Deuteration of water in protoplanetary discs during luminosity outbursts: model predictions for FU Ori discs

Abstract

Luminosity outbursts of FU Ori-type objects (FUors) allow us to observe in the gas the molecules that are typically present in the ice in protoplanetary discs. In particular, the fraction of deuterated water, which is usually is mostly frozen in the midplane of a protoplanetary disc, has been measured for the first time in the gas of the disc around a FUor V883 Ori. We test the hypothesis that the observed high HDO/H$_{2}$O ratio in the V883 Ori protoplanetary disc can be explained by luminosity outbursts of different amplitude, including a series of two consecutive outbursts. Using the ANDES astrochemical code, we modelled the distributions of water and deuterated water abundances under the action of luminosity outbursts of different amplitudes (from 400 to 10 000 $L_{\odot}$) and at different stellar luminosities at the pre-outburst stage. We show that the best agreement with the observed HDO/H$_{2}$O profile is obtained for outburst amplitudes of 2 000 and 10 000 $L_{\odot}$, while the observed bolometric luminosity of V883 Ori does not exceed 400 $L_{\odot}$. We discuss possible reasons for this discrepancy, including the presence of past luminosity outbursts, the age of the star, and the influence of additional heating mechanisms in the midplane of the protoplanetary disc. We also consider how the high observed $\rm HDO/H_{2}O$ ratio may be related to the evolution of the chemical composition of the ice in the protoplanetary disc and the chemical processes activated under outburst conditions.