Spitzer Observations of Molecular Hydrogen in Interacting Supernova Remnants

Abstract

With Spitzer IRS, we have obtained sensitive low-resolution spectroscopy from 5 to 35 μm for six supernova remnants (SNRs) that show evidence of shocked molecular gas: Kes 69, 3C 396, Kes 17, G346.6–0.2, G348.5–0.0, and G349.7+0.2. Bright pure rotational lines of molecular hydrogen are detected at the shock front in all remnants, indicative of radiative cooling from shocks interacting with dense clouds. We find the excitation of H_2 S(0)-S(7) lines in these SNRs requires two nondissociative shock components: a slow 10 km s^(–1) C-shock through clumps of density 10^6 cm^(–3), and a faster 40-70 km s^(–1) C-shock through a medium of density 10^4 cm^(–3). The ortho-to-para ratio for H_2 in the warm shocked gas is typically found to be much less than the LTE value, suggesting that these SNRs are propagating into cold quiescent clouds. Additionally, a total of 13 atomic fine-structure transitions of Ar^+, Ar^(++), Fe^+, Ne^+, Ne^(++), S^(++), and Si^+ are detected. The ionic emitting regions are spatially segregated from the molecular emitting regions within the IRS slits. The presence of ionic lines with high appearance potential requires the presence of much faster, dissociative shocks through a lower density medium. The IRS slits are sufficiently wide to include regions outside the SNR which permits emission from diffuse gas around the remnants to be separated from the shocked emission. We find the diffuse H_2 gas projected outside the SNR is excited to a temperature of 100-300 K with a warm gas fraction of at least 0.5%-15% along the line of sight.

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