Optical Spectroscopy of the Somewhat Peculiar Type IIb Supernova 2001ig

TL;DR

This paper presents optical spectra of supernova 2001ig over a year, revealing its transition from Type II to Type Ib and analyzing its ejecta composition with models indicating a predominantly oxygen core.

Contribution

It provides the first comprehensive spectral sequence of SN 2001ig from early to late times and models its inner ejecta composition.

Findings

SN 2001ig transitions from Type II to Type Ib.

Late-time spectra show a high magnesium to oxygen ratio.

Ejecta contain about 1.15 solar masses, mostly oxygen.

Abstract

Here we present previously unpublished optical spectra of supernova (SN) 2001ig, a Type IIb SN, from about a week after explosion until nearly one year later. The earliest spectrum consists of only a few broad absorption features, but soon more common Type II SN features including hydrogen P-Cygni profiles and helium absorption become apparent. At later times, as the H features fade and the He I absorption becomes more prominent, we observe the SN to transition from a Type II to a Type Ib. Finally, observations after 250 days past explosion show a nebular-phase SN spectrum with one of the largest magnesium to oxygen intensity ratios ever seen. Additionally, we present models of the late-time spectra which indicate that the inner ejecta consist of ~1.15 M_sun of material, most of which (by mass) is in the form of oxygen, with ~0.13 M_sun of Ni-56 and essentially no hydrogen.