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

TL;DR

This paper presents a new set of early spectra for six Type Ic supernovae, highlighting SN 2019ewu's unique features such as strong carbon signatures and weak oxygen lines, enhancing understanding of supernova diversity.

Contribution

It provides the largest publicly available pre-maximum spectra sample for SNe Ic and introduces new templates for rapid identification of young supernovae.

Findings

SN 2019ewu exhibits strong carbon features and weak oxygen lines.

The dataset increases pre-maximum spectra availability by 25%.

New SNID templates facilitate faster supernova classification.

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 pre-maximum light follow-up spectra of Type Ic supernovae (SNe Ic), probably arising from the most stripped massive stars, remain rare despite their importance. In this paper 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 pre-maximum spectra, of which 8 are observed more than a week before V-band maximum light. This dataset increases the total number of publicly available pre-maximum 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 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 supernovae in the literature and find one SN in our sample that stands out. SN 2019ewu has a unique combination of features for a 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 H$\alpha$, and weak or non-existent O I 7774 absorption feature until maximum light.