Spitzer SPECTROSCOPY OF THE GALACTIC SUPERNOVA REMNANT G292.0+1.8: STRUCTURE AND COMPOSITION OF THE OXYGEN-RICH EJECTA

We present mid-infrared (5−40 μm) spectra of shocked ejecta in the Galactic oxygen-rich supernova remnant G292.0+1.8 , acquired with the IRS spectrograph on board 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 Mg2SiO4 (forsterite) dust in the radiatively shocked O-rich ejecta, or collisional excitation of PAHs 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 non-radiative 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 twocomponent silicate dust model. The temperature of the hot dust component (Md ∼ 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 MO/MNe ≈ 3, MO/MSi & 61 and MO/MS ≈ 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. Subject headings: ISM: individual (G292.0+1.8), ISM: kinematics and dynamics, shock waves, plasmas, ISM: cosmic rays, supernova remnants