1998A&A...330..232F

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 ({dot}(M)>2x10^–6M_☉/yr) the C₂H abundance seems to be correlated with the expansion velocity. Thus, the mean value of the C₂H abundance is 1.2x10^–5 in C-rich stars with V_e≤22km/s (hereafter ``low-V<sub>e</sub>'' stars) and a factor of 5 larger, ∼5x10<sup>–5</sup>M<sub>☉</sub>/yr, in those with V<sub>e</sub>≥22km/s (hereafter ``high-V_e'' stars). Some explanations are suggested to account for the large C₂H abundances observed in ``high-V<sub>e</sub>'' stars. The emission of the HCN lines is optically thick in most C-rich stars. The apparent variations of the HCN abundance with {dot}(M) and V<sub>e</sub> 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)∼3x10<sup>–5</sup>. From H<sup>13</sup>CN observations, we have obtained HCN abundances ranging from 3 to 30x10<sup>–5</sup> in ``high-V_e'' stars. While HCN abundances ∼ 3x10^–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)∼3x10^–5 toward the prototypical star IRC+10216, we estimate a C₂H₂/HCN abundance ratio ∼ 1. The C₂H₂/HCN ratio in ``high-V<sub>e</sub>'' stars ranges from ∼ 0.08 to 7. The large dispersion found in the HCN abundances toward ``high-V_e'' 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 low mass-loss rate, the C₂H₂/HCN ratio is a factor of 10 lower than in IRC+10216.