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Astron. Astrophys. 357, 669-676 (2000)

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3. Results for the isotopic ratios

Assuming (i) that the lines are optically thin and (ii) that both isotopomers have the same excitation conditions (this is most likely, because their abundances are very similar), the derivation of the molecular abundance ratios from the integrated intensity ratios is straightforward: only a frequency correction (in [FORMULA]) has to be applied to take into account the frequency dependence of the line strengths (Kahane et al. 1988; 1992). The rms 1 [FORMULA] errorbars on these ratios include two statistically independent uncertainties: a 10% calibration uncertainty and a "fit uncertainty". The results are reported in Table 2. We have also included in this table the molecular isotopic ratios obtained by Cernicharo et al. (1987) and two of the four ratios derived by Cernicharo et al. (2000) in the 2mm survey (two ratios based on tentatively detected lines have been ruled out). The five measurements appear to be compatible and no systematic difference between NaCl and AlCl (which would indicate an unlikely isotopic fractionation effect) can be seen. We conclude that these molecular abundance ratios reflect the elemental 35Cl/37Cl ratio. Using weightings inversely proportional to the individual errorbars, we derive an average 35Cl/37Cl ratio of 2.30 [FORMULA] 0.24, significantly smaller than the solar value of 3.13. With the significantly reduced errorbars provided by the new data (the average of the previous data was 2.4 [FORMULA] 0.4), we have been able to bring the first observational evidence that the AGB circumstellar envelope IRC+10216 presents deviations from the solar system isotopic composition that are not limited to the well known CNO isotopes. Furthermore, the accuracy of our 35Cl/37Cl isotopic ratio measurement allows now a meaningful comparison with theoretical predictions.


Table 2. Observed isotopic ratios towards IRC+10216.
a) the references are the following: (1) this paper; (2) Cernicharo et al. 1987; (3) Cernicharo et al. 2000; (4) Kahane et al. 1988; (5) Kahane et al. 1992; (6) Guélin et al. 1995
b) from Anders & Grevesse 1989
c) average value derived from the above ratios (see text)
d) due to the non negligeable opacity of the 28Si bearing lines, only a lower limit could be derived.

In Table 2 we have also reported the silicon and sulfur ratios derived from the 2mm survey (Cernicharo et al. 2000), which represent a significant improvement compared to previous estimates (Kahane et al. 1988) and, to our knowledge, the most accurate existing measurements of silicon and sulfur isotopic ratios in an evolved AGB star. We have further included the isotopic composition of C (Cernicharo et al. 2000), N (Kahane et al. 1988), O (Kahane et al. 1992), and Mg (Guélin et al. 1995).

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© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000