the burning front of fuel and causes the extinction of the flame before it reaches the center. As a consequence the amount of unburnt carbon can be as high as in mass at the center of the star. During the subsequent thermally pulsing SAGB phase, the high temperature at the base of the convective envelope prevents the development of thermohaline instabilities associated with ³He burning as found in low-mass red giant stars.

Conclusions. In contrast to the case of low-mass RGB stars, thermohaline mixing is unable to alter the surface composition of SAGB stars. We also emphasize that if the SAGB star evolves into an electron-capture supernovae, the ¹²C remaining in the core may alter the hydrodynamical explosion and modify the explosive nucleosynthesis.