Zooming into the water snow line: High-resolution water observations of the HL Tau disk

¹ Dipartimento di Fisica, Università degli Studi di Milano, Via Celoria 16, 20133 Milano, Italy
² Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile
³ Max-Planck-Institut für Astronomie, Königstuhl 17, 67117 Heidelberg, Germany
⁴ INAF – Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy
⁵ European Southern Observatory (ESO), Karl-Schwarzschild-Straße 2, 85748 Garching bei München, Germany

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 8 October 2025
Accepted: 19 November 2025

Abstract

Context. Water is one of the central molecules for the formation and habitability of planets. In particular, the region where water freezes out, the water snow line, could be a favorable location for planets to form in protoplanetary disks.

Aims. We aimed to spatially resolve the water emission in the HL Tau disk using high-resolution ALMA observations of the H₂O 183 GHz line (E_u = 205 K). We compared the spatially resolved H₂O emission with that of H¹³CO⁺, a chemical tracer of the water snow line, to observationally test their anticorrelation. In addition, we aimed to quantify the fraction of the water reservoir hidden by optically thick dust at ALMA wavelengths versus far- and mid-IR wavelengths.

Methods. We used high-resolution ALMA observations to spatially resolve the H₂O 3_1,3–2_2,0 line at 183 GHz, H¹³CO⁺ J = 2–1, and the SO 4₄–3₃ transition in the HL Tau disk. A rotational diagram analysis was used to characterize the water reservoir seen with ALMA and compare it to the reservoir visible at mid- and far-IR wavelengths.

Results. We find that the H₂O 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 snow line being located inside the central beam (≲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 due to optically thick dust hiding the emission, whereas 35–70% is visible with ALMA. An anticorrelation between the H₂O and H¹³ CO⁺ emission is found, but it is likely caused by optically thick dust hiding the H¹³CO⁺ 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 H₂O emission.

Conclusions. Spatially resolved observations of H₂O lines at (sub)millimeter wavelengths provide valuable constraints on the location of the water snow line while probing the bulk of the gas-phase reservoirs.