Discovery of the propargyl radical (CHCCH) 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 (CHCCH). This species was observed in the cold dark cloud TMC-1 using the Yebes 40m telescope. The six strongest hyperfine components of the 2-1 rotational transition, lying at 37.46 GHz, were detected with signal-to-noise ratios in the range 4.6-12.3 σ. We derive a column density of 8.7 × 10 cm for CHCCH, which translates to a fractional abundance relative to H of 8.7 × 10. This radical has a similar abundance to methyl acetylene, with an abundance ratio CHCCH/CHCCH 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 CHCCH with neutral atoms. According to the chemical model, the propargyl radical is essentially formed by the C + CH reaction and by the dissociative recombination of CH ions with = 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.