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 ground-based spectrally resolved water emission at R = 30,000–100,000 over infrared wavelengths covered by the JWST (2.9–12.8 μm). Two new surveys with iSHELL and the VISIR are combined with previous spectra from the CRIRES to cover parts of multiple rovibrational and rotational bands observable within telluric transmission bands, for a total of ≈160 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 rovibrational bands at 2–9 μm to the rotational lines at 12 μm, and partly match broad and narrow emission components, respectively, as extracted from rovibrational 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 (≈1018 cm−2) from a range of excitation temperatures (≈800–1100 K); and (3) the new 5 μm spectra demonstrate that slab model fits to the rotational lines at >10 μm strongly overpredict the rovibrational emission bands at <9 μm, implying vibrational excitation not in thermodynamic equilibrium. 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.

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 deblended 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 2023): T ∼ 1000 K for the rovibrational 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 .…”

Spiers Memorial Lecture: Astrochemistry at high resolution

“…35,78–81 However, even methanol turns out to be a species difficult to detect in protoplanetary disks, 82 maybe because of the large dust opacity that absorbs much of its emission 83 or the relatively low temperature. 84 Fortunately, the new observations of JWST are already giving plenty of information on the inner regions of the disk, 85–87 an appetizer of which is also reported in this volume ( e.g. , van Dishoeck et al and Kamp et al ).…”

Water-rich Disks around Late M Stars Unveiled: Exploring the Remarkable Case of Sz 114

“…The most prominent characteristic of this spectrum is the forest of water lines, from the hot bending rovibrational band at ∼6.6 μm to the cooler pure rotational lines at longer wavelengths (see, e.g., Figure 1 in Banzatti et al 2023b). In relation to water emission, Sz 114 is very different from other mid-to-late M-star disks observed with Spitzer/IRS (Pascucci et al 2013) and the similarly late M5 object recently observed with MIRI-MRS (J160532; Tabone et al 2023) in which water emission was either very weak or absent.…”

“…Most of the volatile oxygen and carbon are carried by H 2 O and CO, while most nitrogen is likely in the undetected form of gas-phase N 2 (e.g., Pontoppidan et al 2014, and references therein). For the brightest disks, high-resolution (R ∼ 25,000-95,000) ground-based telescopes targeted few accessible transitions and spectroscopically confirmed that infrared molecular emission mostly traces gas within a few au from the star (e.g., Pontoppidan et al 2010;Mandell et al 2012;Najita et al 2018;Salyk et al 2019;Banzatti et al 2023b). This is the gas inside the water-ice line (hereafter the snowline) that is accreted by those gaseous planets, including sub-Neptunes, within ∼1 au (e.g., Bean et al 2021;Izidoro et al 2022).…”