Spectral evolution of the recurrent nova RS Ophiuchi during the 2006 outburst and some mysterious phenomena

Spectral evolution in the optical region of the recurrent nova RS Oph during the 2006 outburst was monitored at the Asiago Astrophysical Observatory. A short-lived flare-up of He I emission lines, which was clearly different from the helium flash of classical novae, was observed on March 7, that is 22 days after the outburst and 3.8 mag below maximum luminosity. The coronal emission line [Fe XIV] 5303 was first detected 35 days after the outburst. Its maximum intensity relative to Hβ was observed 80 days after the outburst, then the intensity declined rapidly. Another coronal line [Ar XI] 6919 showed a similar variation, while [Fe X] 6374 faded more slowly. When the former two coronal lines started to fade about 90 days after the outburst, the emission lines of [O I], [N II], [S II], and [O III] started to strengthen rapidly. Probably, the collisional shock front on the high velocity ejecta reached the outer boundary of the circumstellar envelope and a free expansion started at that time. The variations in intensity of the coronal lines [Fe XIV] and [Ar XI] coincided with that of the flux of super-soft X-rays, suggesting that also the latter originated in the collisional shock front. Many prominent emission lines were composed of a central narrow component, blue- and red-shifted wings, and a very broad component. Unidentified emission lines at 6105.5 and 6181.2 Å are identified as O IV 4s2P0–4p2S, 6105.9 and 6183.4 Å, respectively. On the other hand, new unidentified, probably coronal, emission lines are found at 7711 ± 2 Å and 7774 ± 2 Å. A weak trace of the Raman emission band at 6830 Å was detected when the emission lines of [Fe VII] were not seen. It seems that the lines of O IV at about 1033 Å contributed, in the place of O VI 1032 Å, to the Raman scattering in a low ionization condition. Splittings in two parts of the narrow emission components of H I, He I, and the coronal lines were observed. The splittings were largest on April 8, when the separations between the two peaks were 48 km s-1 for [Fe XIV] 5303, 37 km s-1 for Hβ, and 33 km s-1 for He I 5876. On the other hand, the narrow emission components of Fe II, [N II], [O III], O IV, and [Fe VII] did not show any splitting