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  Here we present previously unpublished optical spectra of supernova (SN)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_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.
