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
2.058$\mu$m. 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 $^{56}$Ni 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
$M_g = -20.06\pm0.16$ mag).
