The interface between the stellar wind and interstellar medium around R Cassiopeiae revealed by far-infrared imaging*
T. Ueta1, R. E. Stencel1, I. Yamamura2, K. M. Geise1, A. Karska3,4, H. Izumiura5, Y. Nakada6, M. Matsuura7,8, Y. Ita2,9, T. Tanabé6, H. Fukushi6, N. Matsunaga6, H. Mito10 and A. K. Speck11
Department of Physics and Astronomy, University of Denver,
2112 E. Wesley Ave., Denver, CO 80208, USA e-mail: email@example.com
2 Institute of Space and Aeronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510, Japan
3 Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraat 1, 85748 Garching, Germany
4 Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
5 Okayama Astrophysical Observatory, National Astronomical Observatory, Kamogata, Asakuchi, Okayama 719-0232, Japan
6 Institute of Astronomy, School of Science, University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan
7 Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
8 Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
9 National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
10 Kiso Observatory, Institute of Astronomy, School of Science, University of Tokyo, Mitake, Kiso, Nagano 397-0101, Japan
11 Department of Physics & Astronomy, University of Missouri, Columbia, MO 65211, USA
Accepted: 6 November 2009
Aims. The circumstellar dust shells of intermediate initial-mass (~1 to 8 ) 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.
Methods. 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.
Results. Our data show that (1) the bow shock cone of 1 – 5 × 10-5 dust mass is inclined at 68° 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-1, consistent with an environment conducive to dust heating by shock interactions.
Key words: circumstellar matter / infrared: stars / stars: AGB and post-AGB / stars: individual: R Cas / stars: mass loss / ISM: kinematics and dynamics
© ESO, 2010