Unravelling the nature of explosion of thermonuclear supernovae

Abstract

Type Ia supernovae (SNe Ia) result from thermonuclear explosions in binary systems consisting of at least one carbon-oxygen white dwarf. They are crucial for the understanding of galactic chemical composition as they are the principal producers of Iron (Fe)-group elements in the Universe, sites for cosmic ray production, probes of low mass star formation rates at high redshifts, understanding the binary star evolution, constraining the dark energy equation of state and also as standardizable candles to measure astronomical distances. The rate of decline in normal Type Ia SNe (0.85 < Δm15(B) < 1.70 mag) is correlated with the absolute magnitude in B-band. The radioactive decay of 56Ni to 56Co and finally to 56Fe supplies the energy during the maximum of the light curve and its subsequent evolution. The luminosity increases with more 56Ni produced in the explosion. While most SNe Ia follow the luminosity decline rate relation, it is important to note that a good fraction of supernova events of thermonuclear origin do not follow this relation. The over-luminous super-Chandrasekhar SNe Ia lie at the brighter end of the Δm15(B) - MB relation. They are slowly declining objects. The SNe Iax (SN 2002cx-like) are a peculiar class of thermonuclear explosions with low luminosity and low kinetic energy and fast declining compared to other SNe Ia classes. There are other classes of thermonuclear explosions, such as the bright Ia’s that show features due to circumstellar interaction in their spectra; the fast declining and faint Ia (also called .Ia); the Ca-rich events; and the SN 2002es-like slowly declining but sub-luminous objects. To understand the nature of the explosion of these events, and to understand the diversity, various progenitor scenarios are proposed for SNe Ia - the exploding white dwarf (WD) can have a non-degenerate star (single degenerate, SD) or another white dwarf (double degenerate, DD) as its binary companion. In the SD scenario, the white dwarf can accrete matter from a red-giant, sub-giant/main- sequence or a He star. A range of explosion models is proposed to explain the observed diversity in SNe Ia. The three-dimensional subsonic pure deflagration in a Mch mass white dwarf (WD) produces explosions that explain SNe Iax. The subsonic deflagration turns to a supersonic detonation due to turbulence called deflagration-to-detonation transition (DDT). The DDT models are capable of explaining normal SNe Ia. Double detonation in a sub-Mch WD has been proposed to explain subluminous SNe like the Ca-rich Ia, faint .Ia, and SN 2002es-like events. Detonation due to the merger of WDs can explain normal Ia, sub-luminous events, and bright events having super-Mch ejected mass. This thesis aims to understand the observed diversity/homogeneity of thermonuclear supernovae in terms of the various explosion mechanisms and progenitors. We have performed long-term monitoring of different kinds of thermonuclear explosions using the 2.0 m Himalayan Chandra Telescope (HCT), 3.6 m Devasthal Optical Telescope (DOT), 0.7 m GROWTH-India Telescope (GIT) in optical wavelengths. We have also used archival UV-optical data from the SWIFT observatory and public data in optical from the Zwicky Transient Facility (ZTF). Analytical modelling of light curves of individual supernovae have been used to estimate the explosion parameters like 56Ni mass, kinetic energy (Ek), ejecta mass (Mej). Numerical models have been generated with publicly available radiative transfer codes like TARDIS, and SEDONA to understand the density profile of the ejecta, abundances of elements in the ejecta of the supernovae. This has been compared with various explosion mechanisms and progenitor scenarios.