FAUST. X. Formaldehyde in the protobinary system [BHB2007] 11: Small-scale deuteration

Context. Deuterium in H-bearing species is enhanced during the early stages of star formation. However, only a small number of high-spatial-resolution deuteration studies exist towards protostellar objects, leaving the small-scale structures of these objects unrevealed and understudied.
Aims: We aim to constrain the deuterium fractionation ratios in a Class 0/I protostellar object in formaldehyde (H₂CO), which has abundant deuterated isotopologues in this environment.
Methods: We used the Atacama Large Millimeter Array (ALMA) within the context of the Large Program Fifty AU STudy of the chemistry in the disk/envelope system of Solar-like protostars (FAUST) to observe the Class 0/I protobinary system [BHB2007] 11, whose emission components are embedded in circumstellar disks that have radii of 2 to 3 au. The system is surrounded by a complex filamentary structure (the so-called streamers) connected to the larger circumbinary disk. In this work, we present the first study of formaldehyde D-fractionation towards this source with detections of H₂CO 3(0,3)-2(0,2), combined with HDCO 4(2,2)-3(2,1), HDCO 4(1,4)-3(1,3) and D₂CO 4(0,4)-3(0,3). These observations probe the structures of the protobinary system, enabling us to resolve multiple velocity components associated with the methanol hot spots also uncovered by FAUST data, as well as the colder external envelope. In addition, based on the kinematics seen in our observations of the H₂CO emission, we propose the presence of a second large-scale outflow.
Results: The results derived from our ALMA observations agree with the current literature in that we only find the deuterated species HDCO and D₂CO in the central regions of the core, while undeuterated H₂CO is found more ubiquitously. From our radiative transfer modelling, we the column density of H₂CO to be in the range of (3-8) × 10¹⁴ cm⁻² and that of HDCO to be within (0.8−2.9) × 10¹³ cm⁻². The column density for the single detected velocity component of D₂CO is within (2.6-1.3) × 10¹² cm⁻². This yields an average D/H ratio for formaldehyde in [BHB2007] 11 of 0.02 _{- 0.01} ^{+ 0.02} from HDCO. The results of our kinematic model suggest that the dynamic feature is inconsistent with a streamer-like nature given the flat and outflowing velocity relation; we therefore tentatively conclude that the feature is an asymmetric molecular outflow launched by a wide-angle disk wind.