Infrared Studies of Molecular Shocks in the Supernova Remnant HB21. I. Thermal Admixture of Shocked H2 Gas in the North

• Published in: The Astrophysical journal 2009-03, Vol.693 (2), p.1883-1894
• Main Authors: Shinn, Jong-Ho, Koo, Bon-Chul, Burton, Michael G, Lee, Ho-Gyu, Moon, Dae-Sik
• Format: Article
• Language: English
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1088/0004-637X/693/2/1883

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 mm (S7), 11 mm (S11), and 15 mm (L15) band images and the WIRC H2 u = 1 -> 0 S(1) 2.12 mm 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 H2 gas-whose infinitesimal H2 column density has a power-law relation with the temperature T, d N ~ T -b dT-with n(H2)~103 cm-3, b ~ 3, and N(H2; T > 100 K) ~3X1020 cm-2. 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 H2 u = 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 u = 1 level of H2.