Red vs Blue: Early observations of thermonuclear supernovae reveal two distinct populations?

TL;DR

This study identifies two distinct early populations of Type Ia supernovae based on their color evolution, linking them to different spectral types and luminosity characteristics, and discusses potential physical processes behind these observations.

Contribution

It reveals the existence of two early color populations in Type Ia supernovae and associates them with specific spectral types and luminosity features, providing new insights into supernova diversity.

Findings

Early-blue supernovae are all 1991T/1999aa-like with luminous, slow-declining light curves.

Early-red supernovae are mostly Branch CN or CL types, with faster color evolution.

Viewing-angle effects in the companion collision model are inconsistent with observations.

Abstract

We examine the early phase intrinsic $(B-V)_{0}$ color evolution of a dozen Type~Ia supernovae discovered within three days of the inferred time of first light ($t_{first}$) and have $(B-V)_0$ color information beginning within 5 days of $t_{first}$. The sample indicates there are two distinct early populations. The first is a population exhibiting blue colors that slowlybevolve, and the second population exhibits red colors and evolves more rapidly. We find that the early-blue events are all 1991T/1999aa-like with more luminous slower declining light curves than those exhibiting early-red colors. Placing the first sample on the Branch diagram (i.e., ratio of \ion{Si}{2} $\lambda\lambda$5972, 6355 pseudo-Equivalent widths) indicates all blue objects are of the Branch Shallow Silicon (SS) spectral type, while all early-red events except for the 2000cx-like SN~2012fr are of the Branch Core-Normal (CN) or CooL (CL) type. A number of potential processes contributing to the early emission are explored, and we find that, in general, the viewing-angle dependance inherent in the companion collision model is inconsistent with all SS objects with early-time observations being blue and exhibiting an excess. We caution that great care must be taken when interpreting early-phase light curves as there may be a variety of physical processes that are possibly at play and significant theoretical work remains to be done.