This research is based on observations with AKARI, a JAXA project with the participation of ESA, and Spitzer, which is operated by the JPL/Caltech under a contract with NASA.We report here on the discovery of an extended far-infrared shell around the AGB star, R Cassiopeia, made by AKARI and Spitzer. The extended, cold circumstellar shell of R Cas spans nearly 3′ and is probably shaped by interaction with the interstellar medium. This report is one of several studies of well-resolved mass loss histories of AGB stars under AKARI and Spitzer observing programs labeled “Excavating Mass Loss History in Extended Dust Shells of Evolved Stars (MLHES)”

Speck, Angela K.

University of Missouri: MOspace

arXiv:0905.0750v1  [astro-ph.GA]  6 May 2009**FULL TITLE**ASP Conference Series, Vol. **VOLUME**, **YEAR OF PUBLICATION****NAMES OF EDITORS**Mass Loss History of the AGB star, R CasToshiya Ueta1, Robert E. Stencel1, Issei Yamamura2, HideyukiIzumiura3, Yoshikazu Nakada4,5, Mikako Matsuura6, Yoshifusa Ita2,6,Toshihiko Tanabe´4, Hinako Fukushi4, Noriyuki Matsunaga6, HiroyukiMito5, and Angela K. Speck71 Dept. of Physics and Astronomy, University of Denver, USA2 Institute of Space and Astronautical Science, JAXA3 Okayama Astrophysical Observatory, NAOJ4 Institute of Astronomy, Schooof Science, University of Tokyo5 Kiso Observatory, Institute of Astronomy, University of Tokyo6 National Astronomical Observatory of Japan7 Dept. of Physics & Astronomy, University of Missouri, USAAbstract. We report here on the discovery of an extended far-infrared shellaround the AGB star, R Cassiopeia, made by AKARI and Spitzer. The ex-tended, cold circumstellar shell of R Cas spans nearly 3′ and is probably shapedby interaction with the interstellar medium. This report is one of several stud-ies of well-resolved mass loss histories of AGB stars under AKARI and Spitzerobserving programs labeled “Excavating Mass Loss History in Extended DustShells of Evolved Stars (MLHES)”.1. IntroductionDeutsch (1956) discussed the existence of blueshifted circumstellar cores in thespectrum of the red supergiant star α Her, constituting direct evidence for highrates of mass loss. Since that time, numerous observations have elucidatedthe magnitude and ubiquity of mass loss across the upper right side of theHertzsprung-Russell diagram. The high rates of mass loss among AGB starsrival evolutionary timescales, affecting stellar evolutionary tracks substantially.Therefore, the high rates of mass loss deserve careful determination. These factshave befuddled theorists, who are struggling with the basic challenge of how tolift so much mass away from the gravitational hold of the star.During the 1980’s, observations with the Infrared Astronomical Satellite(IRAS) demonstrated that extended infrared shells of evolved stars - the antic-ipated effect of continuous dusty mass loss (e.g. Stencel, Pesce, & Hagen Bauer1988; Young, Phillips, & Knapp 1993a,b) - were present. During the 1990’s,the Infrared Space Observatory and ground-based infrared work began to refinethose results (e.g. Izumiura et al. 1997; Meixner et al. 1999), indicating varia-tions in the mass loss rate over time and resulting in shells and structure as mightbe predicted in stellar evolution calculations for AGB stars (e.g. Iben 1995). Thisdecade, we are fortunate to have higher resolution and sensitivity instrumentslike the AKARI Infrared Astronomy Satellite (AKARI: Murakami et al. 2007)12 Ueta et al.and the Spitzer Space Telescope (Spitzer: Werner et al. 2004) that can morecarefully map the mass loss history of evolved stars.This report is the first of several studies of well-resolved mass loss historiesof AGB stars under AKARI and Spitzer observing programs labeled “Excavat-ing Mass Loss History in Extended Dust Shells of Evolved Stars (MLHES)”. Inthis contribution, we will explore whether the detection of an extended (∼ 3′radius) dust shell around the oxygen-rich AGB star, R Cas, can be interpretedin terms of thermal pulses, in order to constrain mass loss histories from thisevolved star. In parallel with the mass loss history of evolved stars, evidencefor the interaction of circumstellar and interstellar medium (ISM) is growing,with new observations of bow shocks around R Hya (Ueta et al. 2006) and Mira(Martin et al. 2007; Ueta 2008), plus theoretical considerations of the phenom-ena (Villaver, Garc´ıa-Segura, & Manchado 2003; Wareing et al. 2007). Becausenature is intrinsically complicated, we will examine evidence for both mass losshistory and interaction with the ISM in the case of the extended infrared shellof R Cas.2. R Cas: the Star and its Circumstellar ShellThe Mira type variable, R Cas (HD224490), is an oxygen-rich star with a periodof 431 days and estimated mass loss rate of 10−7 M⊙ yr−1. This star shows an ex-tended circumstellar shell originally detected by IRAS at 60µm, having angularextent at least 4.′3 (Young, Phillips, & Knapp 1993a), or 2.′8 (Bauer & Stencel1994) using careful deconvolution of the point-spread-function (PSF). The linearextent of the shell depends on the distance determination.We observed R Cas in the four bands at 65, 90, 140 and 160µm using the Far-Infrared Surveyor (FIS: Kawada et al. 2007) on-board AKARI on 2007 January16 as part of the MLHES Mission Program (PI: I. Yamamura). Observationswere made with the FIS01 (compact source photometry) scan mode, in whichtwo strips of forward and backward scans (at 0.5s reset rate) were done with a70′′ spacing.R Cas was also observed at 70µm using MIPS aboard Spitzer on 2008February 18 as part of the Cycle 4 GO project, Spitzer-MLHES (PID 40092,PI: T. Ueta). We mapped a 15′ × 20′ region with a series of exposures inphotometry/fixed-cluster-offset mode, while avoiding the central star that isbrighter than the saturation limit of the MIPS arrays.3. Preliminary Results and DiscussionFigure 1 shows a Spitzer image at 70µm (top left) and AKARI images at 90and 140µm (top middle and right, respectively). These far-infrared images showthe extended circumstellar shell around R Cas, which is roughly circular of ra-dius 140 to 165′′. The shell’s emission structure consists of the relatively flat“plateau” region on the west side (of surface brightness ∼ 15 to 20 MJy sr−1at 65 to 90µm and < 10 MJy sr−1 at 140µm) and the region of higher surfacebrightness (emission core) on the east side around the central star. We imme-diately see that the extended shell is off-centered, i.e., position of the centralstar does not coincide with the geometric center of the extended circular shell.Mass Loss History of the AGB star, R Cas 3Figure 1. R Cas images in the far-infrared and derived quantities. [Top]Spitzer 70µm (PSF subtracted, left) and AKARI 90µm (WIDE-S, middle) and140µm (WIDE-L, right) maps. [Bottom] Optical depth map at 70µm (left)and dust temperature maps derived from Spitzer and AKARI 90µm map(middle) and from AKARI 90 and 140µm maps (right). The line emanatingfrom the central star’s position indicates the direction of the star’s propermotion. The color image is available from the version on arXiv.Also, there appears to be an enhancement of the surface brightness along theperiphery of the shell (especially recognizable in the Spitzer 70µm and the dusttemperature maps). This density enhancement may be due to a fast low densitywind shock-merging with a slower, high density wind which was caused duringan era of higher/enhanced mass loss, as has been hydrodynamically predicted(e.g. Steffen, Szczerba, & Scho¨nberner 1998).Hipparcos measured the proper motion of (µα, µδ) = (85.5± 0.8, 17.5± 0.7)mas yr−1 (van Leeuwen 2007). This translates to the position angle of 78.◦4±0.◦7east of north, which is indicated as a line drawn from the position of the starin Figure 1. This direction agrees remarkably well with the direction alongwhich there is a positive gradient of surface brightness. In other words, thebrighter surface brightness in the eastern edge of the shell appears to arise frominteractions between AGB winds and the ISM, where a bow shock is expectedas discovered around another AGB star, R Hya (Ueta et al. 2006).4 Ueta et al.The shape of the circumstellar shell was fit by an ellipse. The best-fit semi-major axis and eccentricity pair, (a, ǫ), is a = 33.6 pix = 165.′′3 and ǫ = 0.3:indeed, the R Cas shell is not quite circular in projection. According to the best-fit ellipse, the distance from the ellipse center to one of the foci is c = a× ǫ = 9.9pix = 48.′′8. This means that the central star is 48.′′8 displaced from the ellipsecenter over the course of the shell expansion. At the preferred distance for RCas, 176 pc, the semi-major and semi-minor axes correspond to 0.12 to 0.14 pc.At the measured CO expansion rate of 12 km s−1 (Bujarrabal, Fuente, & Omont1994), a crossing time of the shell is therefore roughly 104 years.If the elongation of the R Cas shell is solely due to the motion of the centralstar with respect to the shell (in an otherwise stationary local environment), thestar must have been moving roughly at 5 mas yr−1, which is much smaller thanthe observed motion of 86.5 mas yr−1. Thus, the shaping of the shell is NOTself-inflicted as in interactions between fast and slow AGB winds expected in theAGB shells (Steffen, Szczerba, & Scho¨nberner 1998). Rather, the shall shapingappears to be a result of interactions between AGB winds emanating from themoving star AND the ISM flow local to R Cas. As shown by Ueta et al. (2009)in this volume, one can deduce the direction of the ISM flow local to R Casgiven the direction of the proper motion by fitting the shell/bow structure inthe leading, brighter eastern edge. While similar analyses would allow probingof the 3-D dynamics of the ISM local to these wind-ISM interacting shells, theseshells do not preserve mass loss history beyond the wind crossing time of theshell. In the case of R Cas, mass loss history can be probed only up to 104 yrago from the internal structure of the shell.Acknowledgments. This research is based on observations with AKARI,a JAXA project with the participation of ESA, and Spitzer, which is operatedby the JPL/Caltech under a contract with NASA.ReferencesBauer, W. H., & Stencel, R. E. 1994, AJ, 107, 2233Bujarrabal, V., Fuente, A., & Omont, A. 1994, A&A, 285, 247Deutsch, A. 1956, ApJ, 123, 210Iben, I. Jr. 1995, Phys. Rep., 250, 2Izumiura, H., et al. 1997, A&A, 323, 449Kawada, M., et al. 2007, PASJ59, S389van Leeuwen, F. 2007, A&A, 474, 653Meixner, M., et al. 1999, ApJS, 122, 221Martin, D., et al. 2007, Nat, 448, 780Murakami, H., et al. 2007, PASJ, 59, S369Schro¨der, K.-P. et al. 1998, A&A, 335, L9Steffen, M., Szczerba, R., & Scho¨nberner, D. 1998, A&A, 337, 149Stencel, R. E., Pesce, J. E., & Hagen Bauer, W. 1988, AJ, 95, 141Vassiliadis, E., & Wood, P. 1993, ApJ, 413, 641Ueta, T. 2008, ApJ, 687, L33Ueta, T., et al. 2006, ApJ, 648, L39Ueta, T. et al. 2009, this volumeVillaver, E., Garc´ıa-Segura, G., & Manchado, A. 2003, ApJ, 585, 49Werner et al. 2004, ApJS, 154, 1Young, K., Phillips, T. G., & Knapp, G. R. 1993, ApJS, 86, 517Young, K., Phillips, T. G., & Knapp, G. R. 1993, ApJ, 409, 725Mass Loss History of the AGB star, R Cas 5Wareing, C. J., et al. 2007, ApJ, 670, L125Wilkin, F. 1996, ApJ, 459, 1234

Mass Loss History of the AGB star, R Cas

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