MULTIWAVELENGTH STUDIES OF NOVA SYSTEMS

Novae are interacting binary star systems consisting of a Roche-lobe filling secondary on or near the main sequence, losing hydrogen-rich material through the inner-Lagrangian point onto an accretion disc surrounding the white dwarf primary. The mass losing secondary in some recurrent novae is a late giant star. Nova systems serve as valuable astrophysical laboratories in the studies of physics of accretion onto the compact, evolved objects, thermonuclear runaways on semi-degenerate surface which give insight into nuclear reaction networks, and line formation and transfer processes in moving atmospheres. This work presents study of sixteen novae belonging to different spectral classes. The detailed temporal evolution of seven of these sixteen novae are discussed. The optical data were primarily obtained from the 2 m Himalayan Chandra Telescope (HCT) and 2.3 m Vainu Bappu Telescope (VBT). Different types of novae can be distinguished using their spectra. The spectral evolution of these systems are studied and are used to derive the evolution of the radiation source and use them in testing the theoretical models of nova outburst. The physical conditions in the ejecta and their evolution are also be derived using the spectral evolution of these systems as they exhibit a varying range of physical conditions like ionization parameters and densities with time. The observations of high resolution spectra of some novae are used to get an insight into the geometry of the nova shell. Two novae are also studied in UV and X-ray regimes and one of them in radio wavelength regime too. The multiwavelength observations are used to study the physical properties of the nova ejecta following the outburst, while the quiescence data are used to understand the accretion and binary properties of the nova system. The optical spectroscopic evolution of few classical and recurrent novae like 2011 outburst of T Pyx, V339 Del, 2016 eruption of M31N 2008-12a, V5856 Sgr, V612 Sct, V906 Car are presented. Multiwavelength temporal evolution of 2018 outburst of M31N 2008-12a and the 2019 outburst of V3890 Sgr are discussed. The ejecta geometry of several novae such as V477 Sct, V2362 Cyg, V5587 Sgr, V5588 Sgr, T Pyx, V2676 Oph, V339 Del, V1369 Cen, V2944 Oph, V5856 Sgr, M31N 2008-12a, V3665 Oph, V906 Car, V5857 Sgr and V3890 Sgr are discussed. The ejected mass of the CNe are estimated to be in the range of 10-4 to 10-6 MSun and for RNe in the order of 10-6 MSun except for M31N 2008-12a where the value is in the order of 10-8 MSun. Most of the novae have overabundance values of nitrogen, oxygen and helium compared to solar values sensitive to the evolutionary phase of the system. In Fe II class of novae, the abundance of iron is also more than the solar abundance value. The morphology of the ejecta for three different spectral class of novae Fe II, He/N and hybrid are found to be similar. All the novae in the sample have bipolar structures with cones and with or without equatorial rings. In He/N and one of the hybrid novae in the sample, enhanced bipolar emission is seen. The spatial distribution of elemental lines in all the novae is identical irrespective of the spectral class. The high ionization forbidden lines always arise from the outer regions due to the optical thin ejecta in the outer regions. The helium lines are always from the inner regions of the ejecta. Fe II emission, if present, is from the equatorial rings, and the hydrogen Balmer lines arise from all the components in the ejecta