document: type ia supernovae ( sne ia ) , characterized by no @xmath1 but strong si lines in the spectra at the maximum brightness , are brighter than most of sne classified into the other types and exhibit uniform light curves . 
   thus they are used as a standard candle to measure distances to remote galaxies . 
   a plausible explosion model for sne ia is the accreting white dwarf ( wd ) model , in which a white dwarf in a binary system accretes material from the companion star , increases its mass , usually up to the chandrasekhar mass limit ( @xmath2 ) , and then explodes ( e.g. , * ? ? ? 
   . there have been significant progresses in the accreting wd model since @xcite introduced the stellar wind from the wd while it accretes materials from the companion . 
   their model succeeded in sustaining a stable mass transfer in the progenitor systems of sne ia . according to their model 
   , there are two main evolutionary paths leading to sne ia , the super soft x - ray source ( sss ) channel and the symbiotic channel @xcite . 
   accordingly their model predicts which companion stars lead to sne ia .    the above evolutionary scenario for sne ia has not been confirmed by observations , which will require the identification of the companion star that should remain in the vicinity of the explosion site . 
   @xcite argued that their group identified a g2iv star as the companion of tycho brahe s supernova remnant ( snr ) by measuring the velocities and distances of stars in the vicinity of the center of the snr . 
   they concluded that this g2iv star was moving much faster than the other neighbor stars and that the distance to this star seemed consistent with the distance to tycho s snr . 
   although @xcite has argued that the observed velocity of tycho g might correspond to the velocity of stars belonging to the thick disk population , the expected stellar mass in thick disk stars within the cone with 2.87 arcsec radius ( which corresponds to the angular distance from the center of tycho s snr to the tycho g star ) , at 3 kpc from earth , is only 2 @xmath3 , which makes the thick disk star alternative very unlikely . for this estimate 
   we use the density in the vicinity of tycho s snr @xcite .    although the coincidence of the kinematic characteristics of tycho g with its being at the position and distance of the snr appears significant , confirmation by other means would nonetheless be very useful .    in this paper 
   we propose a direct method to prove that the companion star is located inside the sn ejecta . 
   a hint was dropped by observations for a star called s - m star discovered by @xcite near a type ia snr 1006 . 
   @xcite proved that the s - m star was not the companion of this supernova by investigating features of fe ii absorption lines in the ultraviolet ( uv ) spectrum . 
   very broad wings were observed in both blue and red sides of the absorption lines . 
   the line width was a few thousand km / s much larger than the thermal velocity of stellar atmosphere , which is thought to be @xmath410 km / s . 
   the broad wings are likely to be formed by fe ii in the ejecta of snr 1006 ; photons in the blue wing are absorbed by the matter ejected toward us , and those in the red wing away from us . 
   thus it was proved that the s - m star is located behind the snr 1006 ejecta .    when a star is inside the ejecta of sne ia , photons emitted from the star are absorbed only by the ejecta moving toward us . 
   hence the broad wing must be present only in the blue side . 
   the absorption line with only blue wing enables us to identify companion stars of sne ia . 
   @xcite used the uv range to observe the s - m star with the faint object spectrograph on the hubble space telescope . 
   however , in addition to difficulties in uv observations from the ground , companion stars on the evolutionary paths suggested by the above mentioned scenario @xcite may not be bright in the uv range 
   . then we will focus on absorption lines in the visible range . 
   furthermore the corresponding transitions need to be from the ground state because most fe ions in the ejecta are expected to be in the ground state . 
   thus only fe i can produce such absorption lines in sne ia ejecta .    in this paper , we estimate the amount of fe i in the ejecta of tycho s snr by taking account of collisional processes in non - equilibrium and ionizations by photons emitted from the shocked ejecta , and calculate spectra of a star located at the center of a snr and discuss whether we can identify the feature of fe i absorption lines in the spectrum of the companion star . 
   [ cols="^,^,^,^,^,^ " , ]     [ tbl : ew ]    even when we find a star that exhibits the absorption feature discussed in this paper , there is a chance that a star other than the companion star happens to be inside the ejecta . 
   a star existing in the vicinity of a sn ia gets a fraction of the explosion energy and is accelerated . 
   if the companion star similar to tycho g star with the mass of @xmath5 1.2 @xmath6 and the radius of @xmath7 2 @xmath8 is located at the distance of @xmath9 5.5 @xmath8 from the progenitor , the size of the roche lobe is comparable to the size of the companion . 
   thus the star in this situation will get the maximum velocity after the explosion . 
   then the orbital velocity before the explosion is @xmath10 km / s . in a 3-d hydrodynamical simulation of sne ia by @xcite 
   , they obtain a plausible kick velocity @xmath11 km / s . therefore , including the orbital velocity , the velocity of the companion star becomes @xmath12 km / s . if the companion star has been moving away from the explosion site at that speed , the companion star is now at the distance of @xmath130.08 pc from the center . 
   the stellar mass density in the neighborhood of tycho s snr estimated from a model of the galaxy @xcite is less than 0.03 @xmath14 . 
   thus the expected stellar mass inside this volume is only @xmath15 . 
   therefore if we find a star showing absorption lines with broad blue wings in the spectrum , it is likely that the star is the companion star . 
   as a consequence , we have demonstrated that there exhibit very deep absorption lines with unique shapes in the spectrum of the companion star located at the center of a young snr such as tycho s snr . 
   there are , however , a few factors that might reduce or even erase these distinct absorption features .    first , the number of fe i in the freely expanding ejecta is very sensitive to the number of ionizing photons emitted from the shocked ejecta . 
   an increase in the number of ionizing photons by a few factors might decrease the number of fe i by a few orders of magnitude or more . 
   since the main source of ionizing photons is o in the outer ejecta , the distributions of o and density in the outer ejecta need to be known precisely . 
   unfortunately , these regions in w7 have some problems to reproduce the observed optical spectra ( * ? ? ? 
   * and references therein ) . due to this uncertainty in the explosion model , 
   it is not conclusive if the companion star of tycho s sn will exhibit unique fe i absorption features discussed in this paper . 
   nevertheless , it is true that every sn ia has a period during which the companion star has the distinct absorption features discussed above because the ejecta become cool enough to have plenty of fe i for a time after the optical brightening . 
   second , it is assumed in our calculations that ions in the shocked region are ionized to fe@xmath16 , si@xmath17 , o@xmath18 , c@xmath19 immediately after the shock passes following the procedure taken by @xcite . 
   since ions in lower ionization stages are a strong source of ionizing photons , ionization may be more advanced in real young snrs . 
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summary: 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 @xmath0 nm and 385.9911 nm if a carbon deflagration sn model @xcite 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 .