Helium-rich Superluminous Supernovae From the Zwicky Transient Facility

Helium is expected to be present in the ejecta of some hydrogen-poor superluminous supernovae (SLSN-I). However, so far only one event, PTF10hgi has been identified with He features in its photospheric spectra (Quimby et al. 2018). We present the discovery of a new He-rich SLSN-I, ZTF19aawfbtg (SN2019hge) at z=0.0866. This event has more than 10 optical spectra at phases from -41 to +103 days relative to the peak, most of which match well with that of PTF10hgi. Confirmation comes from a near-IR spectrum taken at +34 days, revealing He I features with P-Cygni profiles at 1.083 and 2.058μm. Using the optical spectra of PTF10hgi and SN2019hge as templates, we examine 70 SLSN-I discovered by ZTF in the first two years of operation and found additional five SLSN-I with distinct He-features. The excitation of He I atoms in normal core collapse supernovae requires non-thermal radiation, as proposed by previous studies. These He-rich events can not be explained by traditional ^56Ni mixing model because of their blue spectra, high peak luminosity and slow rising time scales. Magnetar models offer a possible solution since pulsar winds naturally generate high energy particles as sources of non-thermal excitation. An alternative model is ejecta interaction with H-poor CSM which may be supported by the observed light curve undulations. These six SLSN-Ib appear to have relatively low-peak luminosities (rest-frame M_g = -20.06±0.16 mag).