SIMBAD references

1998A&A...330..232F - Astronomy and Astrophysics, volume 330, 232-242 (1998/2-1)

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


Abstract (from CDS):

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 Ve≤22km/s (hereafter ``low-Ve'' stars) and a factor of 5 larger, ∼5x10–5M/yr, in those with Ve≥22km/s (hereafter ``high-Ve'' 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 C-rich stars. The apparent variations of the HCN abundance with {dot}(M) 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)∼3x10–5. From H13CN observations, we have obtained HCN abundances ranging from 3 to 30x10–5 in ``high-Ve'' 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-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 low mass-loss rate, the C2H2/HCN ratio is a factor of 10 lower than in IRC+10216.

Abstract Copyright:

Journal keyword(s): stars: abundances - circumstellar matter - stars: AGB and post-AGB - radio lines: stars - stars: individual: IRC +10216

Simbad objects: 35

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