Near-infrared Extinction due to Cool Supernova Dust in Cassiopeia A

Abstract

We present the results of extinction measurements toward the main ejecta shell of the Cassiopeia A supernova (SN) remnant using the flux ratios between the two near-infrared (NIR) [Fe ii] lines at 1.26 and 1.64 μm. We find a clear correlation between the NIR extinction (E(J-H)) and the radial velocity of ejecta knots, showing that redshifted knots are systematically more obscured than blueshifted ones. This internal “self-extinction” strongly indicates that a large amount of SN dust resides inside and around the main ejecta shell. At one location in the southern part of the shell, we measure E(J-H) by the SN dust of 0.23 ± 0.05 mag. By analyzing the spectral energy distribution of thermal dust emission at that location, we show that there are warm (∼100 K) and cool (∼40 K) SN dust components and that the latter is responsible for the observed E(J-H). We investigate the possible grain species and size of each component and find that the warm SN dust needs to be silicate grains such as MgSiO₃, Mg₂SiO₄, and SiO₂, whereas the cool dust could be either small (≲0.01 μm) Fe or large (≳0.1 μm) Si grains. We suggest that the warm and cool dust components in Cassiopeia A represent grain species produced in diffuse SN ejecta and in dense ejecta clumps, respectively.