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

TL;DR

This paper reports the first detection of the cyanomethyl molecule CH$_2$CN in a protoplanetary disk, revealing insights into its formation and distribution, and comparing it with related molecules like CH$_3$CN.

Contribution

The study provides the first observational detection of CH$_2$CN in a protoplanetary disk and analyzes its abundance, temperature, and distribution, advancing understanding of complex molecule formation in disks.

Findings

CH$_2$CN detected in TW Hya disk with a column density of ~8.4×10^{12} cm$^{-2}$.

Rotational temperature of CH$_2$CN is around 40 K, consistent across the disk.

CH$_2$CN/CH$_3$CN ratio ranges between 4 and 10, matching chemical model predictions.

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 $(6.3\pm 0.5)\times10^{12}$ cm$^{-2}$, which is rescaled to reflect a 3:1 ortho-para ratio, resulting in a total column density, $N_{\rm tot}$, of $(8.4\pm 0.7)\times10^{12}$ cm$^{-2}$. We calculate a disk-average rotational temperature, $T_{\rm{rot}}$ = $40 \pm 5$ K, while a radially resolved analysis shows that $T_{\rm{rot}}$ 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.