Detection of the propargyl radical at λ 3 mm

Abstract

We report the detection of the propargyl radical (CH2CCH) in the cold dark cloud TMC-1 in the λ 3 mm wavelength band. We recently discovered this species in space toward the same source at a wavelength of λ 8 mm. In those observations, various hyperfine components of the 20,2–10,1 rotational transition, at 37.5 GHz, were detected using the Yebes 40 m telescope. Here, we used the IRAM 30 m telescope to detect ten hyperfine components of the 50,5–40,4 rotational transition, lying at 93.6 GHz. The observed frequencies differ by 0.2 MHz with respect to the predictions from available laboratory data. This difference is significant for a radio-astronomical search for CH2CCH 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 millimeter wavelengths. Moreover, we recommend that future searches for CH2CCH in cold interstellar clouds be carried out at λ 3 mm rather than at λ 8 mm. The 50,5–40,4 transition is about five times more intense than the 20,2–10,1 one 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 CH2CCH in TMC-1 to 9.9 ± 1.5 K, which indicates that the rotational levels of this species are thermalized at the gas kinetic temperature. The revised value of the column density of CH2CCH (including ortho and para species) is (1.0 ± 0.2) × 1014 cm−2, and thus the CH2CCH/CH3CCH abundance ratio is revised slightly higher, approaching one. This study opens the door to future detections of CH2CCH in other cold interstellar clouds, making it possible to further investigate the role of this very abundant hydrocarbon radical in the synthesis of large organic molecules, such as aromatic rings.

A new protonated molecule discovered in TMC-1: HCCNCH+

“…The IRAM 30m data correspond to observations in the 3 mm band and include data from our 3 mm line survey of TMC-1 (Marcelino et al 2007;Cernicharo et al 2012) and data taken in September 2021, which are described in Agúndez et al (2022) and Cabezas et al (2022a). Briefly, in the observations carried out in 2021 the 3 mm EMIR receiver was used connected to a fast Fourier transform spectrometer, providing a spectral resolution of 48.84 kHz.…”

Discovery of five cyano derivatives of propene with the QUIJOTE line survey

“…Cernicharo et al 2021a,b,c,d and references therein). Propargyl, CH 2 CCH, has been found to be one of the most abundant hydrocarbon radicals in this source (Agúndez et al 2021(Agúndez et al , 2022a. Other hydrocarbons such as CH 2 CCHCCH and CH 2 CHCCH have been found to have large abundances as well (Cernicharo et al 2021b,d).…”

Discovery of CH2CCHC4H and a rigorous detection of CH2CCHC3N in TMC-1 with the QUIJOTE line survey

“…The sensitivity of QUIJOTE is a factor ∼50 larger than that of previous TMC-1 Q-band line surveys (Kaifu et al 2004), which has permitted us to address the search of molecules with a very low dipole moment (Cernicharo et al 2021a). Taking into account the large abundances derived from QUIJOTE's data for hydrocarbons such as CH 2 CHCCH (Cernicharo et al 2021d), CH 2 CCH (Agúndez et al 2021(Agúndez et al , 2022, CH 2 CCHCCH (Cernicharo et al 2021e), H 2 C 5 (Cabezas et al 2021), c-C 6 H 4 (Cernicharo et al 2021a), c-C 5 H 6 , and c-C 9 H 8 (Cernicharo et al 2021d), other similar species could be present in the survey, such as allenyl diacetylene (CH 2 CCHC 4 H). Rotational spectroscopy for this species has been measured in the laboratory by McCarthy et al (2020).…”