Shocked Molecular Gas in the Supernova Remnant HB 21

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Abstract
We have carried out 12CO J = 2-1 line observations of the supernova remnant (SNR) HB 21 in order to search for evidence of interaction with molecular clouds. We mapped the eastern half (80' × 110') of the SNR almost completely. Molecular gas appears to be distributed mainly along the boundary of the SNR, but the overall distribution has little correlation either with the distortion of the SNR boundary or with the distribution of radio brightness. Along the eastern boundary, where the SNR was considered to be interacting with molecular clouds in previous studies, we have not found any strong evidence for the interaction. Instead, we detected broad (20-40 km s-1) CO emission lines in the northern and southern parts of the SNR. In the northern area, the broad-line emitting cloud is composed of a small (~2' or 0.5 pc), very bright, U-shaped part and several clumps scattered around it. There is a significant enhancement of radio emission with a flat (-0.28 ± 0.17) spectral index possibly associated with this cloud. In the southern area, the broad-line emitting cloud is filamentary and appears to form an elongated loop of ~30' in extent. Small (12 or 0.3 pc), bright clumps are seen along the filamentary structure. We have obtained sensitive J = 1-0 and J = 2-1 spectra of 12CO and 13CO molecules toward several peak positions. The intensity of 12CO J = 2-1 emission is low (Tmb < 7 K) and the ratio of 12CO J = 2-1 to J = 1-0 integrated intensities is high (1.6-2.3), which suggests that the emission is from warm, dense, and clumpy gas. We have applied a large velocity gradient analysis to derive their physical parameters. The detected broad CO lines are believed to be emitted from the fast-moving molecular gas swept up by the SNR shock. The small (20 km s-1) shock velocity suggests that the shock is a nondissociating C-shock. We discuss the correlation of the shocked molecular gas with the previously detected, shocked atomic gas and the associated infrared emission.
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