2017ApJ...841...62M

Early polarization observations of type Ia supernovae (SNe Ia) may reveal the geometry of supernova ejecta, and then put constraints on their explosion mechanism and their progenitor model. We performed a literature search of SNe Ia with polarization measurements and determined the polarization and relative equivalent width (REW) of the Si II 635.5 nm absorption feature at -5 days after the maximum light. We found that either the distribution of observed polarization degree is bimodal, i.e., the broad-line SNe Ia have systematically higher polarization than all other SNe Ia, or that all types of SNe Ia share the same polarization sequence, i.e., the polarization of the Si II 635.5 nm absorption feature increases with the REW. We also discussed the potential effects that the discovery might have on the explosion mechanism and progenitor model of SNe Ia.