Formaldehyde and methanol deuteration in protostars: fossiles from a past fast high density pre-collapse phase

TL;DR

This study uses a detailed gas-grain model to explain the high deuteration levels of formaldehyde and methanol in protostars, revealing they form during a rapid, dense pre-collapse phase and emphasizing the importance of abstraction reactions.

Contribution

The paper introduces a comprehensive multilayer gas-grain model to explain observed deuteration levels, highlighting the significance of a fast pre-collapse phase and specific chemical reactions.

Findings

High deuteration occurs during a rapid pre-collapse phase at 5×10^6 cm^-3 density.

D and H abstraction reactions are essential for reproducing observed deuteration ratios.

The pre-collapse phase lasts only a few thousand years.

Abstract

Extremely high deuteration of several molecules have been observed around low mass protostars since a decade. Among them, formaldehyde and methanol present particularly high deuteration, with observations of abundant doubly and triply deuterated forms. Both species are thought to be mainly formed on interstellar grains during the low temperature and dense pre-collapse phase by H and D atom additions on the iced CO. We present here a theoretical study of the formaldehyde and methanol deuteration obtained with our gas-grain model, GRAINOBLE. This model takes into account the multilayer nature of the mantle and explores the robustness of the results against the uncertainties of poorly constrained chemical and surface model parameters. The comparison of the model predictions with the observations leads to two major results: i) the observed high deuteration is obtained during the last phase of the pre-collapse stage, when the density reaches 5 10^6 cm^-3, and this phase is fast, lasting only several thousands years. ii) D and H abstraction and substitution reactions are crucial in making up the observed deuteration ratios; This work shows the power of chemical composition as a tool to reconstruct the past history of protostars.