On the Interpretation of Mid-Infrared Absorption Lines of Gas-Phase H$_2$O as Observed by JWST/MIRI

Abstract

Ro-vibrational absorption lines of H$_2$O in the 5-8 $\mu$m wavelength range selectively probe gas against the mid-infrared continuum emitting background of the inner regions of YSOs and AGN and deliver important information about these warm, dust-obscured environments. JWST/MIRI detects these lines in many lines of sight at a moderate spectral resolving power of $R\sim3500$ (FWHM of 85 km/s). Based on our analysis of high-resolution SOFIA/EXES observations, we find that the interpretation of JWST/MIRI absorption spectra can be severely hampered by the blending of individual transitions and the lost information on the intrinsic line width or the partial coverage of the background continuum source. In this paper, we point out problems such as degeneracy that arise in deriving physical properties from an insufficiently resolved spectrum. This can lead to differences in the column density by two orders of magnitude. We emphasize the importance of weighting optically thin and weak lines in spectral analyses and provide recipes for breaking down the coupled parameters. We also provide an online tool to generate the H$_2$O absorption line spectra that can be compared to observations.