Interaction between the Supernova Remnant HB 21 and Molecular Clouds

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Abstract
We have carried out 12CO J = 1-0 observations toward the mixed-morphology supernova remnant (SNR) HB 21 using the SRAO 6 m telescope. The whole area of the SNR was covered using full-beam grid spacing (2') and partly using half-beam spacing (1'). We discovered shocked clouds with broad (Δv 10 km s-1) emission lines in the north, northwest, and center of the SNR. The large (~20') cloud in the northwest has a bow-shaped morphology and enhanced radio emission along the cloud boundary. Toward the central, X-ray-bright area of the remnant, sensitive CO observations have revealed small broad-line clouds. They are probable candidates for the evaporating clouds, which may be responsible for the enhanced thermal X-rays in the central area. Shocked clouds have also been discovered in the northern V-shaped radio structure. Our results suggest that the unusual radio features and the central thermal X-ray enhancements of HB 21 might be the result of an interaction with molecular clouds. High-resolution mappings of two shocked, 1' sized clumps were made with the NRO 45 m telescope. For the U-shaped clump in the north, we have found that temperature and density increase near the bottom of the U-shape. The observed features coincide with the theoretical expectation for a cloud swept by a shock wave propagating from the south to the north. For the other shocked clump in the south, the central velocity systematically changes with a gradient of 11 km s-1 pc-1. We suggest 1.7 kpc as the distance to HB 21 by considering all available distance estimates.

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