Intrinsic color diversity of nearby Type Ia supernovae

It has been reported that the extinction law for Type Ia supernovae (SNe Ia) may be different from the one in the Milky Way, but the intrinsic color of SNe Ia and the dust extinction are observationally mixed. In this study, we examine photometric properties of SNe Ia in the nearby universe (z ≲ 0.04) to investigate the SN Ia intrinsic color and the dust extinction. We focus on the Branch spectroscopic classification of 34 SNe Ia and morphological types of host galaxies. We carefully study the distribution of their peak colors on the B - V, V - R color-color diagram, as well as the color excess and absolute magnitude deviation from the stretch-color relation of the bluest SNe Ia. We find that SNe Ia which show the reddest color occur in early-type spirals and the trend holds when divided into Branch sub-types. The dust extinction becomes close to the Milky Way like extinction if we exclude some peculiar red Broad Line (BL) sub-type SNe Ia. Furthermore, two of these red BLs occur in elliptical galaxies, a less-dusty environment, suggesting intrinsic color diversity in BL sub-type SNe Ia.