ALMA Spectral Survey of An eruptive Young star, V883 Ori (ASSAY): Freshly Sublimated Complex Organic Molecules (COMs) in the Keplerian Disk

ABSTRACT :
Investigating complex organic molecules (COMs) inventory in protoplanetary disks has been challenging as the COMs emission is suppressed by being trapped in an icy mantle of dust grains. However, a Keplerian disk around an eruptive young star V883 Ori is heated, causing COMs to be freshly sublimated in a more extended area. We present an investigation of COMs in the disk based on a spectral survey of V883 Ori carried out with ALMA in Band 6 (220.7~274.9 GHz), being able to spatially resolve COMs emitting area. Using a Local Thermodynamic Equilibrium (LTE) spectrum modeling tool combined with molecular databases, we identified about 3,600 molecular emission lines in disk-averaged spectra, discovering 23 COMs in the disk. We estimated the column densities of COMs detected through LTE line fitting method utilizing only isolated, optically thin lines. The most distinct chemical phenomenon found in the disk is that nitrogen-bearing COMs beyond methyl cyanide (CH3CN) are missing, whereas oxygen-bearing COMs, except for the CH2OH-bearing molecules, are abundant. The missing CH2OH-bearing COMs may indicate the water-ice dominant, rather than CO-ice dominant, environment for forming COMs at the grain surfaces. We also explored column density ratios of COMs relative to methanol (COM/CH3OH) and compared them with those of various objects in different evolutionary stages, from Class 0 hot corinos to a Solar System comet 67P/Churyumov-Gerasimenko, to examine the effect of evolution on the COM compositions. In general, the COM/CH3OH ratios in V883 Ori are higher than in the hot corinos and hot cores, while they are comparable to the cometary values. This may indicate that the planet-forming material chemically evolves in the disk environment rather than keeping the pristine compositions inherited from the cold prestellar cores. In addition, as found in the comet 67P/Churyumov-Gerasimenko, nitrogen might also be trapped as ammonium salt within the dust grains in the V883 Ori disk, which may prevent the formation of nitrogen-bearing COMs.