SUBARU HIGH-RESOLUTION SPECTROSCOPY OF STAR G IN THE TYCHO SUPERNOVA REMNANT

Abstract

It is widely believed that Type Ia supernovae (SN 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 explosion, providing an opportunity to understand the progenitors of Type Ia supernovae.In this paper we argue that rotation is a generic signature expected of most non-giant donor stars that is easily measurable. \citep{2004Natur.431.1069R} examined stars in the center of the remnant of SN 1572 (Tycho's SN) and showed evidence that a subgiant star (Star G by their naming convention) near the remnant's centre was the system's donor star. We present high-resolution (R \simeq 40000) spectra taken with the High Dispersion Spectrograph on Subaru of this candidate donor star and measure the star's radial velocity as $79\pm 2$ \kms with respect to the LSR and put an upper limit on the star's rotation of 7.5 \kms. In addition, by comparing images that were taken in 1970 and 2004, we measure the proper motion of Star G to be $\mu_l = -1.6 \pm 2.1$ \masyr and $\mu_b = -2.7 \pm 1.6$ \masyr. 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's SN that is not associated with the supernova 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.