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

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.