We performed Spitzer Infrared Spectrograph mapping observations covering nearly the entire extent of the Cassiopeia A supernova remnant (SNR), producing mid-infrared (5.5Y35 m) spectra every 5 00 Y10 00 . Gas lines of Ar, Ne, O, Si, S, and Fe, and dust continua were strong for most positions. We identify three distinct ejecta dust populations based on their continuum shapes. The dominant dust continuum shape exhibits a strong peak at 21 m. A line-free map of 21 m peak dust made from the 19Y23 m range closely resembles the [Ar ii], [O iv], and [Ne ii] ejecta-line maps, implying that dust is freshly formed in the ejecta. Spectral fitting implies the presence of SiO 2 , Mg protosilicates, and FeO grains in these regions. The second dust type exhibits a rising continuum up to 21 m and then flattens thereafter. This ''weak 21 m'' dust is likely composed of Al 2 O 3 and C grains. The third dust continuum shape is featureless with a gently rising spectrum and is likely composed of MgSiO 3 and either Al 2 O 3 or Fe grains. Using the least massive composition for each of the three dust classes yields a total mass of 0.020 M . Using the most massive composition yields a total mass of 0.054 M . The primary uncertainty in the total dust mass stems from the selection of the dust composition necessary for fitting the featureless dust as well as 70 m flux. The freshly formed dust mass derived from Cas A is sufficient from SNe to explain the lower limit on the dust masses in high-redshift galaxies.

Using Infrared Array Camera (IRAC) images at 3.6, 4.5, 5.8, and 8 µm from the GLIMPSE Legacy science program on the Spitzer Space Telescope, we searched for infrared counterparts to the 95 known supernova remnants that are located within galactic longitudes 65 • > |l| > 10 • and latitudes |b| < 1 • . Eighteen infrared counterparts were detected. Many other supernova remnants could have significant infrared emission but are in portions of the Milky Way too confused to allow separation from bright H II regions and pervasive mid-infrared emission from atomic and molecular clouds along the line of sight. Infrared emission from supernova remnants originates from synchrotron emission, shock-heated dust, atomic fine-structure lines, and molecular lines. The detected remnants are G11.G348.5-0.0, and G349.7+0.2. The infrared colors suggest emission from molecular lines (9 remnants), fine-structure lines (3), and PAH (4), or a combination; some remnants feature multiple colors in different regions. None of the remnants are dominated by synchrotron radiation at mid-infrared wavelengths. The IRAC-detected sample emphasizes remnants interacting with relatively dense gas, for which most of the shock cooling occurs through molecular or ionic lines in the mid-infrared.

We observed the oxygen-rich Large Magellanic Cloud ( LMC) supernova remnant N132D (SNR 0525À69.6), using all instruments on board the Spitzer Space Telescope, IRS, IRAC, and MIPS (Infrared Spectrograph, Infrared Array Camera, and Multiband Imaging Photometer for Spitzer). The 5 Y 40 m IRS spectra toward the southeastern shell of the remnant show a steeply rising continuum with [Ne iii] and [O iv], as well as PAH emission. We also present the spectrum of a fast moving ejecta knot, previously detected at optical wavelengths, which is dominated by strong [Ne iii] and [O iv] emission lines. We interpret the continuum as thermal emission from swept-up, shock-heated dust grains in the expanding shell of N132D, which is clearly visible in the MIPS 24 m image. A 15Y20 m emission hump appears superposed on the dust continuum, and we attribute this to PAH CÀCÀC bending modes. We also detect the well-known 11.3 m PAH CÀH bending feature, and find the integrated strength of the 15Y20 m hump about a factor of 7 stronger than the 11.3 m band in the shell of the remnant. IRAC 3Y9 m images do not show clear evidence of large-scale, shell-like emission from the remnant, partly due to confusion with the ambient ISM material. However, we identified several knots of shocked interstellar gas based on their distinct infrared colors. We discuss the bright infrared continuum and the polycyclic aromatic hydrocarbon features with respect to dust processing in young supernova remnants. Subject headingg s: infrared: ISM -ISM: individual ( N132D) -Magellanic Clouds -supernova remnants