Interaction between the Supernova Remnant HB 21 and Molecular Clouds

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 (?v 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.