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

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 40 m 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.