2007CBET.1010....2G

So far, the lightcurve has been highly reminiscent of that displayed by Nova Cas 1995 = V723 Cas (cf. Munari et al. 1996, A.Ap. 315, 166). V5558 Sgr has shown large spectral changes in data obtained with the 1.82-m telescope at Asiago and the 0.6-m telescope of the Schiapparelli Observatory in Varese. At maximum on July 10, the spectra were still characterized by a smooth underlying continuum with numerous weak absorptions, while Balmer series and FeI I lines were in emission, the strongest of which belong to multiplets 42, 48, 49, and 74. On July 12, He I started emerging in emission. By July 14, the forest of weak absorptions had gone, the emission lines reinforced, and this trend was followed on July 16. Comparing July 12 and 17 spectra, the integrated flux ratio He I 587.6-nm/Fe II 501.8-nm (multiplet 42) remained constant at 2.2, while the He I 587.6-nm/H_beta ratio increased from 0.32 to 0.52. The most relevant spectral changes, however, were those displayed by the line profiles. Since discovery and until a few days ago, the emission lines were very sharp, with P-Cyg absorption displaced by just -120/-130 km/s. At maximum brightness on July 10, the P-Cyg absorption component of the H_alpha profile still had a mere velocity of -130 km/s. Then, by July 12 a fast, hot wind emerged, with suddenly bloated-up emission-line profiles that peaked to a FWHM of 1150 km/s on July 14, still with the sharp, low-velocity absorption superimposed. By July 17 the FWHM of the fast, hot wind had decreased to 850 km/s, and by July 20 to 780 km/s. This internal high-velocity wind is expected to slam onto the exterior, slower, and mostly neutral wind. This should produce a shock front.