Inferring acetylene abundances from C_2H: the C_2H_2/HCN abundance ratio

Details Inferring acetylene abundances from C_2H: the C_2H_2/HCN abundance ratio Inferring acetylene abundances from C_2H: the C_2H_2/HCN abundance ratio

Feb 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998a%26a...330..232f&link_type=abstract

Astronomy and Astrophysics, v.330, p.232-242 (1998)

Astronomy and Astrophysics

Astrophysics

17

Stars: Abundances, Circumstellar Matter, Stars: Agb And Post-Agb, Radio Lines: Stars, Stars: Irc +10216

Scientific paper

We have carried out a survey of C_2H and HCN in a wide sample of evolved stars using the 30-m IRAM telescope. Significant variations of the C_2H abundance are found between the observed stars. Low C_2H abundances, X(C_2H)<10(-5) , are observed in S-stars, and detached envelopes. In massive C-rich stars (dot {M} > 2 10(-6) ispfmmode {{ispt M}_\odotyr^{-1) the C_2H abundance seems to be correlated with the expansion velocity. Thus, the mean value of the C_2H abundance is 1.2 10(-5) in C-rich stars with V_e<= 22 kms(-1) (hereafter ``low-V_e'' stars) and a factor of 5 larger, ~ 5 10(-5) ispfmmode {{ispt M}_\odotyr^{-1, in those with V_e >= 22 kms(-1) (hereafter ``high-V_e'' stars). Some explanations are suggested to account for the large C_2H abundances observed in ``high-V_e'' 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_e 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 H(13) CN observations, we have obtained HCN abundances ranging from 3 to 30 10(-5) in ``high-V_e'' 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 C_2H data and assuming X(HCN) ~ 3 10(-5) toward the prototypical star IRC+10216, we estimate a C_2H_2/HCN abundance ratio ~ 1. The C_2H_2/HCN ratio in ``high-V_e'' 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 C_2H_2/HCN ratio with the expansion velocity in massive C-rich stars. In C-rich stars with low mass-loss rate, the C_2H_2/HCN ratio is a factor of 10 lower than in IRC+10216.

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