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.

11 pages, 5 figures

References in corpus (14)

• The Critical Density and the Effective Excitation Density of Commonly Observed Molecular Dense Gas Tracers
• Photochemistry and astrochemistry: photochemical pathways to interstellar complex organic molecules
• The cometary composition of a protoplanetary disk as revealed by complex cyanides
• Constraining the X-ray and Cosmic Ray Ionization Chemistry of the TW Hya Protoplanetary Disk: Evidence for a Sub-interstellar Cosmic Ray Rate
• Hydrocarbon emission rings in protoplanetary disks induced by dust evolution
• CO and dust properties in the TW Hya disk from high-resolution ALMA observations
• Detection of circumstellar CH2CHCN, CH2CN, CH3CCH and H2CS
• Direct evidence of multiple reservoirs of volatile nitrogen in a protosolar nebula analogue
• The role of C/O in nitrile astrochemistry in PDRs and planet-forming disks
• An unbiased spectral line survey observation toward the low-mass star-forming region L1527
• The Excitation Conditions of CN in TW Hya
• A sensitive 3 mm line survey of L483: a broad view of the chemical composition of a core around a Class 0 object
• The ortho-to-para ratio of H${}_2$Cl${}^{+}$: Quasi-classical trajectory calculations and new simulations in light of new observations
• The ortho-to-para ratio of interstellar NH${}_2$: Quasi-classical trajectory calculations and new simulations

Cited by in corpus (3)

• Gas and Dust Shadows in the TW Hydrae Disk
• Protoplanetary Disk Chemistry
• Ortho-Para Chemistry of H2CO in the Protoplanetary Disk TW Hya