Low- and Medium-Dispersion Spectropolarimetry of Nova V475 Scuti (Nova Scuti 2003): Discovery of an Asymmetric High-Velocity Wind in a Moderately Fast Nova

Abstract

We present low-resolution (R ~ 90) and medium-resolution (R ~ 2500) spectropolarimetry of nova V475 Sct taken with the HBS instrument, mounted on the 0.91 m telescope at the Okayama Astrophysical Observatory, and taken with the Faint Object Camera and Spectrograph, mounted on the 8.2 m Subaru telescope. We estimated the interstellar polarization toward the nova from the steady continuum polarization components and Hα line emission components. After subtracting the interstellar polarization component from the observations, we found that the Hα emission seen on 2003 October 7 was clearly polarized. In the polarized flux spectrum the Hα emission had a distinct red wing extending to ~+4900 km s-1 and a shoulder around +3500 km s-1, showing a constant position angle (P.A.) of linear polarization θ* 155° ± 15°. This suggests that the nova had an asymmetric outflow with a velocity of vwind 3500 km s-1 or more, which is 6 times higher than the expansion velocity of the ionized shell at the same epoch. Such a high-velocity component has not previously been reported for a nova in the moderately fast speed class. Our observations suggest the occurrence of violent mass-loss activity in the nova binary system even during the common-envelope phase. The P.A. of the polarization in the Hα wing is in good agreement with that of the continuum polarization found on 2003 September 26 (p* 0.4%-0.6%), which disappeared within the following 2 days. The uniformity of the P.A. between the continuum polarization and the wing polarization on October 7 suggests that the axis of the circumstellar asymmetry remained nearly constant during the period of our observations.