Near-Infrared Spectroscopy of the Cassiopeia A and Kepler Supernova Remnants

Abstract

Near-infrared spectra (0.95-2.4 μm) of the Cassiopeia A and Kepler supernova remnants (SNRs) are presented. Low-dispersion (R~700) spectra were obtained for five bright fast-moving ejecta knots (FMKs) at two locations on the main shell and for three bright circumstellar knots (quasi-stellar flocculi or QSFs) near the southwest rim of Cas A. The main shell FMKs in Cas A exhibit a sparse near-infrared spectrum dominated by [S II] 1.03 μm emission with a handful of other, fainter emission lines. Among these are two high-ionization silicon lines, [Si VI] 1.96 and [Si X] 1.43 μm, which have been detected in active galactic nuclei and novae but never before in a supernova remnant. The near-infrared spectra of circumstellar QSFs in Cas A show a much richer spectrum, with strong He I 1.083 μm emission and over a dozen bright [Fe II] lines. Observed [Fe II] line ratios indicate electron densities of 5-9×104 cm-3 in the QSFs. The Cas A QSF data are quite similar to the observed spectrum of a bright circumstellar knot along the northwest rim of the Kepler SNR, which also shows strong He I and [Fe II] emission with a measured electron density of 2.5-3×104 cm-3. Finally, we present J- and K-band images of Cas A. The K-band image shows faint diffuse emission that has no optical or midinfrared counterpart but is morphologically similar to radio continuum maps and may be infrared synchrotron radiation.