EX Lupi from quiescence to outburst: Exploring the LTE approach in modeling blended H₂O and OH mid-infrared emission

We present a comparison of archival Spitzer spectra of the strongly variable T Tauri EX Lupi, observed before and during its 2008 outburst. We analyze the mid-infrared emission from gas-phase molecules thought to originate in a circumstellar disk. In quiescence the emission shows a forest of H ₂O 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 outburst emission shows instead remarkable changes: H₂O and OH line fluxes increase, new OH, H ₂, and HI transitions are detected, and organics are no longer seen. We adopt a simple model of a single-temperature slab of gas in local thermal equilibrium, a common approach for molecular analyses of Spitzer spectra, and derive the excitation temperature, column density, and emitting area of H ₂O and OH. We show how model results strongly depend on the selection of emission lines fitted and how this is likely to be attributed to a combination of non-thermal excitation and multiple emission components. Using H₂O lines that can be approximated as thermalized to a single temperature, our results are consistent with a column density decrease in outburst while the emitting area of warm gas increases. A rotation diagram analysis suggests that the OH emission can be explained with two temperature components, which remarkably increase in column density in outburst. The relative change of H₂O and OH emission suggests a key role for UV radiation in the disk surface chemistry.