Detection of ethanol, acetone, and propanal in TMC-1 New O-bearing complex organics in cold sources^★

¹ Instituto de Física Fundamental, CSIC, Calle Serrano 123, 28006 Madrid, Spain
e-mail: marcelino.agundez@csic.es; jose.cernicharo@csic.es
² Institut des Sciences Moléculaires (ISM), CNRS, Univ. Bordeaux, 351 cours de la Libération, 33400 Talence, France
e-mail: jean-christophe.loison@u-bordeaux.fr
³ Laboratoire d’Astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
⁴ Observatorio Astronómico Nacional, IGN, Calle Alfonso XII 3, 28014 Madrid, Spain
⁵ Observatorio de Yebes, IGN, Cerro de la Palera s/n, 19141 Yebes, Guadalajara, Spain

Received: 3 February 2023
Accepted: 27 March 2023

Abstract

We present the detection of ethanol (C₂H₅OH), acetone (CH₃COCH₃), and propanal (C₂H₅CHO) toward the cyanopolyyne peak of TMC-1. These three O-bearing complex organic molecules are known to be present in warm interstellar clouds but had never been observed in a starless core. The addition of these three new pieces to the puzzle of complex organic molecules in cold interstellar clouds highlights the rich chemical diversity of cold dense cores in stages prior to the onset of star formation. The detections of ethanol, acetone, and propanal were made in the framework of QUIJOTE, a deep line survey of TMC-1 in the Q band that is being carried out with the Yebes 40m telescope. We derive column densities of (1.1 ± 0.3) × 10¹² cm⁻² for C₂H₅OH, (1.4 ± 0.6) × 10¹¹ cm⁻² for CH₃COCH₃, and (1.9 ± 0.7) × 10¹¹ cm⁻² for C₂H₅CHO. We investigated the formation of these three O-bearing complex organic molecules with the aid of a detailed chemical model that includes gas and ice chemistry. The calculated abundances at a time around 2 × 10⁵ yr are in reasonable agreement with the values derived from the observations. The formation mechanisms of these molecules in our chemical model are as follows. Ethanol is formed on grains via the addition of atomic carbon on methanol followed by hydrogenation and nonthermal desorption. Acetone and propanal are produced by the gas-phase reaction between atomic oxygen and two different isomers of the C₃H₇ radical, which itself forms from the hydrogenation of C₃ on grains followed by nonthermal desorption. A gas-phase route involving the formation of (CH₃)₂COH⁺ through several ion-neutral reactions followed by its dissociative recombination with electrons also contributes to the formation of acetone.