An ALMA Survey of H2CO in Protoplanetary Disks



H2CO 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 H2CO towards 15 disks covering a range of stellar spectral types, stellar ages, and dust continuum morphologies.

H2CO 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 H2CO in disks is commonly produced through both gas-phase and CO-ice-regulated grain-surface chemistry. We extract disk-averaged and azimuthally-averaged H2CO excitation temperatures and column densities for four disks with multiple H2CO line detections. The temperatures are between 20-50K, with the exception of colder temperatures in the DM Tau disk. These temperatures suggest that H2CO emission in disks is generally emerging from the warm molecular layer, with some contributions from the colder midplane.

Applying the same H2CO excitation temperatures to all disks in the survey, we find that H2CO column densities span almost three orders of magnitude (∼5×1011−5×1014cm−2). The column densities appear uncorrelated with disk size and stellar age, but Herbig Ae disks may have less H2CO compared to T Tauri disks, possibly because of less CO freeze-out. More H2CO observations towards Herbig Ae disks are needed to confirm this tentative trend, and to better constrain under which disk conditions H2CO and other oxygen-bearing organics efficiently form during planet formation.

Jamila Pegues, Karin I. Öberg, Jennifer B. Bergner, Ryan A. Loomis, Chunhua Qi, Romane Le Gal, L. Ilsedore Cleeves, Viviana V. Guzmán, Jane Huang, Jes K. Jørgensen, Sean M. Andrews, Geoffrey A. Blake, John M. Carpenter, Kamber R. Schwarz, Jonathan P. Williams, David J. Wilner
(Submitted on 28 Feb 2020)
Comments: 20 pages, 14 figures, 3 figure sets, 9 tables. Published in ApJ (February 2020)
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Earth and Planetary Astrophysics (astro-ph.EP); Astrophysics of Galaxies (astro-ph.GA)
Journal reference: The Astrophysical Journal, 890, 142 (2020)
DOI: 10.3847/1538-4357/ab64d9
Cite as: arXiv:2002.12525 [astro-ph.SR] (or arXiv:2002.12525v1 [astro-ph.SR] for this version)
Submission history
From: Jamila Pegues
[v1] Fri, 28 Feb 2020 03:29:39 UTC (12,113 KB)
Astrobiology, Astrochemistry

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