2026ApJ...998...40L

We have obtained a high-resolution, JWST NIRSpec 2.87-5.14 μm spectrum of the debris disk around HD 131488. We discover CO fundamental emission indicating the presence of warm fluorescent gas within ∼10 au of the star. The large discrepancy in CO's vibrational and rotational temperature indicates that CO is out of thermal equilibrium and is excited with UV fluorescence. Our UV fluorescence model gives a best fit of 1150 K with an effective temperature of 450, 332, and 125 K for the warm CO gas kinetic temperature within 0.5, 1, and 10 au to the star and a gas vibrational temperature of 8800 K. The newly discovered warm CO gas population likely resides between sub-astronomical unit scales and ∼10 au, interior to the cold CO reservoir detected beyond 35 au with the Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph and Atacama Large Millimeter/submillimeter Array (ALMA). The discovery of warm, fluorescent gas in a debris disk is the first such detection ever made. The detection of warm CO raises the possibility of unseen molecules (H₂O, H₂, etc.) as collisional partners to excite the warm gas. We estimated a lower mass limit for CO of 1.25 × 10^–7M_⊕, which is 10^–5 of the cold CO mass detected with ALMA and HST. We demonstrate that UV fluorescence emerges as a promising avenue for detecting tenuous gas at 10^–7 Earth-mass level in debris disks with JWST.