Schedule Mar 23, 2007
Searching for a Companion Star of Tycho's Type Ia Supernova with Optical Spectroscopic Observations
Yutaka Ihara (Univ. Tokyo)

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.

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