Inferring acetylene abundances from C2H: the C2H2/HCN abundance ratio

We have carried out a survey of C2H and HCN in a wide sample of evolved stars using the 30-m IRAM telescope. Significant variations of the C2H abundance are found between the observed stars. Low C2H abundances, X(C2H)<10−5, are observed in S-stars, and detached envelopes. In massive C-rich stars (Ṁ > 2 10−6 M yr−1) the C2H abundance seems to be correlated with the expansion velocity. Thus, the mean value of the C2H abundance is 1.2 10−5 in C-rich stars with Ve≤ 22 kms−1 (hereafter “low-Ve” stars) and a factor of 5 larger, ∼ 5 10−5 M yr−1, in those with Ve ≥ 22 kms−1 (hereafter “highVe” stars). Some explanations are suggested to account for the large C2H abundances observed in “high-Ve” stars. The emission of the HCN lines is optically thick in most Crich stars. The apparent variations of the HCN abundance with Ṁ and Ve are very likely due to an opacity effect. The most recent determination, based on ISO data, of the HCN abundance in IRC+10216 is X(HCN) ∼ 3 10−5. From H13CN observations, we have obtained HCN abundances ranging from 3 to 30 10−5 in “high-Ve” stars. While HCN abundances ∼ 3 10−5 are found toward AFGL 809 and AFGL 2901 in agreement with the value found in IRC+10216, abundances ≥ 10−4 are estimated in IRC+00365, IRC+10401, IRC+30374 and S Cep. Based on our C2H data and assuming X(HCN) ∼ 3 10−5 toward the prototypical star IRC+10216, we estimate a C2H2/HCN abundance ratio ∼ 1. The C2H2/HCN ratio in “high-Ve” stars ranges from ∼ 0.08 to 7. The large dispersion found in the HCN abundances toward “high-Ve” stars prevent us from detecting any systematic variation of the C2H2/HCN ratio with the expansion velocity in massive C-rich stars. In C-rich stars with lowmass-loss rate, the C2H2/HCN ratio is a factor of 10 lower than in IRC+10216.