Pure hydrocarbon cycles in TMC-1: Discovery of ethynyl cyclopropenylidene, cyclopentadiene, and indene

Abstract

We report the detection for the first time in space of three new pure hydrocarbon cycles in TMC-1: c-C3HCCH (ethynyl cyclopropenylidene), c-C5H6 (cyclopentadiene), and c-C9H8 (indene). We derive a column density of 3.1 × 1011 cm−2 for the first cycle and similar values, in the range (1−2) × 1013 cm−2, for the second and third. This means that cyclopentadiene and indene, in spite of their large size, are exceptionally abundant, only a factor of five less abundant than the ubiquitous cyclic hydrocarbon c-C3H2. The high abundance found for these two hydrocarbon cycles together with the high abundance previously found for the propargyl radical (CH2CCH) and other hydrocarbons, such as vinyl and allenyl acetylene (Agúndez et al. 2021, A&A, 647, L10; Cernicharo et al. 2021a, A&A, 647, L2; Cernicharo et al. 2021b, A&A, 647, L3), start to allow us to quantify the abundant content of hydrocarbon rings in cold dark clouds and to identify the intermediate species that are probably behind the in situ bottom-up synthesis of aromatic cycles in these environments. While c-C3HCCH is most likely formed through the reaction between the radical CCH and c-C3H2, the high observed abundances of cyclopentadiene and indene are difficult to explain through currently proposed chemical mechanisms. Further studies are needed to identify how five- and six-membered rings are formed under the cold conditions of a cloud such as TMC-1.

The mystery of unidentified infrared emission bands

“…Two pure hydrocarbon cyclic molecules: cyclopentadiene ( -C 5 H 6 , a 5-ring molecule) and indene ( -C 9 H 8 , a bicyclic molecule with both a 5- and 6-membered ring) have been detected in TMC-1 (Cernicharo et al. 2021 ; Burkhardt et al. 2021 ).…”

Detection of deuterated methylcyanoacetylene, CH2DC3N, in TMC-1

“…Methylcyanoacetylene, CH 3 C 3 N, also known as cyanopropyne or methylpropionitrile, has been detected with high abundance in the cold dark cloud TMC-1 (Broten 1984) and more recently by Marcelino et al (2021) using a high sensitivity line survey on TMC-1 gathered with the Yebes 40m radio telescope (see, e.g., Cernicharo et al 2021d). Hence, the deuterated isotopologues of CH 3 C 3 N are good candidates to be observed in this source using the same line survey.…”

CH3-Terminated Carbon Chains in the GOTHAM Survey of TMC-1: Evidence of Interstellar CH3C7N

“…The ongoing GOTHAM survey (GBT Observations of TMC-1: Hunting for Aromatic Molecules) along with a deep Q-band survey being conducted on the Yebes 40 m radio telescope (Cernicharo et al 2020) have thus far proven to be very useful for rigorous studies of exotic molecules in TMC-1. With many new detections of various carbon-bearing molecules, their isomers, and cyclic/polycyclic species, these projects have effectively expanded our knowledge of the interstellar molecular carbon reservoir before star formation occurs (Xue et al 2020;Loomis et al 2021;McGuire et al 2018;McCarthy et al 2020;McGuire et al 2020;McGuire et al 2021;Cabezas et al 2021;Cernicharo et al 2021). In this work, we present a detailed analysis of CH 3 -terminated carbon chains toward TMC-1 as well as the first detection of methylcyanotriacetylene (CH 3 C 7 N, Chen et al (1998)) using the second data release of GOTHAM.…”