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We present a comparison of archival Spitzer spectra of the strongly variableT Tauri EX Lupi, observed before and during its 2008 outburst. We analyze themid-infrared emission from gas-phase molecules thought to originate in acircumstellar disk. In quiescence the emission shows a forest of H2O lines,highly excited OH lines, and the Q branches of the organics C2H2, HCN, and CO2,similar to the emission observed toward several T Tauri systems. The outburstemission shows instead remarkable changes: H2O and OH line fluxes increase, newOH, H2, and HI transitions are detected, and organics are no longer seen. Weadopt a simple model of a single-temperature slab of gas in local thermalequilibrium, a common approach for molecular analyses of Spitzer spectra, andderive the excitation temperature, column density, and emitting area of H2O andOH. We show how model results strongly depend on the selection of emissionlines fitted, and that spectrally-resolved observations are essential for acorrect interpretation of the molecular emission from disks, particularly inthe case of water. Using H2O lines that can be approximated as thermalized to asingle temperature, our results are consistent with a column density decreasein outburst while the emitting area of warm gas increases. A rotation diagramanalysis suggests that the OH emission can be explained with two temperaturecomponents, which remarkably increase in column density in outburst. Therelative change of H2O and OH emission suggests a key role for UV radiation inthe disk surface chemistry.

Ex Lupi from Quiescence to Outburst: Exploring the LTE Approach in Modelling Blended H2O and OH Mid-Infrared Emission