On the evidence of extra mixing in models of 8 M⊙computed with the new solar abundances

Abstract

Stars more massive than about 3Mare known to experience loops in the HR diagram during core helium burning phase. Except for very massive stars the extent of loops increases with the stellar mass. We show that a stellar evolution track for a 8Mstar computed with the new solar abundances (2) shows only a very tiny loop located near the red giant branch. An overshooting below the convective envelope is required to obtain a H-discontinuity located deep enough in the -gradient region and thus to allow the development of a loop in the HR diagram. When the central temperature reaches about 10 8 K, intermediate and massive stars ignite helium in a non-degenerate core at the tip of the RGB, reversing the upward climbing along the RGB. The star is composed of a He burning core and a H burning shell, both these regions producing the total luminosity of the star. The presence of a H shell allows the core to grow, and this phase is thus characterized by a long lifetime (about 20% of the core H burning phase while the star is almost two orders of magnitude brighter). Therefore the star has a large probability to be observed in that phase. The relative importance of one nuclear burning region to another plays an important role in the formation of a loop in the HRD (e.g. (5)). 2. WHAT ABOUT THE NEW SOLAR ABUNDANCES? We have compared the core He burning phase for 8Mmodels computed either with GN93 mixture (3) or with the new solar chemical composition, AGSS09 (2), each with their solar metallicity. The models have been computed with the CLES evolutionary code (7) with the OPAL opacities and with the Ledoux's criterion for the appearance of a convective zone. For the sake of clarity we will refer to the models in the following as GN1 for models computed with GN93, Z = 0.02, X = 0.70, and as AG1 for models computed with AGSS09, Z = 0.014, X = 0.72 (both models computed without overshooting). The blue or red evolution of He-burning stars has been widely investigated (e.g. (4) and references therein). As expected, the track computed with GN1 presents a loop during core He-burning, however, this loop is absent for AG1 track. It is well known that the location of the H discontinuity at the maximum extent of the convective envelope plays an important role in the formation of a loop during the late phases of core helium burning (e.g. (1, 6, 8)). This is indeed the encounter of the H-shell with this discontinuity that suddenly increases the contribution of the H-shell to the total luminosity with a consecutive heating of the envelope and the formation of a loop. If the discontinuity is too superficial, the whole core He- burning takes place before such an encounter and no loop can be formed.