AAS 200th meeting, Albuquerque, NM, June 2002
Session 15. SNRs and PNe
Display, Monday, June 3, 2002, 9:20am-6:30pm, SW Exhibit Hall

[15.02] Two Epochs of HST-WFPC2 Imaging: Proper Motion of a Shock Front in the Cygnus Loop

R. Sankrit, W. P. Blair (JHU), J. C. Raymond (CfA)

We present H\alpha images of a Balmer filament in the northeast Cygnus Loop obtained at two epochs with the Wide Field and Planetary Camera-2 (WFPC2) on board the Hubble Space Telescope (HST). The first image was obtained in 1997, and showed the delicate structure of the shock front as a nearly edge-on sheet, gently rolling along our line of sight'' (Blair et al. 1999, AJ 118,942). The second image, obtained four years after the first, shows that the shock front has moved outward but that the substructure of the filament is essentially unchanged. From the two WFPC2 images, we derive a proper motion of ~0.08~arcsec/yr. This agrees with the results of Blair et al.~(1999) where the proper motion was measured from POSS I data (1953) and our earlier H\alpha image. The new measurement confirms that the distance to the Cygnus Loop is ~ 440\times(vs/170)~pc, where vs is the shock velocity in km/s. The two results are consistent with little or no deceleration of the filament over the last 50 years.

Support for this work was provided by STScI grant GO-09080001A to the Johns Hopkins University.