A new heavy anion in IRC + 10216: Theory favors C₁₀H⁻ versus C₉N^−★

¹ Consejo Superior de Investigaciones Científicas, Instituto de Física Fundamental, Calle Serrano 121, 28006 Madrid, Spain
e-mail: jr.pardo@csic.es; jose.cernicharo@csic.es
² Centro de Desarrollos Tecnológicos, Observatorio de Yebes (IGN), 19141 Yebes, Guadalajara, Spain
³ Observatorio Astronómico Nacional (OAN, IGN), Madrid, Spain
⁴ Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406, Saint-Martin-d’Hères, France

Received: 24 March 2023
Accepted: 26 June 2023

Abstract

Continuing Q-band (31–50 GHz) integrations on IRC +10216 with the Yebes 40 m telescope have now reached sub-millikelvin noise with hundreds of new lines arising above an average 3σ detection limit of 0.71 mK (as low as 0.45 mK in the lower frequency end). The recent discovery of the C₇N⁻ anion and the relatively large abundance of the HC₉N member of the cyanopolyyne family opens the door for searching in the data for harmonically related series of lines belonging to singlet species, with intensities close to the detection limit, that could belong to heavier anions. One such series has been found with rotational quantum numbers from J = 52 − 51 up to J = 74 − 73, with at least 15 distinguishable features clearly detected as isolated or partly blended. There are two main candidates for the carriers of the series: C₉N⁻ and/or C₁₀H⁻, for which our high-level-of-theory ab initio calculations predict a quite compatible rotational constant of ~300 MHz. In this paper we discuss, based on our ab initio calculations and also on chemical models, which of these two candidates is the most likely carrier. There is more evidence for the C₁₀H⁻ candidate. It would be, to date, the heaviest anion discovered in space through a series of detected individual lines.