Hubble Space Telescope WFPC2 imaging of Cassiopeia A

The young galactic supernova remnant Cassiopeia A was imaged with Wide Field Planetary Camera 2 (WFPC2) aboard the Hubble Space Telescope through filters selected to capture the complete velocity range of the remnant's main shell in several emission lines. The primary lines detected, along with the specific WFPC2 filters used, were [O III] lambda lambda 4959, 5007 (F450W), [N II] lambda 6583 (F658N), [S II] lambda lambda 6716, 6731 + [O II] lambda lambda 7319, 7330 + [O I] lambda lambda 6300, 6364 (F675W), and [S III] lambda lambda 9069, 9532 (F850LP). About three-quarters of the remnant's 4' diameter main shell was imaged with all four filters in three WFPC2 pointings, with most remaining shell regions imaged in just the F675W filter via three additional pointings. Considerable detail is observed in the reverse-shocked ejecta with typical knot scale lengths of 0.2-0.4 (1-2 x 10(16) cm). Both bright and faint emission features appear highly clumped with little in the way of a smooth, diffuse emission component detected. Strong differences in [S III] and [O III] line intensities, indicating chemical abundance differences, are also seen, particularly in knots located along the bright northern limb and near the base of the northeastern jet. A line of curved overlapping filaments in the remnant's northwestern rim appears to mark the location of the remnant's reverse shock front in this region. The morphology of some finger-like ejecta structures elsewhere suggest cases in which the reverse shock front is encountering the remnant's clumped ejecta. Large velocity shears (similar or equal to 1000 km s(-1)), possibly associated with the formation of these Rayleigh-Taylor-like features, are found in the line profiles of several emission lines (e.g., [S III] lambda lambda 9069, 9532 and [Cl II] lambda 8679) in ground-based, optical spectra of knots near the remnant's center. The [N II] images of the remnant's circumstellar knots (QSFs) reveal them to be 0.1-0.6 thick knots and filaments, often with diffuse edges facing away from the center of expansion. Three-color composite images of the whole remnant and certain sections, along with individual filter enlargements of selected regions of the bright optical shell, are presented and discussed.