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We present an atlas of Spitzer Space Telescope Infrared Spectrograph ( IRS) spectra of highly luminous, compact, mid-IR sources in the Large Magellanic Cloud. Sources were selected on the basis of IR colors and 8 m (Midcourse Space Experiment) fluxes indicative of highly evolved, intermediate-to-high-mass stars with current or recent mass loss at large rates. We determine the chemistry of the circumstellar envelope from the mid-IR continuum and spectral features and classify the spectral types of the stars. In the sample of 60 sources, we find 21 red supergiants (RSGs), 16 C-rich asymptotic giant branch (AGB) stars, 11 H ii regions, 4 likely O-rich AGB stars, 4 Galactic O-rich AGB stars, 2 OH /IR stars, and 2 B[e] supergiants with peculiar IR spectra. We find that the overwhelming majority of the sample AGB stars (with typical IR luminosities of $10 4 L ) have C-rich envelopes, while the O-rich objects are predominantly luminous RSGs with L IR $ 10 5 L . For both classes of evolved star (C-rich AGB stars and RSGs), we use the near-to-mid-IR spectral energy distributions to determine mean bolometric corrections to the stellar K-band flux densities. For carbon stars, the bolometric corrections depend on the IR color, whereas for RSGs, the bolometric correction is independent of IR color. Our results reveal that objects previously classified as planetary nebulae on the basis of IR colors are, in fact, compact H ii regions with very red IRS spectra that include strong atomic recombination lines and polycyclic aromatic hydrocarbon emission features. We demonstrate that the IRS spectral classes in our sample separate clearly in IR color-color diagrams that use combinations of Two Micron All Sky Survey data and synthetic Spitzer Infrared Array Camera and Multiband Imaging Photometer for Spitzer fluxes derived from the IRS spectra. On this basis we suggest diagnostics to identify and classify, with high confidence levels, IR-luminous evolved stars and compact H ii regions in nearby galaxies using Spitzer and near-IR photometry.

Stars with masses in the range 1-8 solar masses (M(\circ)) live ordinary lives for approximately 10(9)-10(10) years, but die extraordinary deaths. First, during their death throes as asymptotic giant branch (AGB) stars they eject, over 10(4)-10(5) years, half or more of their mass in slowly expanding, spherical winds, and then, in a short (a few 100-1,000 years) and poorly understood phase, they are transformed into aspherical planetary nebula. Recent studies support the idea that high-speed, jet-like flows play a crucial role in this transformation. Evidence for such outflows is indirect, however; this phase is so short that few nearby stars are likely to be caught in the act. Here we report the discovery of a newly launched, high-speed jet-like outflow in the nearby AGB star, V Hydrae. We have detected both proper motions and ongoing evolution in the jet. These results support a model in which the jet is driven by an accretion disk around an unseen, compact companion. We also find a central, dense equatorial disk-like structure which may enable and/or enhance the formation of the accretion disk.

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