In the PN phase, stars more massive than 0.8 return to the ISM material that has been processed in the stellar interior. This matter mixes with the surrounding medium and modifies the original abundances of elements. The contribution of PNe to the Galactic chemical evolution is particularly important for 3He which, together with deuterium, plays a fundamental role in testing the standard Big Bang nucleosynthesis model. While the evolution of deuterium is well understood, that of 3He still encounters serious problems which cast doubts on the usefulness of this isotope as a test of Big Bang nucleosynthesis models (e.g. Galli et al. 1995; for a different opinion on deuterium see Mullan & Linsky 1999). Observations of 3He toward PNe and HII regions have resulted in abundances that differ by almost two orders of magnitude: 3He/H in PNe (Rood et al. 1992, Balser et al. 1997), and 3He/H in HII regions (Balser et al. 1994, Rood et al. 1995). The latter value is also representative of the 3He abundance in the presolar material (Geiss 1993) and the local ISM (Gloeckler & Geiss 1996). However, the abundance in PNe agrees with the predictions of standard stellar evolution models for stars of mass 1-1.5 (see the review by Rood et al. 1998). The main question is then: if low-mass stars are net producers of 3He and return it to the ISM during the PN phase, why don't we observe a much higher abundance in HII regions and in the solar system material, as all standard Galactic evolutionary models predict (see e.g. Tosi 1996)?
An interesting solution to this problem involves the existence of a nonstandard mixing mechanism (or Cool Bottom Processing, hereafter CBP) which operates during the red giant phase of stars with . In addition to decreasing the amount of 3He in the stellar envelope, this process affects the abundances of other important elements, including carbon, as first suggested by Hogan (1995), Charbonnel (1995) and Wasserburg et al. (1995). In particular, the ratio of 12C/13C in the envelope is predicted to be much lower than in the standard case. For a 1 star, the predicted ratio is about 5 against the standard value of 25-30 in the red giant branch (RGB) and of 20-40 in the asymptotic giant branch (AGB). However, the discrepancy becomes larger for more massive stars where the 12C/13C ratio can reach 100 in the AGB phase (Charbonnel 1995; Weiss et al. 1996; Forestini & Charbonnel 1997, hereafter FC; van den Hoek & Groenewegen 1997, herafter HG; Marigo 1998; Boothroyd & Sackmann 1999, hereafter BS).
From an observational viewpoint, it is important to obtain accurate measurements of the isotopic ratio in those PNe where the 3He abundance has been determined. Should these objects show a high value of 12C/13C, then no modifications to the standard stellar models would be required. Otherwise, one has to invoke another selective process (mixing, diffusion etc.) that operates on some isotopes but not on 3He. However, the number of PNe with 3He measurements is small (e.g. Balser et al. 1999), whereas the suggested physical processes should be quite general and should affect the nucleosynthetic yields of all stars of mass less than . Hence, it is critical to measure the carbon isotopic ratio in a sample of PNe as large as possible.
The molecular envelopes of PNe have been studied extensively at near infrared and millimeter wavelengths (see e.g. Kastner et al. 1996, Huggins et al. 1996, Bachiller et al. 1997). These observations have shown that massive envelopes ( ) containing a rich variety of molecular species are commonly found around PNe. CO is the most widely observed species, and the 12C/13C isotopic ratio has been measured toward several PNe (Bachiller et al. 1989, 1997; Cox et al. 1992). These initial studies have shown that the 12C/13C ratio is in the range 10-20.
Our project consists of two parts. In the first one, we have carried out high quality observations of 12CO and 13CO in six PNe that have been searched for 3He emission. In the second run, a larger sample of nebulae with strong 12CO line emission has been observed in the 13CO lines in order to determine the isotopic ratio in PNe without 3He measurements. Galli et al. (1997, hereafter GSTP) have argued that extra-mixing processes must be at work in more than 70% of low-mass stars ( ) in order to reconcile the predictions of the Galactic evolution of 3He with the observational constraints. We set out this experiment to determine the isotopic ratio in a relatively large sample of PNe.
© European Southern Observatory (ESO) 2000
Online publication: March 17, 2000