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Description
We report the detection of the propargyl radical (CH_2_CCH) in the cold dark cloud TMC-1 in the lambda 3mm wavelength band. We recently discovered this species in space toward the same source at a wavelength of lambda 8mm. In those observations, various hyperfine components of the 202-101 rotational transition, at 37.5GHz, were detected using the Yebes 40m telescope. Here, we used the IRAM 30m telescope to detect ten hyperfine components of the 505-404 rotational transition, lying at 93.6GHz. The observed frequencies differ by 0.2MHz with respect to the predictions from available laboratory data. This difference is significant for a radioastronomical search for CH_2_CCH in interstellar sources with narrow lines. We thus included the measured frequencies in a new spectroscopic analysis to provide accurate frequency predictions for the interstellar search for propargyl at mm wavelengths. Moreover, we recommend that future searches for CH_2_CCH in cold interstellar clouds are carried out at lambda 3mm, rather than at lambda 8 mm. The 505-404 transition is about five times more intense than the 202-101 transition in TMC-1, which implies that detecting the former requires about seven times less telescope time than detecting the latter. We constrain the rotational temperature of CH_2_CCH in TMC-1 to 9.9+/-1.5K, which indicates that the rotational levels of this species are thermalized at the gas kinetic temperature. The revised value of the column density of CH_2_CCH (including ortho and para species) is (1.0+/-0.2)x10^14^cm^-2^, and thus the CH_2_CCH/CH_3_CCH abundance ratio is revised from slightly below one to nearly one. This study opens the door for future detections of CH_2_CCH in other cold interstellar clouds, making possible to further investigate the role of this very abundant hydrocarbon radical in the synthesis of large organic molecules such as aromatic rings.
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