Detection of Broad Hα Emission Lines in the Late-time Spectra of a Hydrogen-poor Superluminous Supernova

Discovered by the intermediate Palomar Transient Factory (iPTF), iPTF13ehe is a hydrogen-poor superluminous supernova (SLSN) at z =0.3434, with properties similar to SN2007bi. The early observations show that iPTF13ehe rises within 83 - 148 days (rest-frame) to reach a peak bolometric luminosity of L^(peak)_(bol) ~1.3x10^(44) erg s^(-1), then decays very slowly at 0.015 magnitude per day. The measured ejecta velocity is ~13000km s^(-1). The inferred explosion characteristics, such as the ejecta mass (67 - 220M⊙), the total radiative and kinetic energy (E_(rad) ~10^(51) erg, E_(kin) ~2x10^(53) erg), is typical of SLSN-R 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 ~4500km s^(-1) and has a ~300km s^(-1) blue-ward shift relative to the narrow component. We interpret this broad Hα emission line as the interaction between the supernova ejecta and a H-rich circumstellar medium (CSM) shell, located at a distance of ~4 x 10^(16) cm from the explosion site. This ejecta-CSM interaction can produce the observed Hα luminosity of ~2x10^(41) erg s^(-1) and 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 CSM shell implies that its Thomson scattering optical depth is likely ≤ 1, thus setting upper limits on the CSM mass ≤ 30M⊙ and the volume number density ≤ 4 x 10^8 cm ^(-3). The early-time spectra do not show any H emission lines from this CSM shell, indicating that most of the H-atoms are already neutral and the shell is optically thin to the visible light. We predict that this shell should produce Lyα absorption in the UV spectra. Of the existing models, a Pulsational Pair Instability Supernova (PPISN) model can naturally explain the observed 30M⊙ H-shell, ejected from a progenitor star with an initial mass of (95-150)M⊙ about 40 years ago. By searching the PTF database for events similar to iPTF13ehe, we estimate that about 10% of all SLSNe-I may have late-time Balmer emission lines from ejecta interactions with H-rich shells