Near-infrared studies of V5558 Sgr: an unusually slow nova with multiple outbursts

TL;DR

This study presents near-infrared spectroscopic observations of the slow nova V5558 Sgr, revealing its unusual outburst behavior, spectral evolution, and ejecta properties, with implications for understanding slow and hybrid novae.

Contribution

It provides detailed near-infrared spectral analysis of V5558 Sgr, highlighting its hybrid nature and anomalous spectral behavior not seen in typical novae.

Findings

V5558 Sgr exhibited at least five secondary outbursts.

The nova showed a transition from He/N to Fe II type in optical spectra.

Ejecta mass estimated at (6.0 +/- 1.5) x 10^(-4) solar masses.

Abstract

We present near-infrared (1-2.5 micron) JHK photo-spectroscopic results of the unusually slow nova V5558 Sgr (2007). V5558 Sgr showed a slow climb to maximum that lasted for about 60 days and then underwent at least five strong secondary outbursts. We have analyzed the optical light curve to derive large t2 and t3 values of 281 +/- 3 and 473 +/- 3 days respectively. An alternate approach is adopted to derive a distance estimate of 1.55 +/- 0.25 kpc as conventional MMRD 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 Fe II 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 Fe II type from optical spectra. However the near-infrared data do not show such a transition and we discuss this anomalous behavior. 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) x 10^(-4) Msun, assuming a filling factor of unity, from later observations.