Author
Yutaka Ihara, Jin Ozaki, Mamoru Doi, Toshikazu Shigeyama (Tokyo Univ.),Nobunari Kashikawa, Yutaka Komiyama, and Takashi Hattori (NAOJ).
We report our first results of searching for a companion star of a type Ia
supernova (SN Ia) with photometric observations and spectroscopic
observations in Tycho's supernova remnant (SNR Tycho). If a companion star
is found, a evolutionary scenario for the progenitor systems (e.g., Hachisu
et al. 1999) will be supported.
Ruiz-Lapuente et al. (2004) indicated the existence of a companion star
named Tycho(G) in SNR Tycho based on estimated distance and proper
motions, but it was not a direct method to prove the association of the
star with the ejecta. Ozaki & Shigeyama (2006) proposed a method using
characteristic absorption lines of Fe I existing in SNR Tycho to identify
the companion star. Since the supernova ejecta are expanding and Fe I
components are moving toward us, the companion star within SNR Tycho should
have broad absorptions of only blue-shifted components, while stars behind
SNR Tycho exhibit very broad wings in both of the blue and red sides of the
absorption lines.
From photometric observations, we picked up stars which are located close
tothe center of SNR Tycho for spectroscopy. In spectroscopic observations
with Subaru telescope, we obtained spectra of 17 stars in the SNR Tycho
region and compare them with template stellar spectra. We detect
significant absorption lines from two stars at the wavelength expected to
be absorbed by Fe I in the ejecta. In addition, spectra of four stars have
possible absorption lines. Since widths of their absorption lines are
broad, it is likely that the detected absorptions are due to Fe I in the
expanding ejecta of SNR Tycho. However, the observed profiles of the
absorption do not have clear red wings, which are expected for the
background of the SNR.
Though Tycho(G) was raised as a candidate of the companion star of Tycho's
supernova in Ruiz-Lapuente et al. (2004), we do not obtain evidence
supporting their results. On the contrary, a star named Tycho(E) has the
broad absorption line due to Fe I and its projected position is close to
the center of SNR Tycho. Based on our observations, Tycho(E) might be the
candidate as the companion star of Tycho's supernova.