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

Recent observations suggest that planets formation starts early, in protostellar disks of 10⁵ yrs, which are characterized by strong interactions with the environment, e. g. , through accretion streamers and molecular outflows. To investigate the impact of such phenomena on disk physical and chemical properties it is key to understand what chemistry planets inherit from their natal environment. 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 towards the young planet-forming disk IRS~63. H₂CO probes the gas in the disk as well as in a large scale streamer (~1500 au) impacting onto the South-East (SE) disk side. We detect for the first time deuterated formaldehyde, HDCO and D₂CO, in a planet-forming disk, and HDCO in the streamer that is feeding it. This allows us to estimate the deuterium fractionation of H₂CO in the disk: HDCO/H₂CO0. 1-0. 3 and D₂CO/H₂CO0. 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 SE disk side, where the streamer crashes onto the disk. In addition, D₂CO is detected in two spots along the blue- and red-shifted outflow. This suggests that: (i) in the disk, HDCO formation is dominated by gas-phase reactions similarly to H₂CO, while (ii) D₂CO was 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. These findings testify on the key role of streamers in the build-up of the disk both concerning the final mass available for planet formation and its chemical composition.