Detection of circumstellar CH₂CHCN, CH₂CN, CH₃CCH, and H₂CS^*

¹ Departamento de Astrofísica Molecular e Infrarroja, Instituto de Estructura de la Materia, CSIC, Serrano 121, 28006 Madrid, Spain e-mail: [marce;pablo;cerni;pardo]@damir.iem.csic.es
² Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406 St. Martin d'Hères and LERMA/École Normale Supérieure, 24 rue Lhomond, 75231 Paris, France e-mail: guelin@iram.fr

Received: 29 October 2007
Accepted: 6 December 2007

Abstract

Aims.We report on the detection of vinyl cyanide (CH₂CHCN), cyanomethyl radical (CH₂CN), methylacetylene (CH₃CCH), and thioformaldehyde (H₂CS) in the C-rich star IRC +10216. These species, which are all known to exist in dark clouds, were detected for the first time in the circumstellar envelope around an AGB star.

Methods.The four molecules were detected through pure rotational transitions in the course of a λ 3 mm line survey carried out with the IRAM 30-m telescope. The molecular column densities were derived by constructing rotational temperature diagrams. A detailed chemical model of the circumstellar envelope is used to analyze the formation of these molecular species.

Results.We have found column densities in the range 5  10¹²-2  10¹³ cm^-2, which translates to fractional abundances relative to H₂ of several 10^-9. The chemical model is reasonably successful in explaining the derived abundances through gas phase synthesis in the cold outer envelope. We also find that some of these molecules, CH₂CHCN and CH₂CN, are most probably excited through infrared pumping to excited vibrational states.

Conclusions.The detection of these species stresses the similarity between the molecular content of cold dark clouds and C-rich circumstellar envelopes. However, some differences in the chemistry are indicated by partially saturated carbon chains being present in IRC +10216 at a lower level than those that are highly unsaturated, while in TMC-1 both types of species have comparable abundances.