Astron. Astrophys. 355, 69-78 (2000)

1. Introduction

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
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