Document Type


Publication Date


Publication Title

The Astronomical Journal


Department of Physics and Astronomy


We present proper motions for 21 bright main shell and 17 faint, higher velocity, outer ejecta knots in the Cas A supernova remnant and use them to derive new estimates for the remnant's expansion center and age. Our study included 1951–1976 Palomar 5 m prime focus plates, 1988–1999 CCD images from the KPNO 4 m and MDM 2.4 m telescopes, and 1999 HST WFPC2 images. Measurable positions covered a 23 to 41 yr time span for most knots, with a few outer knots followed for almost 48 yr. We derive an expansion center of α(J2000) = 23h23m2777 ± 005, δ(J2000) = 58°48'494 ± 04 (ICRS), with little difference between centers derived using outer or main shell knots. This position is 30 due north of that estimated by van den Bergh & Kamper. It also lies 66 ± 15 almost due north (P.A. = 354°) of the remnant's recently detected central X-ray point source, implying a transverse velocity for the X-ray point source 330 km s-1 at a distance of 3.4 kpc. Using the knots which lie out ahead of the remnant's forward blast wave, we estimate a knot convergent date of A.D. 1671.3 ± 0.9 assuming no deceleration. However, a deceleration of just ~1.6 km s-1 yr-1 over a 300 yr time span would produce an explosion date A.D. 1680, consistent with the suspected sighting of the Cas A supernova by J. Flamsteed.



Original Citation

John R. Thorstensen et al 2001 AJ 122 297