A Catalog of Outer Ejecta Knots in the Cassiopeia A Supernova Remnant

Abstract

Hubble Space Telescope images of the core-collapse supernova remnant Cassiopeia A are used to identify highvelocity knots of ejecta located outside the remnant's main emission shell of expanding debris. These ejecta fragments are found near or ahead of the remnant's forward shock front and mostly lie from 120 00 to 300 00 in radial distance from the remnant's center of expansion. Filter flux ratios when correlated with published spectra show that these knots can be divided into three emission classes: ( 1) knots dominated by [N ii] kk6548, 6583 emissions, (2) knots dominated by [O ii] kk7319, 7330 emissions, and (3) knots displaying filter flux ratios suggestive of [S ii], [O ii], and [Ar iii] k7135 emission line strengths similar to the ''fast-moving knots'' ( FMKs) found in the remnant's bright main shell. Of 1825 knots identified, 444 are strong [N ii] emission knots, 192 are strong [O ii] emission knots, and 1189 are FMK-like knots.In terms of location around the remnant, 972, 207, and 646 knots are found in the remnant's northeast jet, southwest jet, and non-jet regions, respectively. Assuming a distance of 3.4 kpc, derived knot transverse velocities based on proper motion measurements spanning a 9 month interval indicate maximum transverse expansion velocities for these three knot classes of 14,500, 13,500, and 11,500 km s À1 , respectively. We present a catalog of these outlying ejecta clumps comprising finding charts, epoch 2004.2 knot positions, proper motions, photometric filter fluxes, and estimated knot emission type, along with cross-references to previous knot identifications and data. This compilation represents a nearly tenfold increase in the number of outlying, high-velocity ejecta knots identified around the Cassiopeia A remnant.

An HST Survey of the Highest-Velocity Ejecta in Cassiopeia A*

“…Moreover, follow-up optical studies suggested the highest velocity S-rich ejecta were, in fact, arranged in broad fans and not narrow jet-like streams as first thought (Hammell & Fesen 2008). Three-dimensional velocity reconstructions of the remnant's main shell and outlying optical ejecta have established that the NE and SW jet regions are on truly opposites sides of the remnant and conically distributed with opening half-angles of at least ∼ 40…”

“…The most complete previous survey of Cas A's outer ejecta knots was that of Hammell & Fesen (2008) A similar to the "fast-moving knots" (FMKs) commonly found in the main shell.…”

The Density and Mass of Unshocked Ejecta in Cassiopeia a Through Low Frequency Radio Absorption

“…The jet and counterjet do not represent outflow in the classical sense. Instead they describe locations where the fastest-moving ejecta are observed well beyond the plateau and farthest from the explosion center (see, e.g., Hammell & Fesen 2008). To the southeast, iron-rich ejecta extend beyond the Bright Ring and into the plateau.…”

Kinematics of Supernova Remnants: Status of X-Ray Observations

“…Located mostly between 120 " to 300 " from the center of expansion (Hammell & Fesen 2008), the distribution is aspherical showing jets of Si-rich and S-rich ejecta extending out to the largest distances in the northeast and southwest. The optical knots are rapidly expanding, ∼ 0.3 % yr −1 (an expansion parameter m ≈ 1), with proper motions of 1000s of km s −1 on the Bright Ring and up to 14,000 km s −1 for the optical ejecta at the tip of the jets (Fesen et al 2006b).…”