FAUST. XVII. Super deuteration in the planet-forming system IRS 63 where the streamer strikes the disk

Context. Recent observations suggest that planet formation starts early, in protostellar disks of ≤10⁵ yr, which are characterized by strong interactions with the environment, such as through accretion streamers and molecular outflows.
Aims: To investigate the impact of such phenomena on the physical and chemical properties of a disk, it is key to understand what chemistry planets inherit from their natal environment.
Methods: In the context of the ALMA large program Fifty AU Study of the chemistry in the disk/envelope system of solar-like protostars (FAUST), we present observations on scales from ∼1500 au to ∼60 au of H₂CO, HDCO, and D₂CO toward the young planet-forming disk IRS 63.
Results: The H₂CO probes the gas in the disk as well as in a large scale streamer (∼1500 au) impacting onto the southeast disk side. We detected for the first time deuterated formaldehyde, HDCO and D₂CO, in a planet-forming disk and HDCO in the streamer that is feeding it. These detections allowed us to estimate the deuterium fractionation of H₂CO in the disk: [HDCO]/[H₂CO] ∼ 0.1 − 0.3 and [D₂CO]/[H₂CO] ∼ 0.1. Interestingly, while HDCO follows the H₂CO distribution in the disk and in the streamer, the distribution of D₂CO is highly asymmetric, with a peak of the emission (and [D]/[H] ratio) in the southeast disk side, where the streamer crashes onto the disk. In addition, D₂CO was detected in two spots along the blue- and redshifted outflow. This suggests that (i) in the disk, HDCO formation is dominated by gas-phase reactions in a manner similar to H₂CO, while (ii) D₂CO is mainly formed on the grain mantles during the prestellar phase and/or in the disk itself and is at present released in the gas phase in the shocks driven by the streamer and the outflow.
Conclusions: These findings testify to the key role of streamers in the buildup of the disk concerning both the final mass available for planet formation and its chemical composition.