  The circumstellar dust shells of intermediate initial-mass (about 1 to 8
solar masses) evolved stars are generated by copious mass loss during the
asymptotic giant branch phase. The density structure of their circumstellar
shell is the direct evidence of mass loss processes, from which we can
investigate the nature of mass loss. We used the AKARI Infrared Astronomy
Satellite and the Spitzer Space Telescope to obtain the surface brightness maps
of an evolved star R Cas at far-infrared wavelengths, since the temperature of
dust decreases as the distance from the star increases and one needs to probe
dust at lower temperatures, i.e., at longer wavelengths. The observed shell
structure and the star's known proper motion suggest that the structure
represents the interface regions between the dusty wind and the interstellar
medium. The deconvolved structures are fitted with the analytic bow shock
structure to determine the inclination angle of the bow shock cone. Our data
show that (1) the bow shock cone of 1 - 5 x 10^-5 solar masses (dust mass) is
inclined at 68 degrees with respect to the plane of the sky, and (2) the dust
temperature in the bow shock cone is raised to more than 20 K by collisional
shock interaction in addition to the ambient interstellar radiation field. By
comparison between the apex vector of the bow shock and space motion vector of
the star we infer that there is a flow of interstellar medium local to R Cas
whose flow velocity is at least 55.6 km/s, consistent with an environment
conducive to dust heating by shock interactions.
