---
abstract: 'Here we present previously unpublished optical spectra of supernova (SN) 2001ig, a Type IIb SN, from about a week after explosion until nearly one year later.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 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 [$\sim\!\!$ ]{}1.15 [M$_\sun$]{} of material, most of which (by mass) is in the form of oxygen, with [$\sim\!\!$ ]{}0.13 [M$_\sun$]{} of $^{56}$Ni and essentially no hydrogen.'
author:
- 'Jeffrey M. Silverman, Paolo Mazzali, Ryan Chornock, Alexei V. Filippenko, Alejandro Clocchiatti, Mark M. Phillips, Mohan Ganeshalingam, and Ryan J. Foley'
bibliography:
- 'astro\_refs.bib'
title: Optical Spectroscopy of the Somewhat Peculiar Type IIb Supernova 2001ig
---

Introduction {#s:intro}
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It is thought that most high-mass stars ($\gtrsim8$ [M$_\sun$]{}) 