FORMATION AND EVOLUTION OF DUST IN TYPE IIb SUPERNOVAE WITH APPLICATION TO THE CASSIOPEIA A SUPERNOVA REMNANT

The amount and size of dust formed in the ejecta of core-collapse supernovae (CCSNe) and injected into the interstellar medium (ISM) depend on the type of CCSNe through the varying thicknesses of their outer envelopes. Recently Cas A was identified as a Type IIb SN (SN IIb) that is characterized by a small-mass hydrogen envelope. In order to clarify how the amount of dust formed in the ejecta and supplied into the ISM depends on the type of CCSNe, we investigate the formation of dust grains in the ejecta of an SN IIb and their evolution in the shocked gas in the SN remnant (SNR) by considering two sets of density structures (uniform and power-law profiles) for the circumstellar medium (CSM). Based on these calculations, we also simulate the time evolution of thermal emission from the shock-heated dust in the SNR and compare the results with the observations of Cas A SNR. We find that the total mass of dust formed in the ejecta of an SN IIb is as large as 0.167 M but the average radius of dust is smaller than 0.01 mm and is significantly different from those in SNe II-P with massive hydrogen envelopes; in the explosion with the small-mass hydrogen envelope, the expanding He core undergoes little deceleration, so that the gas density in the He core is too low for large-sized grains to form. In addition, the low-mass hydrogen envelope of the SN IIb leads to the early arrival of the reverse shock at the dust-forming region. If the CSM is more or less spherical, then the newly formed small grains would be completely destroyed in the relatively dense shocked gas for the CSM hydrogen density of n H>0.1 cm-3 without being injected into the ISM. However, the actual CSM is likely to be non-spherical, so a portion of the dust grains could be ejected into the ISM without being shocked. We demonstrate that the temporal evolution of the spectral energy distribution (SED) by thermal emission from dust is sensitive to the ambient gas density and structure that affects the passage of the reverse shock into the ejecta. Thus, the SED evolution reflects the evolution of dust through erosion by sputtering and stochastic heating. For Cas A, we consider the CSM produced by the steady mass loss of $\dot{M} \simeq 8 \times 10^{-5}$ M yr-1 during the supergiant phase. Then we find that the observed infrared SED of Cas A is reasonably reproduced by thermal emission from the newly formed dust of 0.08 M , which consists of the 0.008 M shock-heated warm dust and 0.072 M unshocked cold dust.