Proper Motion of Dense Circumstellar Knots in the Supernova Remnant Cassiopeia A

Abstract The young supernova remnant Cassiopeia A is currently interacting with the circumstellar medium expelled by its progenitor star. We analyzed the proper motion of dense circumstellar clumps, known as quasi-stationary flocculi (QSFs), embedded within the remnant using archived images from the Hubble Space Telescope (HST). Our study focused on approximately 40 QSFs that are bright in optical bands. We determined the proper motions within each QSF by cross-correlating the HST images taken 15 yr apart, and presented the results as vector maps that show both the proper-motion magnitudes and directions. For most QSFs, the proper-motion vectors are well aligned, but some show significant variations in expansion direction within the QSF. We derived the mean proper motion of each QSF and investigated its properties. The mean proper-motion vectors of the QSFs predominantly point radially away from the explosion center of Cas A, though not perfectly aligned with a strict radial direction, with corresponding velocities typically in the range of a few hundred kilometers per second. We found no clear correlation between the proper motion and the projected distance from the explosion center, nor between the proper motion and the line-of-sight velocity of the QSFs. This result indicates that the apparent expansion of QSFs is primarily driven by interactions with the supernova blast wave, rather than by their initial ejection. We present an exploratory model of the three-dimensional distribution of QSFs.