Abstract
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.
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