Reddening Measurements and Physical Conditions for Cassiopeia A from Optical and Near-Infrared Spectra

Spectra of five fast-moving knots (FMKs) and two quasi-stationary flocculi (QSFs) in Cas A, covering the wavelength regions 4000-10,500 Å and 4300-6800 Å, respectively, are presented. FMK extinction values derived from observed [S ii] auroral/transauroral line ratios indicate an optical reddening range of A_v ≃ 4.6-5.4 mag. Estimates of QSF extinctions obtained assuming an intrinsic Balmer ratio of Hα/Hβ = 3.0 are larger, lying in the range A_v ≃ 5.3-6.2 mag. While these FMK and QSF reddening measurements are higher than Searle's value of Av = 4.3 mag, they are consistent with the radio-inferred extinction values of Troland et al. for their various locations in Cas A. The FMK reddening values indicate A_v variations of at least ≃0.5 mag, and possibly up to ≃1 mag in Cas A's bright northern and northeastern shell emission. The higher QSF reddening values may be due to line-of-sight extinction variations or collisionally excited Hα emission. Adopting a reddening value ≃1 mag larger than Searle's value brings the dereddened oxygen line ratios of Cas A FMKs into better agreement with other young, oxygen-rich remnants and the 150-200 km s^-1 shock/ionization front models of Sutherland and Dopita. Spectra of a faint FMK exhibiting weak oxygen emission but strong forbidden and permitted calcium lines are also presented. This FMK may represent a rare clump of relatively unmixed material originating from a thin (≃0.05 to 0.10 M_sun), silicon-rich zone in the Cas A progenitor.