A SPITZER SPACE TELESCOPE FAR-INFRARED SPECTRAL ATLAS OF COMPACT SOURCES IN THE MAGELLANIC CLOUDS. I. THE LARGE MAGELLANIC CLOUD

We present far-infrared spectra, λ = 52–93 μm, obtained with the Spitzer Space Telescope in the spectral energy distribution mode of its MIPS instrument, of a representative sample of the most luminous compact far-infrared sources in the Large Magellanic Cloud (LMC). These include carbon stars, OH/IR asymptotic giant branch (AGB) stars, post-AGB objects and planetary nebulae, the R CrB-type star HV 2671, the OH/IR red supergiants (RSGs) WOH G064 and IRAS 05280 − 6910, the three B[e] stars IRAS 04530 − 6916, R 66 and R 126, the Wolf–Rayet star Brey 3a, the luminous blue variable (LBV) R 71, the supernova remnant N 49, a large number of young stellar objects (YSOs), compact H ii regions and molecular cores, and a background galaxy at a redshift z ≃ 0.175. We use the spectra to constrain the presence and temperature of cold dust and the excitation conditions and shocks within the neutral and ionized gas, in the circumstellar environments and interfaces with the surrounding interstellar medium (ISM). First, we introduce a spectral classification scheme. Then, we measure line strengths, dust temperatures, and IR luminosities. Objects associated with star formation are readily distinguished from evolved stars by their cold dust and/or fine-structure lines. Evolved stars, including the LBV R 71, lack cold dust except in some cases where we argue that this is swept-up ISM. This leads to an estimate of the duration of the prolific dust-producing phase (“superwind”) of several thousand years for both RSGs and massive AGB stars, with a similar fractional mass loss experienced despite the different masses. We tentatively detect line emission from neutral oxygen in the extreme RSG WOH G064, which suggests a large dust-free cavity with implications for wind driving. In N 49, the shock between the supernova ejecta and ISM is revealed in spectacular fashion by its strong [O i] λ63 μm emission and possibly water vapor; we estimate that 0.2 M☉ of ISM dust was swept up. On the other hand, some of the compact H ii regions display pronounced [O iii] λ88 μm emission. The efficiency of photoelectric heating in the interfaces of ionized gas and molecular clouds is estimated at 0.1%–0.3%. We confirm earlier indications of a low nitrogen content in the LMC. Evidence for solid state emission features is found in both young and evolved objects, but the carriers of these features remain elusive; some of the YSOs are found to contain crystalline water ice. The spectra constitute a valuable resource for the planning and interpretation of observations with the Herschel Space Observatory and the Stratospheric Observatory For Infrared Astronomy.