2025ApJ...991..128R

We present a spectroscopic analysis of the GM Aur disk using JWST MIRI-MRS, as part of the JWST Disk Infrared Spectral Chemistry Survey (JDISCS). The 1D spectrum exhibits faint dust continuum emission and is relatively poor in molecular gas compared to protoplanetary disks with no inner dust cavities. We identify fundamental CO emission, bright rotationally excited OH lines longward of 9 μm, weak rotational H₂O emission, the methyl cation CH${}_{3}^{+}$, HCO⁺, and tentatively identify CO₂. Otherwise, the spectrum is dominated by H₂ and atomic emission. We model the molecular spectra using slab models and retrieve excitation temperatures, column densities, and emitting areas in a Bayesian framework. The OH line fluxes are used to estimate the photodissociation rate of water. We find that 5.4 × 10⁴⁰ H₂O molecules are photodestroyed every second, which suggests that the highest-energy OH lines trace a region near the 0.2 au dust cavity wall with an incident far-ultraviolet flux of G₀ ∼ 10⁷. The OH emission is compared to DALI models, which point toward a high incident flux of Lyα photons in the OH emitting region. We further use the H I lines to provide an independent estimate of the accretion luminosity and mass accretion rate, in agreement with prior measurements. A comparison with other cavity disks indicates that more evolved systems such as GM Aur may be less volatile rich and are characterized by brighter OH prompt emission and abundant molecular ions.