2015MNRAS.447..806D

We present near-infrared (1-2.5µm) JHK photospectroscopic results of the unusually slow nova V5558 Sgr (2007). V5558 Sgr showed a slow climb to maximum that lasted for about 60d and then underwent at least five strong secondary outbursts. We have analysed the optical light curve to derive large t2 and t3 values of 281±3 and 473±3d, respectively. An alternate approach is adopted to derive a distance estimate of 1.55±0.25 kpc as conventional maximum magnitude-rate of decline relation may not be applicable for a slow nova. In the pre-maxima stage, the spectra showed narrow (FWHM ∼ 400-550 km/s) and strong emission lines of Paschen and Brackett series with prominent P-Cygni components. In the later phase, the spectra show significant changes with the development of strong and broad ( ∼ 1000 km/s) emission lines of Hi, Hei, Oi and Ni and some uncommon FeII emission lines. No evidence of dust formation is seen. V5558 Sgr has been shown to be a rare hybrid nova showing a transition from He/N to FeII type from optical spectra. However, the near-infrared data do not show such a transition and we discuss this anomalous behaviour. A recombination analysis of the Brackett lines allows us to constrain the electron density and emission measure during the early optically thick phase and to estimate the mass of the ejecta to be (6.0±1.5)x10^–4 M_☉, assuming a filling factor of unity, from later observations.