The First Detection of CH${}_2$CN in a Protoplanetary Disk

Abstract

We report the first detection of the molecule cyanomethyl, CH${}_2$CN, in a protoplanetary disk. Until now, CH${}_2$CN had only been observed at earlier evolutionary stages, in the giant molecular clouds TMC-1 and Sgr 2, and the prestellar core L1544. We detect six transitions of ortho-CH${}_2$CN towards the disk around nearby T Tauri star TW Hya. An excitation analysis reveals that the disk-averaged column density, $N$, for ortho-CH${}_2$CN is $(6.3\pm 0.5)\times 10^{12}$ cm${}^{-2}$, which is rescaled to reflect a 3:1 ortho-para ratio, resulting in a total column density, $N_{\mathrm{tot}}$, of $(8.4\pm 0.7)\times 10^{12}$ cm${}^{-2}$. We calculate a disk-average rotational temperature, $T_{\mathrm{rot}}$ = $40\pm 5$ K, while a radially resolved analysis shows that $T_{\mathrm{rot}}$ 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,CH${}_2$CN 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 CH${}_3$CN emission towards the TW Hya disks, since the observed emission gap disappears when convolving our observations with a larger beam size. We obtain a CH${}_2$CN/CH${}_3$CN ratio ranging between 4 and 10. This high CH${}_2$CN/CH${}_3$CN 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.

arXiv comment11 pages, 5 figures

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