Astro & cosmo-chemical consequences of accretion bursts I: the D/H ratio of water

TL;DR

This study investigates how accretion bursts in protostellar discs influence the D/H ratio of water, revealing that such bursts significantly lower the ratio and extend water equilibration regions, impacting our understanding of solar system formation.

Contribution

It introduces 1D simulations including dead zones and infall, showing how accretion bursts alter water isotopic ratios compared to models without bursts.

Findings

Accretion bursts lower the D/H ratio of water in discs.

Water becomes equilibrated with hydrogen gas up to 1-3 AU during bursts.

Solar system constraints suggest different burst histories or disc properties.

Abstract

The D/H ratio of water in protostellar systems is a result of both inheritance from the parent molecular cloud and isotopic exchange in the disc. A possibly widespread feature of disc evolution, ignored in previous studies, is accretion bursts (or FU Orionis outbursts), which may thermally process a large fraction of the water. One proposed underlying mechanism for FU Orionis outbursts relies on the presence of a magnetically dead zone. Here we examine the evolution of (D/H)$_{\rm water}$ in 1D simulations of a disc's evolution that include dead zones and infall from an envelope with given D/H ratio in the infalling water ($\sim 10^{-3}$), and compare the results with similar calculations without dead zones. We find that the accretion bursts result in a significantly lower (D/H)$_{\rm water}$ ratio and a more extended region (radius up to $\sim 1-3$ AU) where water is equilibrated with hydrogen gas (D/H=$2\times 10^{-5}$), when compared to burst-free models. Solar system constraints suggest that our solar nebula either experienced no accretion bursts and had a Schmidt number $\lesssim 0.2$ or had a Schmidt number closer to "nominal" values ($\sim 1$) and experienced several accretion bursts. Finally, future observations of (D/H)$_{\rm water}$ in protoplanetary discs will allow inferences about angular momentum properties of the disc during disc building and the role of accretion bursts.