Structure and evolution of classical Nova Shells

Abstract

This thesis focuses on shells ejected during classical nova events. Novae are due to thermonuclear runaway on the surface of a white dwarf in a binary system. The work herein concentrates on nova shells individually to understand and unify nova shells as a whole. This thesis aims to follow the ageing process of classical nova shells from ejection to centuries post-ejection. This research was undertaken using imaging, spectroscopy and polarimetric observations, as well as morphology, kinematic and photoionisation simulations. The three main results chapters in this thesis integrate the methods listed above to follow the ageing process of expanding nova shells. The first results chapter focuses on the shell of V5668 Sagittarii (2015) from 0 - 822 days post-discovery. A main finding from the examination was that V5668 Sagittarii displays O ii rather than N iii flaring around the 4640 ̊A region. This flaring episode has been commonly seen but misidentified in erupting slow nova systems. A symmetry discovered in the expanding nova shell of GK Persei (1901) is presented in the second results chapter. The revealed shaping of knots in the GK Persei nova shell are attributed to fast chasing dwarf nova winds. In addition, the velocity is determined for the first time of the only known apparent jet in a classical nova system. The low velocity of the apparent jet is attributed to an illuminated lobe of the fossil planetary nebula within which the GK Persei shell resides. The third results chapter includes: the discovery of two shells around known nova producing systems, a time evolution analysis of four of the better-studied classical nova shells, and the finding what could be a nested shell structure from multiple nova ejection episodes. A common axial-symmetry between all of the studied classical nova shells is revealed throughout the thesis and concluding that through considerations of the inclination of the source towards the observer the wide variety of observed nova light curves and spectral characteristics can be reconciled.