2009ApJ...693.1883S

We present near- and mid-infrared observations on the shock-cloud interaction region in the northern part of the supernova remnant HB21, performed with the infrared camera (IRC) aboard the AKARI satellite and the wide-field infrared camera (WIRC) at the Palomar 5 m telescope. The IRC 7 µm (S7), 11 µm (S11), and 15 µm (L15) band images and the WIRC H₂υ = 1 -> 0 S(1) 2.12 µm image show similar shock-cloud interaction features. We chose three representative regions, and analyzed their IRC emissions through comparison with H₂ line emissions of several shock models. The IRC colors are well explained by the thermal admixture model of H₂gas–whose infinitesimal H₂ column density has a power-law relation with the temperature T, d N ∼ T ^–b dT–with n(H₂)∼10³/cm3, b ∼ 3, and N(H₂; T > 100 K) ∼3x10²⁰/cm2. The derived b value may be understood by a bow shock picture, whose shape is cycloidal (cuspy) rather than paraboloidal. However, this picture raises another issue that the bow shocks must reside within ∼0.01 pc size scale, smaller than the theoretically expected. Instead, we conjectured a shocked clumpy interstellar medium picture, which may avoid the size-scale issue while explaining the similar model parameters. The observed H₂υ = 1 -> 0 S(1) intensities are a factor of ∼17-33 greater than the prediction from the power-law admixture model. This excess may be attributed to either an extra component of hot H₂ gas or to the effects of collisions with hydrogen atoms, omitted in our power-law admixture model, both of which would increase the population in the υ = 1 level of H₂.