FAUST XIX. D$_2$CO in the outflow cavities of NGC\,1333 IRAS\,4A: recovering the physical structure of its original prestellar core

TL;DR

This study uses high-resolution ALMA observations to analyze formaldehyde deuteration in the outflow cavities of NGC 1333 IRAS 4A, revealing how the deuteration ratio varies with distance and relates to the core's density profile.

Contribution

It provides new insights into the chemical processes and physical structure of the prestellar core through detailed deuteration ratio analysis in outflow cavities.

Findings

Deuteration ratio decreases from 60-20% to 10% with distance from the protostar.

High deuteration levels suggest shock injection of molecules from dust mantles.

The density profile of the prestellar core influences the observed deuteration distribution.

Abstract

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 ($\sim$\,100\,au) ALMA observations, we investigate the [D$_2$CO]/[HDCO] ratio along the cavity walls of the outflows emanating from IRAS\,4A1. Our analysis reveals a consistent decrease in the deuteration ratio (from $\sim$\,60-20\% to $\sim$\,10\%) with increasing distance from the protostar (from $\sim$\,2000\,au to $\sim$\,4000\,au). Given the large measured [D$_2$CO]/[HDCO], both HDCO and D$_2$CO 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 [D$_2$CO]/[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.