Infant Type Ia Supernovae from the KMTNet I. Multi-Color Evolution and Populations

Abstract

We conduct a systematic analysis of the early multi-band light curves and colors of 19 Type Ia Supernovae (SNe) from the Korea Microlensing Telescope Network SN Program, including 16 previously unpublished events. Seven are detected $\lesssim$ 1 day since the estimated epoch of first light and the rest within $\lesssim$ 3 days. Some show excess emission within $<$ 0.5 days to $\sim$ 2 days, but most show pure power-law rises. The colors are initially diverse before $\sim$ 5 days, but converge to a similar color at $\sim$ 10 days. We identify at least three populations based on 2--5-day color evolution: (1) "early-blues" exhibit slowly-evolving colors consistent with a $\sim$ 17,000 K blackbody; (2) "early-reds" have initially blue $B-V$ and red $V-i$ colors that cannot simultaneously be fit with a blackbody -- likely due to suppression of $B$- and $i$-band flux by Fe II/III and Ca II -- and evolve more rapidly; and (3) "early-yellows" evolve blueward, consistent with thermal heating from $\sim$ 8,000 to 13,000 K. The distributions of early-blue and early-red colors are compatible with them being either distinct populations -- with early-reds comprising (60 $\pm$ 15)% of them -- or extreme ends of one continuous population; whereas the early-yellow population identified here is clearly distinct. Compared to the other populations, early-blues in our sample differ by exhibiting excess emission within 1--2 days, nearly constant peak brightness regardless of $\Delta M_{15}(B)$ after standardization, and shallower Si II features. Early-blues also prefer star-forming host environments, while early-yellows and, to a lesser extent, early-reds prefer quiescent ones. These preferences appear to indicate at least two Type Ia SN production channels based on stellar population age, while early-reds and early-blues may still share a common origin.