Letter to the Editor

Discovery of the propargyl radical (CH₂CCH) in TMC-1: One of the most abundant radicals ever found and a key species for cyclization to benzene in cold dark clouds^⋆

¹ Instituto de Física Fundamental, CSIC, Calle Serrano 123, 28006 Madrid, Spain
e-mail: marcelino.agundez@csic.es, jose.cernicharo@csic.es
² Observatorio Astronómico Nacional, IGN, Calle Alfonso XII 3, 28014 Madrid, Spain
³ Observatorio de Yebes, IGN, Cerro de la Palera s/n, 19141 Yebes, Guadalajara, Spain

Received: 12 February 2021
Accepted: 5 March 2021

Abstract

We present the first identification in interstellar space of the propargyl radical (CH₂CCH). This species was observed in the cold dark cloud TMC-1 using the Yebes 40 m telescope. The six strongest hyperfine components of the 2_0, 2–1_0, 1 rotational transition, lying at 37.46 GHz, were detected with signal-to-noise ratios from 4.6σ to 12.3σ. We derived a column density of 8.7 × 10¹³ cm⁻² for CH₂CCH, which translates to a fractional abundance relative to H₂ of 8.7 × 10⁻⁹. This radical has a similar abundance as methyl acetylene, with an abundance ratio CH₂CCH/CH₃CCH close to one. The propargyl radical is thus one of the most abundant radicals detected in TMC-1, and it is probably the most abundant organic radical with a certain chemical complexity ever found in a cold dark cloud. We constructed a gas-phase chemical model and find calculated abundances that agree with, or fall two orders of magnitude below, the observed value depending on the poorly constrained low-temperature reactivity of CH₂CCH with neutral atoms. According to the chemical model, the propargyl radical is essentially formed by the C + C₂H₄ reaction and by the dissociative recombination of C₃${\mathrm{H}}_{\text{n}}^{+}$ ions with n = 4−6. The propargyl radical is believed to control the synthesis of the first aromatic ring in combustion processes, and it probably plays a key role in the synthesis of large organic molecules and cyclization processes to benzene in cold dark clouds.