Red versus Blue: Early Observations of Thermonuclear Supernovae Reveal Two Distinct Populations?

• Published in: Astrophysical journal. Letters 2018-09, Vol.864 (2), p.L35
• Main Authors: Stritzinger, Maximilian D., Shappee, Benjamin J., Piro, Anthony L., Ashall, Christopher, Baron, E., Hoeflich, Peter, Holmbo, Simon, Holoien, Thomas W.-S., Phillips, M. M., Burns, C. R., Contreras, Carlos, Morrell, Nidia, Tucker, Michael A.
• Format: Article
• Language: English
• Subjects: Color; Light curve; Populations; Silicon; Supernovae; supernovae: general
• ISSN: 2041-8205; 2041-8213
• DOI: 10.3847/2041-8213/aadd46

We examine the early phase intrinsic (B − V)0 color evolution of a dozen SNe Ia discovered within three days of the inferred time of first light (tfirst) and have (B − V)0 color information beginning within five days of tfirst. The sample indicates there are two distinct early populations. The first is a population exhibiting blue colors that slowly evolve, 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 Si ii λλ5972, 6355 pseudo-Equivalent widths) indicates that 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 of the 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.