SIMBAD references

We search for the reasons behind the spectroscopic diversity of hydrogen-poor superluminous supernovae (SLSNe-I) in the premaximum phase. Our analysis is a continuation of the paper of Könyves-Tóth & Vinkó, who disclosed two new subtypes of SLSNe-I characterized by the presence/absence of a W-shaped absorption feature in their premaximum spectra between 4000 and 5000 Å (called Type W and Type 15bn, respectively). However, the physical cause of this bimodality is still uncertain. Here we present premaximum spectral synthesis of 27 SLSNe-I with special attention to the photospheric temperature (T_phot) and velocity (v_phot) evolution. We find that a T_phot limit of 12,000 K separates the Type W and Type 15bn SLSNe-I: Type W objects tend to show T_phot ≥ 12,000 K, while Type 15bn ones have T_phot ≤ 12,000 K. This is consistent with the chemical composition of the studied objects. Another difference between these groups may be found in their ejecta geometry: Type W SLSNe-I may show null polarization, implying spherical symmetry, while the polarization of Type 15bn objects may increase in time. This suggests a two-component model with a spherical outer carbon-oxygen layer and an asymmetric inner layer containing heavier ions. The two subgroups may have different light-curve evolution as well, since six Type W objects show early bumps, unlike Type 15bn SLSNe-I. This feature, however, needs further study, as it is based on only a few objects at present.