The Distribution and Excitation of CH3CN in a Solar Nebula Analog

Ryan A. Loomis, L. Ilsedore Cleeves, Karin I. Öberg, Yuri Aikawa, Jennifer Bergner, Kenji Furuya, V.V. Guzman, Catherine Walsh
(Submitted on 3 May 2018)

Cometary studies suggest that the organic composition of the early Solar Nebula was rich in com- plex nitrile species such a CH3CN. Recent ALMA detections in protoplanetary disks suggest that these species may be common during planet and comet formation, but connecting gas phase mea- surements to cometary abundances first requires constraints on formation chemistry and distributions of these species. We present here the detection of seven spatially resolved transitions of CH3CN in the protoplanetary disk around the T-Tauri star TW Hya. Using a rotational diagram analysis we find a disk-averaged column density of N =1.45+0.19 × 1012 cm−2 and a rotational temperature of T    −0.15 T    =32.7+3.9 K. A radially resolved rotational diagram shows the rotational temperature to be con- rot    −3.4 stant across the disk, suggesting that the CH3CN emission originates from a layer at z/r∼0.3. Through comparison of the observations with predictions from a disk chemistry model, we find that grain-surface reactions likely dominate CH3CN formation and that in situ disk chemistry is sufficient to explain the observed CH3CN column density profile without invoking inheritance from the protostellar phase. However, the same model fails to reproduce a Solar System cometary abundance of CH3CN relative to H2O in the midplane, suggesting that either vigorous vertical mixing or some degree of inheritance from interstellar ices occurred in the Solar Nebula.

Comments:    15 pages, 12 figures, accepted for publication in ApJ
Subjects:    Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as:    arXiv:1805.01458 [astro-ph.EP] (or arXiv:1805.01458v1 [astro-ph.EP] for this version)
Submission history
From: Ryan Loomis 
[v1] Thu, 3 May 2018 17:59:49 GMT (12970kb,D)
https://arxiv.org/abs/1805.01458
Astrobiology Astrochemistry