Infrared studies of molecular shocks in the supernova remnant HB 21: II. Thermal admixture of shocked H₂ gas in the south

Abstract

We present near- and mid-infrared observations on the shock-cloud interaction region in the southern part of the supernova remnant HB 21, performed with the InfraRed Camera (IRC) aboard AKARI satellite and the Wide InfraRed Camera (WIRC) at the Palomar 5 m telescope. The IRC 4 μm (N4), 7 μm (S7), and 11 μm (S11) band images and the WIRC Hυ=1→0S(1) 2.12 μm image show similar diffuse features, around a shocked CO cloud. We analyzed the emission through comparison with the H₂ line emission of several shock models. The IRC colors are well explained by the thermal admixture model of H₂ gas - whose infinitesimal H₂ column density has a power-law relation with the temperature T, dN∼T^-dT - with n(H)∼3.9×10⁴cm^-3,b∼4.2, and N(H;T>100K)∼2.8×10²¹cm^-2. We interpreted these parameters with several different pictures of the shock-cloud interactions - multiple planar C-shocks, bow shocks, and shocked clumps - and discussed their weaknesses and strengths. The observed Hυ=1→0S(1) intensity is four times greater than the prediction from the power-law admixture model, the same tendency as found in the northern part of HB 21 (Paper I). We also explored the limitation of the thermal admixture model with respect to the derived model parameters.