5. Constraints from CNO isotopes
5.1. The 12C /13C ratio
In Fig. 4 we plotted the envelope Cl isotopic ratio versus the 12C/13C ratio, with two different initial ratios at the beginning of the TP-AGB phase: (12C/13C)ini 12, or 24, as representative of low-mass AGB stars (M 2 ) or of higher mass stars, respectively. This important point needs to be elucidated.
IRC+10216 has a 12C /13C ratio of 45 3 (see Table 2). This value, which is typical of C (N-type) stars with measured C/O 1 (e.g., de Laverny & Gustafsson 1998; Lambert et al. 1986; Jorissen et al. 1992) may appear a bit small in the case of IRC+10216, if we accept for it the value C/O = 1.4 inferred from circumstellar modelling (Winters et al. 1994). The estimated C/O ratio is reasonable in the lights of the advanced spectral type of CW Leo (C9.5, see Olofsson et al. 1982) but cannot be trusted at the same level of confidence as the other (observed) ratios discussed here. Therefore, independently of the precise C/O value, we shall simply try to explain how a C-rich atmosphere with the observed 12C /13C ratio can be formed.
It is known that in the red giant phase the 12C /13C ratio is modified by the first dredge up, where a photospheric value 20 - 25 is predicted by canonical models. This implies that later, on the TP-AGB, mixing of pure 12C by TDU would increase this isotopic ratio to values around 100. However, in red giant stars of low mass 12C /13C values lower than the canonical one have been measured spectroscopically (Gilroy 1989). For stars on the red giant branch and initial masses below 2 , an appropriate value for the 12C /13C ratio appears to be between 10 and 15. This indicates that some kind of extramixing, or "cool bottom processing" (CBP) phenomena are at work (Charbonnel 1994, 1995; Wasserburg et al. 1995; Charbonnel et al. 1998), possibly driven by rotational shear (Sweigart & Mengel 1979).
From Fig. 4b it results that CBP, simulated by adopting a 12C /13C value of 12 on the red giant branch, chosen as appropriate for a low mass model, is indeed required to explain the observed carbon isotope ratio in the circumstellar envelope of CW Leo. We notice that in the case of the 1.5 model we predict for this ratio the value of 45 when C/O equals unity (i.e. slightly less than the C/O ratio estimated for CW Leo), at a somewhat earlier phase than the AGB tip. However, in view of the above discussion on C/O this cannot be considered as critical. It would be sufficient in our models to change the parameter of mass loss, increasing it slightly, to obtain C/O 1 and 12C /13C 45 at the very end of the sequence. Another possibility is to choose a slightly higher progenitor mass, resulting in a somewhat larger dilution of He-intershell material with the envelope. In the absence of a precise measurement for the C/O ratio in CW Leo, all we can say is that our 1.5 AGB model can provide a C-rich envelope with the observed 12C /13C ratio. Conversely, in AGB stars of M 2 , this is not possible, as the observational evidence excludes the operation of CBP. In stars with M 2 the 12C /13C ratio reaches about 100 already at C/O 1.
Thanks to CBP, the same low mass star holds to explain the high value of the 14N/15N ratio (Boothroyd et al. 1995), for which only a lower limit exists. In principle, the predicted 12C /13C ratio in the AGB phase might be kept low also for higher stellar masses, but only if a moderate HBB occurs, consuming some of the 12C in the envelope (Guélin et al. 1995; Weigelt et al. 1998).
5.2. The 16O /17O and 16O /18O ratios
Another crucial constraint comes from the oxygen isotopes, and this is a further decisive argument in favour of a low initial mass. Indeed, as shown by Boothroyd et al. (1995) and Lattanzio & Boothroyd (1997), the low value of 16O/17O, and the high value of 16O/18O measured in IRC+10216 (see Table 2) cannot be explained by HBB, even of moderate entity. When represented in a 3-isotope plot displaying the 18O/16O ratio versus the 17O/16O one, the data (see Fig. 2 of Wasserburg et al. 1995) falls in a region that cannot be reached by model curves from HBB calculations. The same authors show instead as those isotopic ratios, which are not accounted for by the canonical first dredge up, are a natural result of CBP. This fact again necessarily implies a low initial mass.
As a final comment, it can be noticed that our conclusion about a low initial mass for CW Leo places it in the already known family of dust-enshrouded low-mass carbon stars. This was indirectly recognized through high precision isotopic measurements in presolar SiC grains recovered from meteorites (Zinner 1997; Hoppe & Ott 1997), while comparisons with nucleosynthesis models similar to the one performed here demonstrated that indeed these grains condensed in the circumstellar envelopes of low-mass carbon stars (Gallino et al. 1997).
© European Southern Observatory (ESO) 2000
Online publication: June 5, 2000