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

Abstract

We present the first identification in interstellar space of the propargyl radical (CH 2 CCH). 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 4.6 σ to 12.3 σ . We derived a column density of 8.7 × 10 13 cm −2 for CH 2 CCH, which translates to a fractional abundance relative to H 2 of 8.7 × 10 −9 . This radical has a similar abundance as methyl acetylene, with an abundance ratio CH 2 CCH/CH 3 CCH 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 CH 2 CCH with neutral atoms. According to the chemical model, the propargyl radical is essentially formed by the C + C 2 H 4 reaction and by the dissociative recombination of C 3 $ \rm{H}_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.

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