2019ApJ...878L...5F

We investigate ZTF18aalrxas, a double-peaked Type IIb core-collapse supernova (SN) discovered during science validation of the Zwicky Transient Facility. ZTF18aalrxas was discovered while the optical emission was still rising toward the initial cooling peak (0.7 mag over 2 days). Our observations consist of multi-band (ultraviolet and optical) light curves (LCs), and optical spectra spanning from ≃0.7 to ≃180 days past the explosion. We use a Monte-Carlo based non-local thermodynamic equilibrium model that simultaneously reproduces both the ⁵⁶Ni-powered bolometric LC and our nebular spectrum. This model is used to constrain the synthesized radioactive nickel mass (0.17 M_☉) and the total ejecta mass (1.7 M_☉) of the SN. The cooling emission is modeled using semi-analytical extended envelope models to constrain the progenitor radius (790-1050 R_☉) at the time of explosion. Our nebular spectrum shows signs of interaction with a dense circumstellar medium (CSM), and this spectrum is modeled and analyzed to constrain the amount of ejected oxygen (0.3-0.5 M_☉) and the total hydrogen mass (≃0.15 M_☉) in the envelope of the progenitor. The oxygen mass of ZTF18aalrxas is consistent with a low (12-13 M_☉) zero-age main-sequence mass progenitor. The LCs and spectra of ZTF18aalrxas are not consistent with massive single-star SN Type IIb progenitor models. The presence of an extended hydrogen envelope of low mass, the presence of a dense CSM, the derived ejecta mass, and the late-time oxygen emission can all be explained in a binary model scenario.