2. Constraints on the destruction of 3He in low-mass stars
Rood et al. (1984) suggested that the destruction of 3He could be related to chemical anomalies such as the very low carbon isotopic ratios observed in low-mass red giants. Indeed, while standard models predict post-dredge up values of the 12C/13C ratio between 20 and 30 (depending on the stellar mass and metallicity; see Charbonnel 1994), Pop II field and globular cluster giants present 12C/13C ratios lower than 10, even down to the near-equilibrium value of 4 in many cases. This discrepancy also exists, but to a lower extent, in evolved stars belonging to open clusters with turnoff masses lower than 2 (see Charbonnel et al. 1998, hereafter CBW98, for references).
The 12C/13C data point out the existence of an extra-mixing process that becomes efficient in low mass RGB stars as soon as they reach the so-called luminosity function bump (Gilroy & Brown 1991, hereafter GB91; Charbonnel 1994; Pilachowski et al 1997; CBW98). Different groups have simulated extra-mixing between the base of the convective envelope and the hydrogen-burning shell in order to reproduce the chemical anomalies in RGB stars. These non-standard models predict that the mechanism which is responsible for the low 12C/13C ratios on the RGB must also lead to the destruction of 3He by a large factor in the bulk of the envelope material (Charbonnel 1995, Wasserburg et al. 1995, Weiss et al. 1996), and confirm the estimation of Hogan (1995).
In this paper, we use the observations of 12C/13C in evolved stars to determine statistically what fraction of low-mass stars experience this extra-mixing on the RGB, and are then expected to destroy their 3He.
© European Southern Observatory (ESO) 1998
Online publication: July 27, 1998