2026ApJ...998...53N

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 ∼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-H₂¹⁸O 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 H₂¹⁸O at 23.6σ and 9.3σ, respectively. Rotational-diagram analysis using a Markov Chain Monte Carlo approach yields T_rot = 116.89 ± 12.81 K and N = (4.90 ± 1.69) × 10¹⁵ cm^–2 for H₂¹⁸O and T_rot = 87.46 ± 4.95 K and N = (4.47 ± 0.62) × 10¹⁵ cm^–2 for HDO. These results imply water vapor abundances of ${N}_{{{\rm{H}}}_{2}{\rm{O}}}/{N}_{{{\rm{H}}}_{2}}\sim 3\times 1{0}^{-7}$-5 × 10^–6 and an HDO/H₂O ratio of (0.4-2.0) × 10^–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 ∼26% of the expected strength. This attenuation can be explained by a more compact, hotter emitting region with an effective radius of ∼53 au and/or frequency-dependent dust absorption that enlarges the apparent inner cavity at a 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.