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

Abstract

This work presents water emission spectra at wavelengths covered by JWST (2.9-12.8 µm) as spectrally-resolved with high resolving powers (R = 30,000-100,000) using ground-based spectrographs. 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 85 disks and ≈ 160 spectra. 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 going from the ro-vibrational bands at 2-9 µm to the rotational lines at 12 µm, 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 curvatures indicative of optically thick emission (≈ 10 18 cm −2 ) from a range of excitation temperatures (≈ 800-1100 K); 3) the new 5 µm spectra demonstrate that slab model fits to the rotational lines at > 10 µm strongly over-predict the ro-vibrational emission bands at < 9 µm, implying non-LTE excitation. We discuss these findings in the context of a emission from a disk surface and a molecular inner disk wind, and provide a list of detailed guidelines to support the analysis and interpretation of spectrally-unresolved JWST spectra.

JWST/MIRI Spectroscopy of the Disk of the Young Eruptive Star EX Lup in Quiescence

“…Compared to previous Spitzer spectra, most of these lines are now de-blended in the MIRI spectrum, with some blending still visible at the shortest wavelengths. In Figure 3, we show two slab models for water using properties previously found from fits to high-resolution ground-based spectra of other T Tauri disks (Banzatti et al 2022a): T ∼ 1000 K for the ro-vibrational lines below 6.6 µm and T ∼ 600 K for the rotational lines at 13-16 µm, and a similar column density of ≈ 10 17 -10 18 cm −2 .…”

Protoplanetary Disk Chemistry

“…From space, the Spitzer mission was instrumental in surveying numerous disks in MIR emission lines (e.g., Carr & Najita 2008, Salyk et al 2011b), but at fairly low spectral resolution (λ/ λ < 700). These observations have been complemented with high-resolution spectroscopy from the ground, which avoids line blending and enables spectrotomographic characterizations of the radial origin of the emission lines (e.g., , Pontoppidan et al 2010, Banzatti et al 2022. Line absorption observations are possible in disks with favorable inclinations and are used to provide complementary constraints on the gas inventory (e.g., Gibb et al 2007, Najita et al 2021.…”

“…Furthermore, in a Keplerian disk, the spectral line shape can be used for spectroastrometry and to map different spectral line regions to different disk radii, providing additional tools to characterize chemical disk structures at smaller scales than are accessible with imaging alone. Spectroastrometry, line excitation modeling, and spectral profile analysis of spectrally resolved lines have been especially important in accessing substructure in the inner disk (e.g., Pontoppidan et al 2008, Fedele et al 2013 (Banzatti et al 2022). Spectral line profile analysis has also been applied to millimeter line observations to map out the molecular emission structure in disks (e.g., Dutrey et al 2008, Rosenfeld et al 2012, Bosman et al 2021c).…”

“…To begin, the disk temperature structure changes over time, sometimes abruptly due to an accretion burst. Such accretion bursts move snowlines outward, while stellar evolution results in an inward migration of snowline locations over time (Ciesla & Cuzzi 2006, Banzatti et al 2015, Cieza et al 2016, Price et al 2021). Furthermore, inward drift of pebbles often occurs on timescales similar to those of desorption, which can readily move the effective snowline inward by a factor of two (Piso et al 2016).…”

“…The outer disk ice reservoir is constrained by interstellar ice observations (setting the initial conditions), a small set of disk ice observations (e.g., Pontoppidan et al 2005, Aikawa et al 2012, comet compositions (Mumma & Charnley 2011, and disk gas-phase observations of sublimated ice. Such disk gas-phase observations are possible following a stellar luminosity burst and in sources where the outer disk is exposed to high levels of stellar radiation due to the combination of a higher-mass star and a large inner cavity (Banzatti et al 2012(Banzatti et al , 2015Cieza et al 2016;Lee et al 2019;Booth et al 2021b;van der Marel et al 2021). Disk and comet observations indicate that, similar to the ISM, disk icy grains consist mainly of H 2 O, CO 2 , CO, NH 3 , and Orich organics.…”