Water vapor emission at the warm cavity wall of the HD 100546 disk as revealed by ALMA

TL;DR

This paper reports the first spatially resolved ALMA observation of the main water isotopologue (H2O 183 GHz) in a transition disk, HD 100546. The emission peaks at the warm cavity wall near ~15 au, consistent with a water snowline set by thermal desorption in an irradiated inner cavity. The results validate thermo-chemical expectations that the snowline lies at the cavity edge and link water vapor to ice desorption of COMs such as methanol. The study demonstrates ALMA's capability to map water vapor and constrain snowline geometry in planet-forming disks, with implications for the chemical environments of nascent planets.

Abstract

We present spatially resolved ALMA observations of the water line at 183 GHz in the disk around the Herbig star HD 100546. The water vapor emission peaks at the inner edge of the warm dust cavity, located ~15 au from the central star. We attribute this to thermal desorption at the water snowline, shifted outward at the dust cavity wall directly heated by the intense radiation. This represents the first spatially resolved image of the water snowline using ALMA observations of the main water isotopologue in a protoplanetary disk. The water emission morphology peaking inside the first dust ring is consistent with previous ALMA detections of oxygen-bearing complex organic molecules in the disk, including thermally desorbed methanol. These findings signal that warm cavities of transition disks provide ideal targets to directly reconstruct the spatial distribution of water vapor and the snowline location with ALMA, and directly connect water vapor emission to ice desorption of complex organic species.

Figures (6)

• Figure 1: First and second panels: 1.7 mm continuum maps of the HD 100546 disk showing the faint outer ring (first), and a zoom in on the inner ring (second). Third panel: integrated intensity map of the H2O 183 GHz line emission. The intensity is expressed as velocity integrated brightness temperature. Pink lines show 3, 4, and 5 $\sigma$ contours, where $\sigma$ is evaluated as standard deviation inside an annulus between $2\farcs3$ and $4\farcs0$. Fourth panel: intensity weighted velocity map of the 183 GHz water line. The color scale is centered at the systemic velocity of 5.7 km s$^{-1}\,$ (white color). The ellipse in the bottom left of each panel represents the beam.
• Figure 2: Radial profile of the integrated intensity of the water emission (blue line) compared to the continuum one (dashed black line). Blue ribbons show the uncertainty on the water radial profile. The light blue region indicates the radial extent of the 3$\mu$m water ice absorption band from Gemini observations honda2016, and the vertical blue line shows the inner edge of the water emission inferred from Herschel observations vandishoeck2021pirovano2022. The hatched region corresponds to the beam semi-major axis of the water cube used to extract the profile. The negative trend of the radial profile beyond 100 au seems to be an artifact due to negative sidelobes in the PSF.
• Figure 3: Disk-integrated spectrum of the H2O line, extracted from a de-projected circle with a radius of 100 au (left panel), and after correcting for the Keplerian rotation (right panel). The vertical red dashed line indicates the systemic velocity of $5.7$ km s$^{-1}\,$, while the blue line on the top right side of each panel is the uncertainty. The dip in the spectrum in the left panel at $\sim-21$ km s$^{-1}\,$ is due to the low transmission at the peak of the atmospheric telluric water line (see Appendix \ref{['app:noise']} for details on the bandpass calibration and its effect on the retrieved water spectra).
• Figure 4: Left: RMS as a function of velocity extracted from a circle with a 500 au radius, for the three EBs and combining all EBs. Right: Water spectrum extracted from a circle with a 50 au radius, for the three EBs (EB0 and EB1 have been shifted by 100 and 200 mJy for clarity). Non-JvM corrected cubes imaged with natural weighting and a channel width of 1.2 km s$^{-1}\,$ were used. The vertical dashed lines correspond to the systemic velocity in the LSRK frame and the vertical dotted lines to $v = 0$ km s$^{-1}\,$ in the TOPO frame.
• Figure 5: Channel maps of the water line at 183 GHz. The systemic velocity is 5.7 km s$^{-1}\,$. White ellipses correspond to the location of the two peaks in the radial profile (see Fig. \ref{['fig:radial_profile']}) at $R = 15$ and $85$ au. Pink contours show 3,4, and 5$\sigma$ levels, where $\sigma$ is evaluated as standard deviation inside an annulus between 250 and 450 au for each channel. The white dashed contours show the expected Keplerian pattern.