A Method to Identify the Companion Stars of Type Ia Supernovae in Young Supernova Remnants

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 blueshifted 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 hydrodynamic calculations for SNRs expanding into a uniform ambient media, taking into account collisional ionizations, excitations, and photoionizations 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 λλ = 371.9935 and 385.9911 nm if a carbon deflagration SN model is taken. However, these features may disappear if we take 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. © 2006. The American Astronomical Society. All rights reserved.