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

TL;DR

This study probes the water snowline in the FUor-like disk of V883 Ori by detecting para-$H_{2}^{18}$O and HDO in ALMA Band 7, complementing previous Band 5/6 measurements to derive water abundances and the HDO/H$_2$O ratio in the warm inner disk. Keplerian-rotation correction and template-based spectral fitting reveal robust detections ($23.6\sigma$ for HDO and $9.3\sigma$ for $H_{2}^{18}$O) with rotational-temperature analyses yielding $T_{rot}\approx117$ K for $H_{2}^{18}$O and $T_{rot}\approx87$ K for HDO, and column densities around $N\sim10^{15.6}$ cm$^{-2}$. The inferred water vapor abundances at the snowline are $N_{H_{2}O}/N_{H_{2}}\sim3\times10^{-7}$–$5\times10^{-6}$, and the HDO/H$_2$O ratios are $(0.4$–$2.0)\times10^{-3}$, consistent with inheritance from protostellar envelopes. However, the Band 7 HDO line is weaker than predicted by extrapolating from Band 6, suggesting either a more compact emitting region or increased dust opacity at higher frequency; higher angular-resolution observations are required to distinguish these possibilities and to precisely map the water snowline.

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}$O at 23.6$σ$ and 9.3$σ$, 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.

Figures (14)

• Figure 1: (a) Observed spectrum around the H$_{2}$$^{16}$O 321 GHz line (black solid line). (b) Spectrum after correction for Keplerian rotation (black solid line). In both (a) and (b), the systemic velocity has not been corrected; the blue dotted lines indicate the source velocity of V883 Ori (4.25 km s$^{-1}$) and mark the expected frequency of the H$_{2}$$^{16}$O transition. The green solid lines represent the combined fitting models of the nearby contaminated lines. The gray solid lines show the residuals obtained by subtracting the model spectra from the observed and Keplerian-corrected spectrum. The vertical black ticks indicate the central frequencies of the nearby contaminated lines (see Line 1-6 in Table \ref{['tab:kep_mask']}). The gray dashed lines indicate the 5$\sigma$ noise level, derived from the standard deviation of spectra taken in an off-source region; (a) from the raw spectrum, and (b) from the Keplerian-rotation-corrected spectrum.
• Figure 2: Similar to Figure \ref{['fig:spectra_h216o']}, but for the H$_2$$^{18}$O 322 GHz line. The blue solid lines show the fitted models for the H$_{2}$$^{18}$O line. The green solid lines represent the combined fitting models of the H$_{2}$$^{18}$O line and the nearby contaminated lines. The vertical black ticks indicate the central frequencies of the nearby contaminated lines (see Line 7--11 in Table \ref{['tab:kep_mask']}).
• Figure 3: Similar to Figure \ref{['fig:spectra_h218o']}, but for the HDO 335 GHz line. The vertical black ticks indicate the central frequencies of the nearby contaminated lines (see Line 12--14 in Table \ref{['tab:kep_mask']}).
• Figure 4: Channel maps around the frequency of the HDO line at 335 GHz. The position of the protostar is marked with a white cross, and the channels closest to the systemic velocity of V883 Ori (4.25 km s$^{-1}$) are indicated by a star in the upper right corner. Thin gray contours represent 1, 2, and 3$\sigma$ levels. To clearly highlight the emission region of the HDO line, the emission is saturated in some panels. The synthesized beam is shown in the lower right corner of the bottom-left panel. The Keplerian mask for the HDO emission is drawn as a thick white line, and the masks corresponding to the blended nearby contaminated lines and COM line; two in blueshifted region and two in redshifted region of the HDO line are shown as thick cyan and red lines, respectively.
• Figure 5: Similar to Figure \ref{['fig:channel_h218o']}, but for the H$_{2}$$^{18}$O line at 322 GHz. The Keplerian mask for the H$_{2}$$^{18}$O emission is drawn as a thick white line, and the masks corresponding to the nearby contaminated lines; two in blueshifted region and three in redshifted region of the H$_{2}$$^{18}$ are shown as thick cyan and red lines, respectively.