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

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 4.6 sigma to 12.3 sigma. We derived a column density of 8.7x10(13) cm(-2) for CH2CCH, which translates to a fractional abundance relative to H-2 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 C-3 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.

Automated summary

The propargyl radical CH2CCH was detected for the first time in interstellar space, showing similar abundance to methyl acetylene and potentially playing a key role in the synthesis of large organic molecules and cyclization processes to benzene in cold dark clouds.