The kinematics and excitation of infrared water vapor emission from planet-forming disks: results from spectrally-resolved surveys and guidelines for JWST spectra

TL;DR

This study combines ground-based spectrally-resolved infrared water vapor observations from multiple surveys to analyze the kinematics and excitation conditions in planet-forming disks, providing guidelines for interpreting upcoming JWST spectra.

Contribution

It offers the first comprehensive analysis of water vapor emission kinematics and excitation across multiple wavelengths, supporting the interpretation of JWST spectra with new observational insights.

Findings

Water lines narrow from 2-9 μm to 12 μm, matching broad and narrow emission components.

Rotation diagrams indicate optically thick emission with excitation temperatures of 800-1100 K.

Non-LTE vibrational excitation is implied by discrepancies in slab model fits.

Abstract

This work presents ground-based spectrally-resolved water emission at R = 30000-100000 over infrared wavelengths covered by JWST (2.9-12.8 $\mu$m). Two new surveys with iSHELL and VISIR are combined with previous spectra from CRIRES and TEXES to cover parts of multiple ro-vibrational and rotational bands observable within telluric transmission bands, for a total of $\approx160$ spectra and 85 disks (30 of which are JWST targets in Cycle 1). The general expectation of a range of regions and excitation conditions traced by infrared water spectra is for the first time supported by the combined kinematics and excitation as spectrally resolved at multiple wavelengths. The main findings from this analysis are: 1) water lines are progressively narrower from the ro-vibrational bands at 2-9 $\mu$m to the rotational lines at 12 $\mu$m, and partly match a broad (BC) and narrow (NC) emission components, respectively, as extracted from ro-vibrational CO spectra; 2) rotation diagrams of resolved water lines from upper level energies of 4000-9500 K show vertical spread and curvatures indicative of optically thick emission ($\approx 10^{18}$ cm$^{-2}$) from a range of excitation temperatures ($\approx 800$-1100 K); 3) the new 5 $\mu$m spectra demonstrate that slab model fits to the rotational lines at $> 10$ $\mu$m strongly over-predict the ro-vibrational emission bands at $< 9$ $\mu$m, implying non-LTE vibrational excitation. We discuss these findings in the context of emission from a disk surface and a molecular inner disk wind, and provide a list of guidelines to support the analysis of spectrally-unresolved JWST spectra.