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 ALMA survey of H${}_2$CO towards 15 disks covering a range of stellar spectral types, stellar ages, and dust continuum morphologies. H${}_2$CO is detected towards 13 disks and tentatively detected towards a 14th. 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 20-50K, with the exception of colder temperatures in the DM Tau disk. These temperatures suggest that H${}_2$CO emission in disks is generally emerging 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 ($\sim 5\times 10^{11}-5\times 10^{14}{\mathrm{cm}}^{-2}$). 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 towards 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.

20 pages, 14 figures, 3 figure sets, 9 tables. Published in ApJ (February 2020)

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