  We propose a method to identify the companion stars of type Ia supernovae
(SNe Ia) in young supernova remnants (SNRs) by recognizing distinct features of
absorption lines due to Fe I appearing in the spectrum. If a sufficient amount
of Fe I remains in the ejecta, Fe I atoms moving toward us absorb photons by
transitions from the ground state to imprint broad absorption lines exclusively
with the blue-shifted components in the spectrum of the companion star. To
investigate the time evolution of column depth of Fe I in the ejecta, we have
performed hydrodynamical calculations for SNRs expanding into the uniform
ambient media, taking into account collisional ionizations, excitations, and
photo-ionizations of heavy elements. As a result, it is found that the
companion star in Tycho's SNR will exhibit observable features in absorption
lines due to Fe I at $\lambda\lambda = 371.9935$ nm and 385.9911 nm if a carbon
deflagration SN model is taken. However, these features may disappear by taking
another model that emits a few times more intense ionizing photons from the
shocked outer layers. To further explore the ionization states in the freely
expanding ejecta, we need a reliable model to describe the structure of the
outer layers.
