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

TL;DR

This work investigates how FU Ori-type luminosity outbursts reshape water chemistry in protoplanetary discs by modelling the HDO/H$_2$O ratio with the ANDES astrochemical code across outburst amplitudes from $400$ to $10^4\,L_{\odot}$, including a two-outburst history. It demonstrates that reproducing the observed high HDO/H$_2$O in V883 Ori—especially in the $60$–$100$ au zone—favors either a past very bright outburst ($\sim 10^4\,L_{\odot}$) or a sequence of two outbursts, rather than the current luminosity of $\sim 400\,L_{\odot}$. The results also show that the inferred snow line position and isotopologue profiles depend strongly on the assumed disc temperature structure, with an alternative scaling $T_{\rm mp} \propto (L_{\rm acc}+L_\star)^{2/7} R^{-3/7}$ providing better simultaneous agreement in some cases. Overall, the study links the present gas-phase deuteration to the ice history and potential past luminosity events, offering insight into water delivery and the chemical legacy imprinted by FUor outbursts in young planetary systems.

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.

Figures (6)

• Figure 1: Evolution of the total luminosity of the accretion source in different models. Three models with different outburst amplitudes: 400, 2 000, and 10 000 $L_{\odot}$ are shown on the left. On the right is the model with two consecutive outbursts with amplitudes of 10 000 and 400 $L_{\odot}$. The time is specified relative to the outburst onset.
• Figure 2: Temperature in the protoplanetary disc midplane with no outburst for stellar ages of $0.1$ and $0.5$$M_{\odot}$ and at different values of accretion luminosity. The horizontal black line indicates the sublimation temperature of water ice ($\approx150$ K). The intersections with it of the dashed temperature profiles correspond to the approximate position of the water snow line at the given luminosities.
• Figure 3: Relative water abundance in the gas and on dust grain surface (in the ice) at different accretion luminosities (400, 2 000, and 10 000 $L_{\odot}$) for stellar ages of $0.1$ and $0.5$ Myr. The red, green, and blue colours denote outbursts with luminosities of 10 000, 2 000, and 400 $L_{\odot}$, respectively. The intersection of ice and gas components defines the snow line. At 10 000 $L_{\odot}$ (red), the ice abundance is low and the H$_2$O and HDO snow lines are absent. For fainter outbursts, the snow lines are at $20-30$ au (400 $L_{\odot}$, blue) and $\sim70$ au (2 000 $L_{\odot}$, green).
• Figure 4: Radial profiles of the HDO/H$_2$O ratio in the gas for different outburst scenarios. Black markers with error bars show observational data for the V883 Ori protoplanetary disc from Tobin2023, with values inside 40 au considered less reliable due to the contribution of the optically thick dust continuum. The blue line corresponds to the time before the outburst ($t=500$ kyr). The red, green, and magenta lines show the simulation results 131 years after the outburst at different maximum luminosities: 400 $L_{\odot}$, 2 000 $L_{\odot}$, and 10 000 $L_{\odot}$, respectively.
• Figure 5: Comparison of model HDO/H$_2$O radial profiles for stellar ages of $0.1$ and $0.5$$M_{\odot}$ with observed data for V883 Ori Tobin2023. The shown scenarios are: single 10 000 $L_{\odot}$ outburst (blue line), single 400 $L_{\odot}$ outburst (green line), the second outburst in the two $10\,000+400$$L_{\odot}$ outbursts scenario (red line), a fading 2 000 $L_{\odot}$ outburst at the moment when the luminosity is $400$$L_{\odot}$ (purple line).