Astro- and cosmochemical 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)water in 1D simulations of a disc's evolution that includes dead zones and infall from an envelope with given D/H ratio in the infalling water (~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)water ratio and a more extended region (radius up to ~1-3 au) where water is equilibrated with hydrogen gas (D/H=2×10-5), when compared to burst-free models. Solar system constraints suggest that our solar nebula either experienced no accretion bursts and had a Schmidt number ≲ 0.2 or had a Schmidt number closer to 'nominal' values (~1) and experienced several accretion bursts. Finally, future observations of (D/H)water in protoplanetary discs will allow inferences about angular momentum properties of the disc during disc building and the role of accretion bursts.