FAUST XIX. D2CO in the outflow cavities of NGC 1333 IRAS 4A: recovering the physical structure of its original prestellar core

Molecular deuteration is a powerful diagnostic tool for probing the physical conditions and chemical processes in astrophysical environments. In this work, we focus on formaldehyde deuteration in the protobinary system NGC 1333 IRAS 4A, located in the Perseus molecular cloud. Using high-resolution (∼100 au) ALMA (The Atacama Large Millimeter/submillimeter Array) observations, we investigate the [D2CO]/[HDCO] ratio along the cavity walls of the outflows emanating from IRAS 4A1. Our analysis reveals a consistent decrease in the deuteration ratio (from ∼60-20 per cent to ∼10 per cent) with increasing distance from the protostar (from ∼2000 to ∼4000 au). Given the large measured [D2CO]/[HDCO], both HDCO and D2CO are likely injected by the shocks along the cavity walls into the gas-phase from the dust mantles, formed in the previous prestellar phase. We propose that the observed [D2CO]/[HDCO] decrease is due to the density profile of the prestellar core from which NGC 1333 IRAS 4A was born. When considering the chemical processes at the base of formaldehyde deuteration, the IRAS 4A's prestellar precursor had a predominantly flat density profile within 3000 au and a decrease of density beyond this radius.