ALMA Band 7 Observations of Water Lines in the Protoplanetary Disk of V883 Ori

Abstract

The FU Orionis star V883 Ori provides a unique opportunity to probe the water snowline in a protoplanetary disk. During an accretion burst, the enhanced stellar luminosity heats the disk, sublimating ices and bringing volatile species into the gas-phase. The water snowline, located at $\sim$80 au in the midplane, represents a key boundary for dust growth and volatile delivery to forming planets. We present Atacama Large Millimeter/submillimeter Array Band 7 observations of V883 Ori that detect two targeted water isotopologue transitions: para-H$_2$$^{18}$O $5_{1,5}$-$4_{2,2}$ at 322 GHz and HDO $3_{3,1}$-$4_{2,2}$ at 335 GHz. After correcting for Keplerian rotation, we detect HDO and H$_2$$^{18}$O at 23.6$\sigma$ and 9.3$\sigma$, respectively. Rotational-diagram analysis using a Markov Chain Monte Carlo approach yields $T_\mathrm{rot}=116.89\pm12.81$ K and $N=(4.90\pm1.69)\times10^{15}\,\mathrm{cm}^{-2}$ for H$_2$$^{18}$O, and $T_\mathrm{rot}=87.46\pm4.95$ K and $N=(4.47\pm0.62)\times10^{15}\,\mathrm{cm}^{-2}$ for HDO. These results imply water vapor abundances of $N_{\mathrm{H_2O}}/N_{\mathrm{H_2}}\sim3\times10^{-7}$-$5\times10^{-6}$ and an HDO/H$_2$O ratio of $(0.4$-$2.0)\times10^{-3}$ just inside the water snowline, broadly consistent with inheritance from protostellar envelopes. The HDO line in Band 7 is significantly weaker than predicted from Band 6 extrapolation, showing only $\sim$26% of the expected strength. This attenuation can be explained by a more compact, hotter emitting region with an effective radius of $\sim$53 au and/or frequency-dependent dust absorption that enlarges the apparent inner cavity at higher frequency. Our results highlight both the diagnostic power of water isotopologue lines and the need for higher angular resolution observations to resolve the water snowline and test these scenarios.