Zooming into the water snowline: high resolution water observations of the HL Tau disk

TL;DR

This work directly maps water emission in HL Tau at high angular resolution using the H2O $183$ GHz line, comparing it with H$^{13}$CO$^+$ and SO to test the snowline tracer paradigm. A radially resolved rotational diagram shows an excitation temperature around $350$ K that is radius-independent, and the inner, optically thick region places the snowline at $\lesssim 6$ au in the probed height; a simple dust-opacity model demonstrates that the bulk of the water reservoir is accessible at (sub-)mm wavelengths (35–70%), while mid- and far-IR wavelengths see only $0.02\%-2\%$ due to dust opacity. The study also highlights how dust can masquerade as snowline signatures in H$^{13}$CO$^+$, while SO traces disk material, a streamer, and an outflow, with possible links to water-productive shocks. Overall, spatially resolved H2O $183$ GHz observations emerge as a robust tool to locate the water snowline and characterize the bulk gas-phase water reservoir in protoplanetary disks.

Abstract

Water is one of the central molecules for the formation and habitability of planets. In particular, the region where water freezes-out, the water snowline, could be a favorable location to form planets in protoplanetary disks. We use high resolution ALMA observations to spatially resolve H$_2$O, H$^{13}$CO$^+$ and SO emission in the HL Tau disk. A rotational diagram analysis is used to characterize the water reservoir seen with ALMA and compare this to the reservoir visible at mid- and far-IR wavelengths. We find that the H$_2$O 183 GHz line has a compact central component and a diffuse component that is seen out to ~75 au. A radially resolved rotational diagram shows that the excitation temperature of the water is ~350 K independent of radius. The steep drop in the water brightness temperature outside the central beam of the observations where the emission is optically thick is consistent with the water snowline being located inside the central beam ($\lesssim 6$ au) at the height probed by the observations. Comparing the ALMA lines to those seen at shorter wavelengths shows that only 0.02%-2% of the water reservoir is visible at mid- and far-IR wavelengths, respectively, due to optically thick dust hiding the emission whereas 35-70% is visible with ALMA. An anti-correlation between the H$_2$O and H$^{13}$CO$^+$ emission is found but this is likely caused by optically thick dust hiding the H$^{13}$CO$^+$ emission in the disk center. Finally, we see SO emission tracing the disk and for the first time in SO a molecular outflow and the infalling streamer out to ~2". The velocity structure hints at a possible connection between the SO and the H$_2$O emission. Spatially resolved observations of H$_2$O lines at (sub-)mm wavelengths provide valuable constraints on the location of the water snowline, while probing the bulk of the gas-phase reservoirs.

Figures (18)

• Figure 1: ALMA Band 5 images of the HL Tau disk and the 321 GHz H2O line. The continuum (top left) and the JvM-corrected integrated intensity maps of the H2O line at 183 GHz imaged with $r=0.0$ (top right) and $r=2.0$ (bottom right) providing high and moderate spatial resolution. The inset in the top right panel presents a zoom of the inner $0\farcs2$ and highlights the approximate water snowline location derived by GuerraAlvarado2024 The bottom left image is the reimaged H2O 321 GHz line originally presented in Facchini2024. The beams are indicated in the bottom left corners of the respective panels. The dust rings and gaps derived from high resolution ALMA observations are indicated with solid and dashed arcs in each panel ALMA2015CarrascoGonzalez2019GuerraAlvarado2024 and a 20 au scalebar is shown in the bottom right corner of the bottom right panel.
• Figure 2: Azimuthally averaged radial profiles of the integrated intensity (left) and brightness temperature (right). The H2O line at 183 GHz imaged with a robust parameter of 0.0 is indicated in blue, the H2O line at 321 GHz in orange, and the continuum emission in black. In the left panel the continuum emission is presented in arbitrary units and the upper limit on the water snowline location is indicated with the vertical red line. Note the difference in the radial axes. The beams are indicated with the horizontal bars in the top right corner.
• Figure 3: Radially resolved rotational diagram analysis of the H2O emission in the HL Tau disk. The excitation temperature (left), column density (middle), and optical depth (right) of the H2O 183 GHz (blue) and 321 GHz (orange) lines are obtained after convolving both lines to the same beam size. The dashed lines are derived under the assumption of fully optically thin emission whereas the solid lines include the correction for the optical depth of the lines. The black scatter points indicate the disk averaged results from Facchini2024 assuming an emitting region of 17 au and the upper limit on the water snowline location is indicated with the vertical red line.
• Figure 4: Rotational diagram summarizing the disk integrated H2O line detections in the HL Tau disk with ALMA in the (sub-)mm (cyan), Herschel PACS in the far-IR (pink), and Gemini North in the mid-IR (orange). The left panel shows the rotational diagram without a correction for the difference in the continuum optical depth for the various lines whereas in the right panel this correction factor derived from a thermochemical model is applied. The shaded points in the right panel are identical to those in the left panel.
• Figure 5: Abundance of gas-phase H2O in a representative model for the HL Tau disk (colored background). The orange, pink, and cyan contours in the left, middle, and right panels indicate the $\tau_{\rm dust}=1$ surface for the lines in Table \ref{['tab:all_H2O']} in the mid-IR, far-IR, and (sub-)mm, respectively. In the middle and right panel the solid and dashed lines indicate the long and short wavelength ends of the range of lines in Table \ref{['tab:all_H2O']} in the far-IR and (sub-)mm, respectively. The water snow surface in the model is indicated with the black contour.