1. The 3He problem
The evolution of 3He in the Galaxy has first been considered to be straightforward, dominated by the net production of this light element by low mass stars (i.e., with masses lower than 2 ). In these objects, initial D is processed to 3He during the pre-main sequence phase. Then, as described by Iben (1967), an 3He peak builds up due to pp-reactions on the main sequence, and is engulfed in the stellar convective envelope during the first dredge-up on the lower red giant branch (RGB). Standard theory predicts that, once in the convective layers of the evolved star, 3He cannot be destroyed because of the too cool temperature in these regions. It is finally ejected in the interstellar medium (ISM) in the latest stages of stellar evolution. In this standard view, the abundance of 3He must increase in the Galaxy as soon as low-mass stars begin to pollute the ISM (Rood et al. 1976). One expects then to have constraints on the cosmological abundance of 3He (Yang et al. 1984).
Recent observations of a few planetary nebulae (PN; Rood et al. 1992, Balser et al. 1997) led to the measurement of 3He in one object, NGC 3242 1. This PN, whose estimated initial mass is 1.20.2 (Galli et al. 1997, hereafter GSTP97), presents a value of 3He/H, in very good agreement with standard predictions (Vassiliadis & Wood 1993, Charbonnel 1995, Dearborn et al. 1996, Weiss et al. 1996 for the most recent computations). This value however differs by almost two orders of magnitude with the 3He abundance in the proto-solar nebula, 3He/H= (Geiss 1993), in the local interstellar cloud, 3He/H= (Gloeckler & Geiss 1996), and in galactic HII regions, 3He/H= (Balser et al. 1994).
These low values are in clear contradiction with the conventional scenario for galactic evolution of the 3He abundance, and cannot be explained if all low mass stars, such as NGC 3242, happen to return all their 3He to the ISM. GSTP97 showed that, in order to fit the galactic constraints, 3He should be destroyed in at least 70% of low-mass stars before they become PN.
© European Southern Observatory (ESO) 1998
Online publication: July 27, 1998