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