Early Blue Excess from the Type Ia Supernova 2017cbv and Implications for Its Progenitor

Abstract

We present very early, high-cadence photometric observations of the nearby Type Ia SN 2017cbv. The light curve is unique in that it has a blue bump during the first five days of observations in the U , B, and g bands, which is clearly resolved given our photometric cadence of 5.7 hr during that time span. We model the light curve as the combination of early shocking of the supernova ejecta against a nondegenerate companion star plus a standard SN Ia component. Our best-fit model suggests the presence of a subgiant star 56 R from the exploding white dwarf, although this number is highly model-dependent. While this model matches the optical light curve well, it overpredicts the observed flux in the ultraviolet bands. This may indicate that the shock is not a blackbody, perhaps because of line blanketing in the UV. Alternatively, it could point to another physical explanation for the optical blue bump, such as interaction with circumstellar material or an unusual nickel distribution. Early optical spectra of SN 2017cbv show strong carbon (C II λ6580) absorption up through day −13 with respect to maximum light, suggesting that the progenitor system contains a significant amount of unburned material. These early results on SN 2017cbv illustrate the power of early discovery and intense follow-up of nearby supernovae to resolve standing questions about the progenitor systems and explosion mechanisms of SNe Ia.

The Early Detection and Follow-up of the Highly Obscured Type II Supernova 2016ija/DLT16am∗

“…First results from the DLT40 survey were presented for the nearby Type Ia SN2017cbv (DLT17u; Hosseinzadeh et al 2017) and for the kilonova DLT17ck (AT 2017gfo Valenti et al 2017a). Additionally, an upper limit on the rate of fast kilonova-like events was calculated from the first year of DLT40 data (Yang et al 2017).…”

Early Observations of the Type Ia Supernova iPTF 16abc: A Case of Interaction with Nearby, Unbound Material and/or Strong Ejecta Mixing

“…Spectra obtained around or after T B, max rarely show C II, as the photosphere has receded from the outermost ejecta, while pre-maximum spectra show evidence for weak C II absorption in ∼1/4 of all normal SNe Ia (e.g., Parrent et al 2011;Thomas et al 2011;Silverman & Filippenko 2012). While the sample of SNe Ia with spectra taken within a few days of explosion is small, SN 2013dy and SN 2017cbv are the only other objects known to have strong C II features like iPTF 16abc (Zheng et al 2013;Hosseinzadeh et al 2017). As a counterexample, SN 2011fe only exhibited weak C II features in its first spectra (Parrent et al 2012).…”

“…Like iPTF 16abc, SN 2012cg (Marion et al 2016) and SN 2017cbv (Hosseinzadeh et al 2017) exhibit B V 0 -( ) colors that are significantly bluer than SN 2011fe at very early epochs. While there are many factors that contribute to the early optical colors of SNe Ia (see Section 5 below), early blue colors are often interpreted as a hallmark of interaction between the SN ejecta and a binary companion.…”

“…Models of this scenario naturally produce C II absorption that is comparable in strength to Si II in the days after explosion (Dessart et al 2014), as was observed in SN 2013dy (Zheng et al 2013) and SN 2017cbv (Hosseinzadeh et al 2017).…”

“…While such emission requires a favorable geometric alignment and is only expected in 10% of SNe Ia (Kasen 2010), many studies have searched for signatures of an ejecta-companion interaction, typically resulting in nondetections (e.g., Hayden et al 2010a;Bianco et al 2011;Bloom et al 2012;Foley et al 2012;Zheng et al 2013;Goobar et al 2015;Im et al 2015;Olling et al 2015;Shappee et al 2016a). Possible exceptions include SN 2012cg, which exhibited excess blue emission in its early-phase light curve (Marion et al 2016; though for an interpretation that does not invoke ejecta-companion interaction, see Shappee et al 2016b), and SN 2017cbv, which shows a clearly resolved "bump" in the early UBg light curves (Hosseinzadeh et al 2017).…”