Spectral evolution of the narrow emission-line components in optical during the 2022 nova eruption of U Scorpii

Abstract There remains debate over whether the accretion disk survives or is entirely disrupted after the nova eruption. In our previous paper (Muraoka et al. 2024, PASJ, 76, 293), we photometrically demonstrated that the surviving accretion disk expanded close to the L1 point during the optical plateau stage and then drastically shrank to the tidal truncation radius after the optical plateau stage ended. With the aim of clarifying the physical mechanism that drives these structural changes, we then conducted systematic analyses of the spectral evolution of the narrow emission-line components in optical over 22 d following the optical peak during the 2022 nova eruption of U Scorpii. Additionally, we present its optical spectrum in quiescence 794 d after the 2022 nova eruption. We find that the single-peaked narrow components of H$\alpha$ and He ii 4686 appeared almost simultaneously between roughly days 6 and 8, preceding the onset of the disk eclipses observed after day 11. This finding suggests that the nova wind near the binary system may be the primary origin of these narrow components and even remained active several days after the nova eruption with a velocity of approximately 1000 km s$^{-1}$, likely driving the expansion of the accretion disk until the end of the optical plateau stage. While the contribution of the rotating accretion disk might be dominated by that of the nova wind in the H$\alpha$ line profile, the outward surface flow from the expanded disk might also contribute to these narrow features during the optical plateau stage, making the single-peaked narrow line profiles more pronounced.