Spectral Evolution of the Unusual Slow Nova V5558 Sgr

TL;DR

This study documents the spectral evolution of the slow nova V5558 Sgr, revealing unique spectral transition patterns and emission line behaviors during its rebrightening phases, providing new insights into nova spectral evolution.

Contribution

First observation of spectral type transition from He/N to Fe II and back in a slow nova during rebrightenings, highlighting complex spectral evolution.

Findings

Spectral type evolved from He/N to Fe II and back during rebrightenings.

P-Cygni absorption velocity decreased at brightness maxima.

Spectral behaviors similar to V723 Cas and other rebrightening novae.

Abstract

We report on the spectral evolution of the enigmatic, very slow nova V5558 Sgr, based on the low-resolution spectra obtained at the Fujii-Bisei Observatory and the Bisei Astronomical Observatory, Japan during a period of 2007 April 6 to 2008 May 3. V5558 Sgr shows a pre-maximum halt and then several flare-like rebrightenings, which is similar to another very slow nova V723 Cas. In our observations, the spectral type of V5558 Sgr evolved from the He/N type toward the Fe II type during the pre-maximum halt, and then toward the He/N type again. This course of spectral transition was observed for the first time in the long history of the nova research. In the rebrightening stage after the initial brightness maximum, we could identify many emission lines accompanied by a stronger absorption component of the P-Cygni profile at the brightness maxima. We found that the velocity of the P-Cygni absorption component measured from the emission peak decreased at the brightness maxima. Furthermore, we compared the spectra of V5558 Sgr with V723 Cas, and other novae which exhibited several rebrightenings during the early phase.