Spitzerspectroscopy of the Galactic Supernova Remnant G292.0+1.8: Structure and Composition of the Oxygen-Rich Ejecta

We present mid-infrared (mid-IR; 5–40 μm) spectra of shocked ejecta in the Galactic oxygen-rich supernova remnant (SNR) G292.0+1.8, acquired with the Infrared Spectrograph onboard the Spitzer Space Telescope . The observations targeted two positions within the brightest oxygen-rich feature in G292.0+1.8. Emission lines of [Ne ii ] λ12.8, [Ne iii ] λλ15.5,36.0, [Ne v ] λ24.3, and [O iv ] λ25.9 μm are detected from the shocked ejecta. In marked contrast to what is observed in Cassiopeia A, no discernible mid-IR emission from heavier species such as Mg, Si, S, Ar, or Fe is detected in G292.0+1.8. We also detect a broad emission bump between 15 and 28 μm in spectra of the radiatively shocked O-rich ejecta in G292.0+1.8. We suggest that this feature arises from either shock-heated Mg 2 SiO 4 (forsterite) dust in the radiatively shocked O-rich ejecta or collisional excitation of polycyclic aromatic hydrocarbons in the blast wave of the SNR. If the former interpretation is correct, this would be the first mid-IR detection of ejecta dust in G292.0+1.8. A featureless dust continuum is also detected from nonradiative shocks in the circumstellar medium around G292.0+1.8. The mid-IR continuum from these structures, which lack mid-IR line emission, is seen in Chandra images as bright X-ray filaments, is well described by a two-component silicate dust model. The temperature of the hot dust component ( M d ∼ 2 × 10 −3 M ☉ ) is ∼115 K, while that of the cold component (roughly constrained to be ≲3 M ☉ ) is ∼35 K. We attribute the hot component to collisionally heated dust in the circumstellar shocks in G292.0+1.8, and attribute the cold component to dust heated by the hard FUV radiation from the circumstellar shocks. Using average O/Ne and O/Si mass ratios measured for a sample of ejecta knots in the X-rays, our models yield line strengths consistent with mass ratios M O / M Ne ≈ 3, M O / M Si ≳ 61, and M O / M S ≈ 50. These ratios (especially the large O/Ne mass ratio) are difficult to reproduce with standard nucleosynthesis models of well-mixed supernova ejecta. This reinforces the conclusions of existing X-ray studies that the reverse shock in G292.0+1.8 is currently propagating into the hydrostatic nucleosynthetic layers of the progenitor star, and has not yet penetrated the layers dominated by explosive nucleosynthetic products.