Origin of Broad He II 4686 Å Emission in Early Spectra of SNe IIP

Abstract

We propose a model for the origin of the broad He II 4686 Å emission in early spectrum of SN 2020jfo (type IIP). The 4686 Å line is emitted presumably by dense fragments embedded into a hot gas of the forward shock wave. The fragments are produced as a result of a heavy braking of the dense low mass shell, at the ejecta boundary and a simultaneous Rayleigh–Taylor instability. The temperature of line-emitting fragments is \({\approx}5\times 10^{4}\) K. Calculations of ionization and excitation of helium and hydrogen accounts for the He II 4686 Å luminosity, large flux ratio of He II 4686 Å/H\(\alpha\) and a significant optical depth of 4686 Å line. We demonstrate that fragments heating by hot electrons of the forward shock compensates cooling via He II 304 Å emission.