Letter to the Editor

Discovery of HC₉N ν₁₉ overtones in IRC+10216

¹ Departamento de Astrofísica Molecular, Instituto de Física Fundamental (IFF-CSIC), Calle Serrano 121, 28006 Madrid, Spain
² Centre National de la Recherche Scientifique, Observatoire de Paris, LIRA, 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), Calle Alfonso XII 3, 28014 Madrid, Spain
⁵ Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, F-38406 Saint Martin d’Hères, France

^★ Corresponding authors: This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 7 February 2026
Accepted: 18 March 2026

Abstract

The sub-milliKelvin sensitivity achieved in the Q-band (31–50 GHz) line survey of IRC+10216 obtained with the Yebes 40m telescope reveals the presence of several new vibrational HC₉N overtones (ν₁₉ ≥ 2) beyond the already reported ν₁₉ = 1. The newly found series of rotational lines appears in seven groups with rotational constants (B) between 291.252 and 293.465 MHz, with integer quantum numbers from J_up = 54 to 81 and upper-level excitation energies up to ∼600 K, well above what is typical of lines found in the outer part of this circumstellar envelope. All evidence, especially the spectral splitting of the ℓ-doubling components and the spacing between B values, unambiguously points toward rotational lines within ν₁₉ = 2 − 8 that appear surprisingly strong, possibly due to very efficient infrared pumping and slow decay via the ν₁₉ mode. The detection of ν₁₉ = 5, 6, 7, and 8 has been possible thanks to collapsed ℓ-doubling components to a single or only two spectral features per rotational transition. To obtain a correct determination of the abundance of HC₉N in IRC+10216 one has to take into account that about ∼70% is estimated to be in the vibrationally excited ν₁₉ states.