Premaximum Spectroscopic Diversity of Hydrogen-poor Superluminous Supernovae

Abstract

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.

On the Formation of the W-shaped O ii Lines in Spectra of Type I Superluminous Supernovae

“…The temperatures decrease with time. It is clear that the only spectra with temperatures higher than T R ∼ 12,000 K show the W-shaped O II lines (see also Könyves-Tóth 2022). The dependence of the W-shaped O II lines on temperature is discussed in more detail in Section 5.1.…”

“…Spectra of SLSNe I tend to show characteristic W-shaped absorption lines around 4000-4500 Å in pre-/near-maximum phases. The O II lines are thought to give the dominant contribution to these features, and thus, these features are called the W-shaped O II lines (Quimby et al 2011b;Chomiuk et al 2011;Lunnan et al 2013;Nicholl et al 2015;Branch & Wheeler 2017;Liu et al 2017;Gal-Yam 2019b;Kumar et al 2020;Könyves-Tóth & Vinkó 2021;Könyves-Tóth 2022). These features are not seen in normal SNe except for a few possible cases: Type Ib supernova (SN) 2008D (Modjaz et al 2009), Type Ibn OGLE-2012-SN-006 (Pastorello et al 2015), and Type IIL SN 2019hcc (Parrag et al 2021).…”