2021A&A...647L..10A

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 37.46GHz, were detected with signal-to-noise ratios from 4.6σ to 12.3σ. We derived a column density of 8.7x10¹³cm^–2 for CH₂CCH, which translates to a fractional abundance relative to H₂ of 8.7x10^–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 C₃Hn⁺ 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.