Letter to the Editor

Detection of the linear SiC₃ and SiC₅ radicals in IRC+10216^⋆

¹ Consejo Superior de Investigaciones Científicas, Instituto de Física Fundamental, C/ Serrano 121, 28006 Madrid, Spain
² Observatorio Astronómico Nacional (OAN, IGN), Calle Alfonso XII 3, 28014 Madrid, Spain
³ Centro de Desarrollos Tecnológicos, Observatorio de Yebes (IGN), 19141 Yebes, Guadalajara, Spain
⁴ Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, F-38406 Saint Martin d’Hères, France

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

Received: 11 July 2025
Accepted: 29 July 2025

Abstract

We detected the linear ³Σ⁻ radicals SiC₃ and SiC₅ toward IRC+10216 using an ultrasensitive line survey gathered with the Yebes 40 m radio telescope. The derived column densities of l-SiC₃ and l-SiC₅ are (3.6 ± 0.4) × 10¹² cm⁻² and (1.8 ± 0.2) × 10¹² cm⁻², respectively. The linear SiC₃ radical is ∼2 times less abundant that its singlet rhomboidal prolate isomer, for which we provide a new analysis based on recent sensitive observations in the Q band (7 mm), and at 3 and 2 mm with the IRAM 30 m telescope. The emission detected from these species arises from the cool external layers of the circumstellar envelope. We speculate whether ion-neutral routes involving SiC_nH_m⁺ cations or neutral-neutral reactions involving Si and SiC₂ could efficiently synthesize these species.