Optical Spectroscopy of the Somewhat Peculiar Type Iib Supernova 2001ig

Here we present previously unpublished optical spectra of supernova (SN) 2001ig, a Type IIb SN, from about a week after explosion until nearly one year later.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⊙ of material, most of which (by mass) is in the form of oxygen, with ∼0.13 M⊙ of Ni and essentially no hydrogen. Subject headings: supernovae: general — supernovae: individual (SN 2001ig, SN 1987K, SN 1993J, SN 2008D, SN 2002ap) — radiative transfer: supernovae