SN 2019ewu: A Peculiar Supernova with Early Strong Carbon and Weak Oxygen Features from a New Sample of Young SN Ic Spectra

Abstract

With the advent of high-cadence, all-sky automated surveys, supernovae (SNe) are now discovered closer than ever to their dates of explosion. However, young premaximum light follow-up spectra of Type Ic SNe (SNe Ic), probably arising from the most-stripped massive stars, remain rare despite their importance. In this Letter, we present a set of 49 optical spectra observed with the Las Cumbres Observatory through the Global Supernova Project for 6 SNe Ic, including a total of 17 premaximum spectra, of which 8 are observed more than a week before V-band maximum light. This data set increases the total number of publicly available premaximum-light SN Ic spectra by 25%, and we provide publicly available SNID templates that will significantly aid in the fast identification of young SNe Ic in the future. We present a detailed analysis of these spectra, including Fe ii 5169 velocity measurements, O i 7774 line strengths, and continuum shapes. We compare our results to published samples of stripped SNe in the literature and find one SN in our sample that stands out. SN 2019ewu has a unique combination of features for an SN Ic: an extremely blue continuum, high absorption velocities, a P Cygni–shaped feature almost 2 weeks before maximum light that TARDIS radiative transfer modeling attributes to C ii rather than Hα, and weak or nonexistent O i 7774 absorption feature until maximum light.

Peculiar Spectral Evolution of the Type I Supernova 2019eix: A Possible Double Detonation from a Helium Shell on a Sub-Chandrasekhar-mass White Dwarf

“…We also compared SN 2019eix with SN 2019ewu, a peculiar Type Ic SN that, like SN 2019eix, lacks an O I absorption feature. However, SN 2019ewu displays distinct characteristics at this epoch, including an extreme blue continuum and high absorption velocities, which are not typically observed in SNe Ic (Williamson et al 2022). These peculiarities are outlined and discussed in greater detail in Section 3.4.…”

“…The striking features of the spectral sequence of SN 2019eix are concentrated in the suppression of the blue color λ < 5000 Å, the absence of an O I feature (This is peculiar for SNe Ic which have been shown to generally have a strong O I 7774 feature; however, an exception has been observed; Fremling et al 2018;Williamson et al 2022), and the strong evolution of Ca II feature. In Figure 7, we plot SN 2019eix along with various types of SNe that match spectroscopically at different epochs.…”

Early-time Ultraviolet and Optical Hubble Space Telescope Spectroscopy of the Type II Supernova 2022wsp

“…The spectral continuum of an SN II peaks in the ultraviolet (UV) in the days and weeks following the explosion, and then continues to shift toward optical wavelengths over the next few months through a combination of cooling and line blanketing. Distinguishing the SN type at early times is challenging, as SN II spectra often resemble those of SNe Ic (see, for example, Williamson et al 2023). Although the optical spectra of corecollapse SNe (CCSNe) have been extensively studied, the sample of UV spectra of CCSNe still remains scarce.…”