  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 AKARI satellite and the Wide InfraRed Camera
(WIRC) at the Palomar 5 m telescope. The IRC 7 um (S7), 11 um (S11), and 15 um
(L15) band images and the WIRC H2 v = 1 -> 0 S(1) 2.12 um 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, dN ~ T^-b dT--with n(H2) ~ 10^3
cm^-3, b ~ 3, and N(H2 ;T > 100K) ~ 3x10^20 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 sizescale issue while explaining the similar model
parameters. The observed H2 v = 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 v = 1 level of H2.
