ZTF Early Observations of Type Ia Supernovae III: Early-Time Colors as a Test for Explosion Models and Multiple Populations

Colors of Type Ia supernovae in the first few days after explosion provide a potential discriminant between different models. In this paper, we present g−r colors of 38 Type Ia supernovae discovered within 3 days from first light by the Zwicky Transient Facility in 2018, a sample which is twice as large as that in the literature. We find that g−r colors are intrinsically rather homogeneous at early phases, with about half of the dispersion in the first 5 days attributable to photometric uncertainties (σ_(noise)∼σ_(int)∼0.25 mag). Colors are nearly constant starting from 5 days after first light (g−r∼−0.15 mag), while the time evolution at earlier epochs is characterized by a continuous range of slopes, from events rapidly transitioning from redder to bluer colors (slope of ∼−0.25 mag day−1) to events with a flatter evolution. The continuum in the slope distribution is in good agreement both with models requiring some amount of ⁵⁶Ni mixed in the outermost regions of the ejecta and with "double-detonation" models having thin helium layers (M_(He)=0.01M⊙) and varying carbon-oxygen core masses. At the same time, four events show evidence for a distinctive "red bump" signature predicted by "double-detonation" models with larger helium masses. We finally identify a significant correlation between the early-time g−r slopes and supernova brightness, with brighter events associated to flatter color evolution (p-value=0.001). The distribution of slopes, however, is consistent with being drawn from a single population, with no evidence for two components as claimed in the literature based on B−V colors.