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We describe the design and performance of the Medium Resolution Spectrometer (MRS) for the JWST-MIRI instrument. The MRS incorporates four coaxial spectral channels in a compact opto-mechanical layout that generates spectral images over fields of view up to 7:7 × 7:7" in extent and at spectral resolving powers ranging from 1300 to 3700. Each channel includes an all-reflective integral field unit (IFU): an "image slicer" that reformats the input field for presentation to a grating spectrometer. Two 1024 × 1024 focal plane detector arrays record the output spectral images with an instantaneous spectral coverage of approximately one third of the full wavelength range of each channel. The full 5-28:5 μm spectrum is then obtained by making three exposures using gratings and pass-band-determining filters that are selected using just two three-position mechanisms. The expected on-orbit optical performance is presented, based on testing of the MIRI Flight Model and including spectral and spatial coverage and resolution. The point spread function of the reconstructed images is shown to be diffraction limited and the optical transmission is shown to be consistent with the design expectations. [ABSTRACT FROM AUTHOR]

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 circurnstellar disk. In quiescence the emission shows a forest of H20 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: H20 and OH line fluxes increase, new OH, H2, and H I 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 H20 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 H20 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 H20 and OH emission suggests a key role for UV radiation in the disk surface chemistry. [ABSTRACT FROM AUTHOR]

A correction to the article "Ex Lupi from quiescence to outburst: Exploring the LTE approach in modeling blended H2O and OH mid-infrared emission," by A. Banzatti, M.R. Meyer, S. Bruderer, V. Geers. I. Pascucci, F. Lahuis, A. Juhaász, T. Henning and P. ábrahám.