Near Infrared studies during maximum and early decline of Nova Cephei 2014 and Nova Scorpii 2015

TL;DR

This study presents near-infrared spectroscopic observations of two novae, revealing their classification, the symbiotic nature of Nova Sco 2015, and analyzing ejecta properties and evolution.

Contribution

It is the first detailed near-infrared study showing Nova Sco 2015 as a rare classical nova with a giant secondary and provides insights into ejecta dynamics and composition.

Findings

Nova Cep 2014 is a Fe II class nova.

Nova Sco 2015 is a He/N class nova and a symbiotic system.

Ejecta velocity decays as t^{-1.13}.

Abstract

We present multi-epoch near-infrared photo-spectroscopic observations of Nova Cephei 2014 and Nova Scorpii 2015, discovered in outburst on 2014 March 8.79 UT and 2015 February 11.84 UT respectively. Nova Cep 2014 shows the conventional NIR characteristics of a Fe II class nova characterized by strong CI, HI and O I lines, whereas Nova Sco 2015 is shown to belong to the He/N class with strong He I, HI and OI emission lines. The highlight of the results consists in demonstrating that Nova Sco 2015 is a symbiotic system containing a giant secondary. Leaving aside the T CrB class of recurrent novae, all of which have giant donors, Nova Sco 2015 is shown to be only the third classical nova to be found with a giant secondary. The evidence for the symbiotic nature is three-fold; first is the presence of a strong decelerative shock accompanying the passage of the nova's ejecta through the giant's wind, second is the H$\alpha$ excess seen from the system and third is the spectral energy distribution of the secondary in quiescence typical of a cool late type giant. The evolution of the strength and shape of the emission line profiles shows that the ejecta velocity follows a power law decay with time ($t^{-1.13 \pm 0.17}$). A Case B recombination analysis of the H I Brackett lines shows that these lines are affected by optical depth effects for both the novae. Using this analysis we make estimates for both the novae of the emission measure $n_e^2L$, the electron density $n_e$ and the mass of the ejecta.