Helium shell flash

PN nucleus, horizontal-branch and white-dwarf regions. The dotted line shows a schematic main sequence and evolutionary track for Population II, while various dashed lines show roughly the Cepheid instability strip, the transition to surface convection zones and the helium-shell flashing locus for Population I. After Pagel (1977). Copyright by the IAU. Reproduced with kind permission from Kluwer Academic Publishers. 

The third dredge-up occurs as a result of the envelope convection extending downward in mass during a helium shell flash, with the result that the products of both the CNO cycle and the 3-a process are brought to the stellar surface (Iben 1975). As a result of the third dredge-up, the stellar surface is enriched in 12C and s-process elements. 

However, there are two sets of observations which indicate that AGB stars do indeed reach the AGB limit Mbol = -7.1. Firstly, the results of WBF and Reid, Glass and Catchpole (1987) for the LPVs show that such stars do reach the AGB limit. Secondly, a luminosity function for red stars in the bar of the LMC obtained by Hughes and Wood (1987) shows that AGB stars extend to Mbol = -7.1, although there is a steep fall-off with luminosity above Mboi = -6. One explanation for the rapid fall-off in the number of AGB stars above Mboi = -6 is provided by Wood and Faulkner (1986) who show that envelope ejection will occur at this point in all but the most massive AGB stars due to the luminosity of the star exceeding the Eddington limit at the base of the hydrogen-rich envelope during the surface luminosity peak of a helium shell flash. 

HASS EJECTION DURING HELIUM SHELL FLASHES FROM A MASSIVE WHITE DWARF... 

Abstract We have simulated the helium shell flashes on an 1.3 M0 white dwarf and estimated the amount of mass loss. Our results may suggest a serious difficulty for the theories of the formation of Type I supernovae and of the accretion-induced formation of neutron stars because a significant amount of envelope mass is ejected during a helium shell flash. 

Figure 1 The ratio of the lost mass to the accreted mass for each cycle of helium shell flashes as a function of the helium accretion rate.

Mass Ejection During Helium Shell Flashes from a Massive White Dwarf... 

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