Circumstellar Light Echo as a Possible Origin of the Polarization of Type IIP Supernovae

Type IIP supernovae (SNe IIP) are the most common class of core-collapse SNe. They often show rapid increase of polarization degree in the late phase. This time evolution is generally believed to originate from the emergence of an inner aspherical core, while an effect of polarized-scattered echoes by circumstellar (CS) dust around the SN may also substantially contribute to this polarization feature. In this study, we examine the effects of the scatted echoes on the SN polarization through radiative transfer simulations for various geometry and amount of CS dust. It has been found that asymmetrically-distributed CS dust, which is generally inferred for red supergiants, could reproduce the observed polarization features. We have applied our results to SNe 2004dj and 2006ov, deriving the geometry and amount of CS dust to explain their observed polarization features in this scenario. For both SNe, the blob-like or bipolar distribution of CS dust rather than the disk-like distribution is favored. The derived dust mass $M_{\mathrm{dust}}$ in the blob model (the bipolar CS dust model) for SNe 2004dj and 2006ov are $\sim 7.5 \times 10^{-4}$ M$_{\odot}$ ($\sim 8.5 \times10^{-4}$ M$_{\odot}$) and $\sim 5.2 \times 10^{-4}$ M$_{\odot}$ ($\sim 1.3 \times10^{-3}$ M$_{\odot}$), respectively. Even in the case where this process would not play a dominant role in the observed polarization signals, this effect should in principle contribute to it, the strength of which depends on the nature of CS dust. Therefore, this effect must be taken into account in discussing multi-dimensional structure of an SN explosion through polarimetric observations.