An ALMA Survey of H 2 CO in Protoplanetary Disks

Abstract

H 2 CO is one of the most abundant organic molecules in protoplanetary disks and can serve as a precursor to more complex organic chemistry. We present an Atacama Large Millimeter/submillimeter Array survey of H 2 CO toward 15 disks covering a range of stellar spectral types, stellar ages, and dust continuum morphologies. H 2 CO is detected toward 13 disks and tentatively detected toward a fourteenth. We find both centrally peaked and centrally depressed emission morphologies, and half of the disks show ring-like structures at or beyond expected CO snowline locations. Together these morphologies suggest that H 2 CO in disks is commonly produced through both gas-phase and CO-ice-regulated grain-surface chemistry. We extract disk-averaged and azimuthally-averaged H 2 CO excitation temperatures and column densities for four disks with multiple H 2 CO line detections. The temperatures are between 20–50 K, with the exception of colder temperatures in the DM Tau disk. These temperatures suggest that H 2 CO emission in disks generally emerges from the warm molecular layer, with some contributions from the colder midplane. Applying the same H 2 CO excitation temperatures to all disks in the survey, we find that H 2 CO column densities span almost three orders of magnitude (∼5 × 10 11 –5 × 10 14 cm −2 ). The column densities appear uncorrelated with disk size and stellar age, but Herbig Ae disks may have less H 2 CO compared to T Tauri disks, possibly because of less CO freeze-out. More H 2 CO observations toward Herbig Ae disks are needed to confirm this tentative trend, and to better constrain under which disk conditions H 2 CO and other oxygen-bearing organics efficiently form during planet formation.