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

We present low-resolution ($R\sim 90$) and medium-resolution ($R\sim 2500$)spectropolarimetry of Nova V475 Sct with the HBS instrument, mounted on the0.91-m telescope at the Okayama Astrophysical Observatory, and with FOCAS,mounted on the 8.2-m Subaru telescope. We estimated the interstellarpolarization toward the nova from the steady continuum polarization componentsand H$\alpha$ line emission components. After subtracting the interstellarpolarization component from the observations, we found that the H$\alpha$emission seen on 2003 October 7 was clearly polarized. In the polarized fluxspectrum, the H$\alpha$ emission had a distinct red wing extending to $\sim+4900$ km s$^{-1}$ and a shoulder around $+3500$ km s$^{-1}$, showing aconstant position angle of linear polarization $\theta_{\rm *}\simeq155\arcdeg\pm 15\arcdeg$. This suggests that the nova had an asymmetric outflowwith a velocity of $v_{\rm wind}\simeq 3500$ km s$^{-1}$ or more, which is sixtimes higher than the expansion velocity of the ionized shell at the sameepoch. Such a high-velocity component has not previously been reported for anova in the `moderately fast' speed class. Our observations suggest theoccurrence of violent mass-loss activity in the nova binary system even duringthe common-envelope phase. The position angle of the polarization in theH$\alpha$ wing is in good agreement with that of the continuum polarizationfound on 2003 September 26 ($p_{\rm *}\simeq 0.4$--0.6 %), which disappearedwithin the following 2 d. The uniformity of the PA between the continuumpolarization and the wing polarization on October 7 suggests that the axis ofthe circumstellar asymmetry remained nearly constant during the period of ourobservations.Comment: 27 pages, 7 figures, accepted for publication in A