Astron. Astrophys. 359, 586-596 (2000)

5. The ¹²C/¹³C-ratio

Taken at face values our derived ¹²CO/¹³CO-ratios support the ¹²C/¹³C-ratios derived by Lambert et al. (1986), Fig 5. However, there are a number of uncertainties in the extrapolation from the circumstellar isotopomer result to the stellar isotope ratio. The properties of the CO molecule suggests that the isotopomer ratio in the gas leaving the star is equal to the isotope ratio. We have based our circumstellar model upon the photodissociation/chemical fractionation results of Mamon et al. (1988), and this has so far proven to give good results in our test cases. Our radiative transfer calculations are detailed and also provide good fits to multi-transition data. We also believe that estimates of isotopomer ratios are far less dependant on the adopted circumstellar model than are individual abundances. The two molecules have essentially the same energy level diagrams, the same transition strengths, and the same collisional cross sections, and hence their relative abundances are much less dependant on the adopted model than their absolute abundances (note that for ¹²CO we actually adopt an abundance). However, one should note that the circumstellar estimates apply to time scales of 10² to 10³ years, but there is no reason to expect that these stars have changed their surface composition over such a short time scale. In conclusion, we believe that our derived ¹²CO/¹³CO-ratios are reliable estimates of the stellar ¹²C/¹³C-ratios. Thus, we support the results obtained by Lambert et al. (1986).

Also for the J-stars, whose origin is uncertain, we estimate ¹²C/¹³C-ratios that are more consistent with those of Lambert et al. (1986) than those of Ohnaka & Tsuji (1999), i.e., ratios that are, at least in principle, possible to obtain within the CNO-cycle. IRAS 15194-5115 also has a low ¹²C/¹³C-ratio, but it differs from the J-stars in the sense that it has a much higher mass loss rate (see also Ryde et al. 1999). This star could be a borderline case with a mass of about 3.5 , where the CNO-cycle produces a low ¹²C/¹³C-ratio, while the temperature is not high enough to effectively convert ¹²C to ¹⁴N (Ventura et al. 1999).

© European Southern Observatory (ESO) 2000

Online publication: July 7, 2000
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