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Astron. Astrophys. 330, 232-242 (1998)

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

A. Fuente 1, J. Cernicharo 2 and A. Omont 3

1 Observatorio Astronómico Nacional (IGN), Campus Universitario, Apdo. 1143, E-28800 Alcalá de Henares (Madrid), Spain
2 Consejo Superior de Investigaciones Cientficas (CSIC) Serrano 123, E-28006 Madrid, Spain
3 Institut d'Astrophysique de Paris (CNRS), 98 bis Bd Arago, F-75014 Paris, France

Received 21 March 1997 / Accepted 24 September 1997

Abstract

We have carried out a survey of C2 H and HCN in a wide sample of evolved stars using the 30-m IRAM telescope. Significant variations of the C2 H abundance are found between the observed stars. Low C2 H abundances, X(C2 H) [FORMULA] 10-5, are observed in S-stars, and detached envelopes. In massive C-rich stars ([FORMULA] [FORMULA] 2 10-6 M yr-1) the C2 H abundance seems to be correlated with the expansion velocity. Thus, the mean value of the C2 H abundance is 1.2 10-5 in C-rich stars with [FORMULA] [FORMULA] 22 kms-1 (hereafter "low- [FORMULA] " stars) and a factor of 5 larger, [FORMULA] 5 10-5 M yr-1 , in those with [FORMULA] [FORMULA] 22 kms-1 (hereafter "high- [FORMULA] " stars). Some explanations are suggested to account for the large C2 H abundances observed in "high- [FORMULA] " stars.

The emission of the HCN lines is optically thick in most C-rich stars. The apparent variations of the HCN abundance with [FORMULA] and [FORMULA] 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) [FORMULA] 3 10-5. From H13 CN observations, we have obtained HCN abundances ranging from 3 to 30 10-5 in "high- [FORMULA] " stars. While HCN abundances [FORMULA] 3 10-5 are found toward AFGL 809 and AFGL 2901 in agreement with the value found in IRC+10216, abundances [FORMULA] 10-4 are estimated in IRC+00365, IRC+10401, IRC+30374 and S Cep. Based on our C2 H data and assuming X(HCN) [FORMULA] 3 10-5 toward the prototypical star IRC+10216, we estimate a C2 H2 /HCN abundance ratio [FORMULA] 1. The C2 H2 /HCN ratio in "high- [FORMULA] " stars ranges from [FORMULA] 0.08 to 7. The large dispersion found in the HCN abundances toward "high- [FORMULA] " stars prevent us from detecting any systematic variation of the C2 H2 /HCN ratio with the expansion velocity in massive C-rich stars. In C-rich stars with low mass-loss rate, the C2 H2 /HCN ratio is a factor of 10 lower than in IRC+10216.

Key words: stars: abundances – circumstellar matter – stars: AGB and post-AGB – radio lines: stars – stars: IRC +10216

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© European Southern Observatory (ESO) 1998

Online publication: January 8, 1998
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