The First Detection of CH2CN in a Protoplanetary Disk

Abstract

Abstract We report the first detection of the molecule cyanomethyl, CH2CN, in a protoplanetary disk. Until now, CH2CN had only been observed at earlier evolutionary stages, in the molecular clouds TMC-1, Sgr2, and L483, in the prestellar core L1544, and toward the protostar L1527. We detect six transitions of ortho-CH2CN toward the disk around nearby T Tauri star TW Hya. An excitation analysis reveals that the disk-averaged column density, N , for ortho-CH2CN is (6.3 ± 0.5) × 1012 cm−2, which is rescaled to reflect a 3:1 ortho-para ratio, resulting in a total column density, N tot, of (8.4 ± 0.7) × 1012 cm−2. We calculate a disk-average rotational temperature, T rot = 40 ± 5 K, while a radially resolved analysis shows that T rot remains relatively constant across the radius of the disk. This high rotation temperature suggests that in a static disk and if vertical mixing can be neglected, CH2CN is largely formed through gas-phase reactions in the upper layers of the disk, rather than solid-state reactions on the surface of grains in the disk midplane. The integrated intensity radial profiles show a ring structure consistent with molecules such as CN and DCN. We note that this is also consistent with previous lower-resolution observations of centrally peaked CH3CN emission toward the TW Hya disks, since the observed emission gap disappears when convolving our observations with a larger beam size. We obtain a CH2CN/CH3CN ratio ranging between 4 and 10. This high CH2CN/CH3CN is reproduced in a representative chemical model of the TW Hya disk that employs standard static disk chemistry model assumptions, i.e., without any additional tuning.