Letter to the Editor

Detection of SiC₆ in IRC +10216 through Q-band lines

¹ Consejo Superior de Investigaciones Científicas, Instituto de Física Fundamental, C/ Serrano 121, 28006 Madrid, Spain
² Centre National de la Recherche Scientifique, Observatoire de Paris, 77 Avenue Denfert Rochereau, 75014 Paris, France
³ Centro de Desarrollos Tecnológicos, Observatorio de Yebes (IGN), 19141 Yebes, Guadalajara, Spain
⁴ Observatorio Astronómico Nacional (OAN, IGN), Madrid, Spain
⁵ Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, F-38406 Saint Martin d’Hères, France

^⋆ Corresponding authors: jr.pardo@csic.es, jose.cernicharo@csic.es

Received: 11 June 2025
Accepted: 14 July 2025

Abstract

Ongoing Q-band (31–50 GHz) integrations on IRC +10216 with the Yebes 40m telescope have now reached the level of sub-mK noise, with hundreds of new lines arising above the 3σ detection limit. Among them is a new, harmonically related series with integer quantum numbers ranging from J_up = 26 to 40, which has allowed it to be assigned to SiC₆. We clearly detected a total of nine isolated (or otherwise not critically blended) spectral features, with peak intensities around or slightly below 1 mK in the T_A^* scale, thanks to the extremely low noise in the spectra. Five additional lines were seen to be partially blended with lines from other species, but could still be relatively well modeled due to the uniformity of the characteristic U-shaped line profiles emerge from the envelope of IRC+10216. As a result, we built a rotation diagram for SiC₆, providing a column density of ∼8 × 10¹¹ cm⁻² and a rotational temperature around 19 K. A similar analysis was done for SiC₄, ²⁹SiC₄, and ³⁰SiC₄. Together, these insights offer the first view at the chemistry of linear silicon carbides in IRC+10216.