2009ApJ...701.1665K

It is widely believed that Type Ia supernovae (SNe Ia) originate in binary systems where a white dwarf accretes material from a companion star until its mass approaches the Chandrasekhar mass and carbon is ignited in the white dwarf's core. This scenario predicts that the donor star should survive the supernova (SNe) explosion, providing an opportunity to understand the progenitors of SNe Ia. In this paper, we argue that rotation is a generic signature expected of most nongiant donor stars that is easily measurable. Ruiz-Lapuente et al. examined stars in the center of the remnant of SN 1572 (Tycho SN) and showed evidence that a subgiant star (Star G by their naming convention) near the remnant's center was the system's donor star. We present high-resolution (R ≃ 40, 000) spectra taken with the High Dispersion Spectrograph on Subaru of this candidate donor star and measure the star's radial velocity as 79±2 km/s with respect to the local standard of rest and put an upper limit on the star's rotation of 7.5 km/s. In addition, by comparing images that were taken in 1970 and 2004, we measure the proper motion of Star G to be µ_l= -1.6±2.1 mas/yr and µ_b= -2.7±1.6 mas/yr. We demonstrate that all of the measured properties of Star G presented in this paper are consistent with those of a star in the direction of Tycho SN that is not associated with the SN event. However, we discuss an unlikely, but still viable scenario for Star G to be the donor star, and suggest further observations that might be able to confirm or refute it.