Detection of Broad H alpha Emission Lines in the Late-Time Spectra of a Hydrogen-Poor Superluminous Supernova

iPTF13ehe is a hydrogen-poor superluminous supernova (SLSN) at z = 0.3434, with a slow-evolving light curve and spectral features similar to SN2007bi. It rises in 83–148 days to reach a peak bolometric luminosity of ~1.3 × 1044 erg s−1, then decays slowly at 0.015 mag day−1. The measured ejecta velocity is ~ 13,000 km s−1. The inferred explosion characteristics, such as the ejecta mass (70–220 M⊙), and the total radiative and kinetic energy (Erad ~ 1051 erg, Ekin ~ 2 × 1053 erg), are typical of slow-evolving H-poor SLSN events. However, the late-time spectrum taken at +251 days (rest, post-peak) reveals a Balmer Hα emission feature with broad and narrow components, which has never been detected before among other H-poor SLSNe. The broad component has a velocity width of ~4500 km s−1 and a ~300 km s−1 blueward shift relative to the narrow component. We interpret this broad Hα emission with a luminosity of ~2 × 1041 erg s−1 as resulting from the interaction between the supernova ejecta and a discrete H-rich shell, located at a distance of ~4 × 1016 cm from the explosion site. This interaction causes the rest-frame r-band LC to brighten at late times. The fact that the late-time spectra are not completely absorbed by the shock-ionized H-shell implies that its Thomson scattering optical depth is likely ≤1, thus setting upper limits on the shell mass ≤30 M⊙. Of the existing models, a Pulsational Pair Instability supernova model can naturally explain the observed 30 M⊙ H-shell, ejected from a progenitor star with an initial mass of (95–150) M⊙ about 40 years ago. We estimate that at least ~15% of all SLSNe-I may have late-time Balmer emission lines