Abstract
H$_2$CO ice on dust grains is an important precursor of complex organic molecules (COMs). H$_2$CO gas can be readily observed in protoplanetary disks and may be used to trace COM chemistry. However, its utility as a COM probe is currently limited by a lack of constraints on the relative contributions of two different formation pathways: on icy grain-surfaces and in the gas-phase. We use archival ALMA observations of the resolved distribution of H$_2$CO emission in the disk around the young low-mass star DM Tau to assess the relative importance of these formation routes. The observed H$_2$CO emission has a centrally peaked and radially broad brightness profile (extending out to 500 AU). We compare these observations with disk chemistry models with and without grain-surface formation reactions, and find that both gas and grain-surface chemistry are necessary to explain the spatial distribution of the emission. Gas-phase H$_2$CO production is responsible for the observed central peak, while grain-surface chemistry is required to reproduce the emission exterior to the CO snowline (where H$_2$CO mainly forms through the hydrogenation of CO ice before being non-thermally desorbed). These observations demonstrate that both gas and grain-surface pathways contribute to the observed H$_2$CO in disks, and that their relative contributions depend strongly on distance from the host star.
Highlights
The chemical structure of protoplanetary disks is set by a combination of gas and grain-surface chemistry, regulated by the temperature, density, and radiation properties of the disk (e.g. Aikawa et al 2002)
Grain-surface chemistry is especially important for the production of complex organic molecules (COMs), and for the prebiotic potential of planetary bodies forming in the disk (e.g. Garrod et al 2008; Garrod 2013; Mumma & Charnley 2011)
Based on Atacama Large (sub-)Millimeter Array (ALMA) observations and detailed chemical modeling, we have shown that there are both gas-phase and grain-surface contributions to the observed H2CO gas in DM Tau
Summary
The chemical structure of protoplanetary disks is set by a combination of gas and grain-surface chemistry, regulated by the temperature, density, and radiation properties of the disk (e.g. Aikawa et al 2002). The chemical structure of protoplanetary disks is set by a combination of gas and grain-surface chemistry, regulated by the temperature, density, and radiation properties of the disk Radiation impinging upon the disk from both the central star and the interstellar radiation field results in strong radial and vertical temperature gradients, differentiating the disk into three chemical layers: a cold midplane, a hot atmosphere, and a warm molecular layer at intermediate heights, (Aikawa et al 1996; Bergin et al 2007). Grain-surface chemistry is especially important for the production of complex organic molecules (COMs), and for the prebiotic potential of planetary bodies forming in the disk An alternative approach to estimating the COM abundance in disks is to constrain the chemistry that produces them, by observing smaller organic molecules with a grain-surface formation
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