Dimethyl Ether Emission Tracing Icy Organic Sublimates in the MWC 480 Protoplanetary Disk

ABSTRACT :
Determining the availability of complex organic molecules (COMs) at the epoch of planet formation provides insights into the chemical evolution of the interstellar medium (ISM) and the origin of organic materials in our Solar System. We present sensitive Atacama Large Millimeter/submillimeter Array observations of the disk around the Herbig Ae star MWC 480, where we detect dimethyl ether (CH3OCH3) emission for the first time in this disk. The spatially unresolved, compact (<25 au in radius) nature, the broad line width (~30 km/s), and the high excitation temperature (~200 K) are consistent with thermal sublimation of COMs in the warm inner disk. Despite the detection of CH3OCH3, methanol (CH3OH), the most abundant COM in the ISM, has not been detected, from which we constrain the column density ratio of CH3OCH3/CH3OH>7. This high ratio may indicate the reprocessing of COMs during the disk phase, as well as the effect of the physical structure in the inner disk. We also find that this ratio is higher than in COM-rich transition disks recently discovered, indicating substantial chemical reprocessing of COMs in the full disk of MWC 480 while the COM emission in the transition disks predominantly tracing the sublimates of inherited ice.