The Distribution and Chemistry of H$_2$CO in the DM Tau Protoplanetary Disk

TL;DR

This study uses ALMA observations and models to show that both gas-phase and grain-surface chemistry contribute to H$_2$CO distribution in the DM Tau protoplanetary disk, with their roles varying with distance from the star.

Contribution

It provides the first observational evidence quantifying the relative roles of gas and grain-surface formation pathways of H$_2$CO in a protoplanetary disk.

Findings

Both gas and grain-surface chemistry are needed to explain H$_2$CO distribution.

Gas-phase H$_2$CO explains the central peak in emission.

Grain-surface chemistry accounts for emission beyond the CO snowline.

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.