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 micron) 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] 12.8, [Ne III] 15.5, 36.0, [Ne V] 24.3 and [O IV] 25.9 are detected from the shocked ejecta. 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 microns 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 is well described by a two-component dust model. The temperature of the hot dust component (mass ~ 0.002 Solar masses) is ~ 115 K, while that of the cold component (> = 3 Solar masses) 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. Our models yield mid-IR line strengths consistent with M(O)/M(Ne) ~ 3, M(O)/M(Si) >~ 61 and M(O)/M(S) ~ 50. These ratios are difficult to reproduce with standard nucleosynthesis models of well-mixed SN ejecta (abridged).

💡 Research Summary

This paper presents a comprehensive mid‑infrared spectroscopic study of the Galactic oxygen‑rich supernova remnant (SNR) G292.0+1.8, using the Infrared Spectrograph (IRS) aboard the Spitzer Space Telescope. The authors targeted two positions that correspond to the brightest optical knot of O‑rich ejecta, acquiring low‑ and high‑resolution spectra covering the 5–40 µm range. After careful background subtraction and extraction, the spectra reveal strong ionic lines of neon and oxygen: