Letter to the Editor

Detection of the SiNC radical in IRC+10216

¹ IRAM, 300 rue de la piscine, 38406 Saint-Martin-d'Hères, France e-mail: guelin@iram.fr
² Institute of Astronomy and Astrophysics, Academia Sinica, 128 Section 2, PO Box 1-87, Nankang, Taipei 115, Taiwan
³ Instituto de Estructura de la Materia, C/Serrano 121, 28006 Madrid, Spain
⁴ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA

Received: 29 July 2004
Accepted: 6 September 2004

Abstract

Following discovery of the free radical SiCN in the C-star envelope IRC+10216, we report the detection in the same source of its isomer SiNC. The microwave spectra of SiNC and SiCN were studied in the laboratory and their rotational transition frequencies are accurately known. The ground fine structure state of SiNC, , gives rise to a series of rotational transitions, spaced by 12.8 GHz, each with Λ-doubling. Five weak lines are detected with the IRAM 30-m telescope at the frequencies of the (e), (f), (f) and (e) and (f) rotational transitions. Other SiNC lines from these or adjacent rotational transitions are found to be blended with stronger lines from known molecules.
The lines assigned to SiNC have a cusped shape, characteristic of species confined to a hollow shell in the outer circumstellar envelope. They are twice weaker than their SiCN counterparts, which have the same shape, and presumably arise in the same region of the envelope. SiNC and SiCN have about the same abundance in IRC+10216, ~4 10^-9 with respect to H₂. This contrasts with HCN, HC₃N and HC₅N, for which the cyanide to isocyanide abundance ratio is >100.