Time dependent mixing in He-burning cores: The case of NGC 1866

Abstract

We examine the helium burning phase of the convective core of intermediate mass stars, and investigate the role of the coupling of nuclear burning and mixing on the extension and duration of the blue loop phase. We compare the theoretical scenario with the distribution of stars in the colour-magnitude (CM) diagram of the Large Magellanic Cloud (LMC) cluster NGC 1866, whose densely populated clump of He-burning stars is equally populated on the blue and red side. We compare the distributions expected by adopting either a diffusive scheme within the instability regions, in which nuclear burning and mixing are self-consistently coupled, or the traditional instantaneous mixing approximation. We analyze with particular care the sensitivity of the results to: a) the e-folding distance with which the velocity of convective eddies decays outside the formal border of the convective regions fixed by the Schwarzschild criterion; b) the convective model adopted to evaluate the temperature gradient; c) the rate of reaction 12C + α → 16O. Models not including convective overshoot are also commented.