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Title:
The Expansion Asymmetry and Age of the Cassiopeia A Supernova Remnant
Authors:
Fesen, Robert A.; Hammell, Molly C.; Morse, Jon; Chevalier, Roger A.; Borkowski, Kazimierz J.; Dopita, Michael A.; Gerardy, Christopher L.; Lawrence, Stephen S.; Raymond, John C.; van den Bergh, Sidney
Affiliation:
AA(6127 Wilder Lab, Department of Physics and Astronomy, Dartmouth College, Hanover, NH 03755.), AB(6127 Wilder Lab, Department of Physics and Astronomy, Dartmouth College, Hanover, NH 03755.), AC(Department of Physics and Astronomy, Arizona State University, Box 871504, Tempe, AZ 85287.), AD(Department of Astronomy, University of Virginia, P.O. Box 3818, Charlottesville, VA 22903.), AE(Department of Physics, North Carolina State University, Raleigh, NC 27695.), AF(Research School of Astronomy and Astrophysics, The Australian National University, Cotter Road, Weston Creek, ACT 2611, Australia.), AG(Astrophysics Group, Imperial College London, Blackett Laboratory, Prince Consort Road, London SW7 2BZ, UK.), AH(Department of Physics and Astronomy, Hofstra University, Hempstead, NY 11549.), AI(Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138.), AJ(Dominion Astrophysical Observatory, Herzberg Institute of Astrophysics, NRC of Canada, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada.)
Publication:
The Astrophysical Journal, Volume 645, Issue 1, pp. 283-292. (ApJ Homepage)
Publication Date:
07/2006
Origin:
UCP
ApJ Keywords:
ISM: Individual: Name: Cassiopeia A, ISM: Kinematics and Dynamics, ISM: Supernova Remnants
DOI:
10.1086/504254
Bibliographic Code:
2006ApJ...645..283F

Abstract

HST images of the young supernova remnant Cas A are used to explore the expansion and spatial distribution of its highest velocity debris. ACS WFC images taken in 2004 March and December with Sloan F625W, F775W, and F850LP filters were used to identify 1825 high-velocity, outlying ejecta knots through measured proper motions of 0.35"-0.90" yr-1, corresponding to Vtrans=5500-14,500 km s-1 assuming d=3.4 kpc. The distribution of derived transverse expansion velocities for these ejecta knots shows a striking bipolar asymmetry with the highest velocity knots (Vtrans>=10,500 km s-1) confined to nearly opposing northeast and southwest ``jets'' at P.A.=45deg-70deg and 230°-270°, respectively. The jets have about the same maximum expansion velocity of ~=14,000 km s-1 and appear kinematically and chemically distinct in that they are the remnant's only S-rich ejecta with expansion velocities above the 10,000-11,000 km s-1 exhibited by outer nitrogen-rich ejecta, which otherwise represent the remnant's highest velocity debris. In addition, we find significant gaps in the spatial distribution of outlying ejecta in directions that are approximately perpendicular to the jets (P.A.=145deg-200deg and 335°-350°). The remnant's central X-ray point source lies some 7" to the southeast of the estimated expansion center (P.A.=169deg+/-8.4d) indicating a projected motion toward the middle of the broad southern ejecta knot gap. Extrapolations of measured 9 month proper motions for all 1825 outer ejecta knots and a selected subsample of 72 bright and compact knots suggest explosion dates (assuming no knot deceleration) of 1662+/-27 and 1672+/-18, respectively. We find some evidence for nonuniform deceleration in different directions around the remnant and find 126 knots located along the northwestern limb among the least decelerated ejecta, suggesting a convergence date of 1681+/-19. A remnant age of around 325 yr would imply a ~=350 km s-1 transverse velocity for the central X-ray point source.

Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under NASA contract NAS5-26555.

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