Observational study of Core-Collapse Supernovae
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The advent of dedicated surveys for studying transient events has invoked a great interest in the study of supernovae (SNe). The rate of discovery of SNe has hence gone up ten-fold relinquishing their diversity. SNe not only helps in probing the end stages of stellar evolution but also help in understanding the cosmic enrichment. SNe plays a significant role in driving the chemical and dynamical evolution of galaxies and have been proposed to be major contributors of dust when the Universe was young. SNe were initially classified into just two types: Type Ia and Type II. However, several decades of research have highlighted that their peculiarities can be depicted accurately only with the help of subclasses, namely Ia, Ib, Ic, Ic-BL, IIP, IIL, IIb, IIn and Ibn. The diversity is primarily driven by differences in the spectroscopic signatures of these objects and in some cases, the photometric features. Based on the explosion mechanisms, SNe are broadly classified into Thermo-nuclear supernovae and Core-collapse supernovae (CCSNe). CCSNe result from the gravitational collapse of the core in stars more massive than 8 M⊙. All subclasses of SNe except Type Ia events are a result of core-collapse. The observational differences in the properties of various subclasses of CCSNe (even within a single subclass) is attributed to the mass, metallicity and environment of its progenitor. The temporal (i.e., light curve) and spectral evolution of a SN allow the inference of critical parameters related to the progenitor. This highlights the importance of studying individual SNe events. The aim of the work performed during the course of the thesis is to study the individual SNe in detail, infer explosion parameters and progenitor properties (such as 56Ni-mass, ejecta mass, explosion energy, and progenitor mass, radius, and metallicity) and understand their inherent peculiarities. This thesis consists of 7 chapters. Chapter 1 gives a generic introduction to supernovae, its classification scheme, and the explosion mechanism. Massive stars and their evolutionary cycle is introduced with emphasis on mass loss. Possible progenitors of CCSNe were discussed depending on where they explode on the HR diagram. An overview on our understanding of Type II SNe till date is also presented. Chapter 2 introduces CCD astronomy and describes the several ground and space-based telescopes and instruments used. A detailed description of photometric and spectroscopic data reduction procedures is also given in this chapter. Chapter3 discusses the basic techniques of estimating explosion parameters of Type II SNe. Several ways of estimating 56Ni-mass are indicated. The distances to the supernovae are estimated using the Standard Candle Method (SCM, Hamuy & Pinto, 2002). In addition, the explosion energy, radius of the progenitor, the nickel mass, and the mass ejected during the explosion are estimated using the observed light curves and the spectra (Hamuy, 2003; Elmhamdi et al.,2003b; Litvinova & Nadezhin, 1985). Chapter 4 describes a detailed study on ASASSN-14dq. The peak bolometric luminosity of ASASSN-14dq combined with its steep decline during the plateau classifies it as a luminous, fast-declining Type IIP SN. The host galaxy UGC 11860 of ASASSN-14dq displays a sub-solar oxygen abundance and is coherent with the presence of weak metal features in the SN spectra during the photospheric phase. The shallower absorption of Hα P-Cygni profile in the early phase spectra of ASASSN-14dq compared to other Type IIP SNe, indicating a hydrogen-poor envelope. All the above features label ASASSN-14dq as a transitional event between the Type IIP and IIL SNe. Chapter 5 is based on Type II SN 2016gfy which is slow-declining in comparison to the extensive sample of Type II SNe in Anderson et al. (2014b). A boxy emission profile ofHαis seen in the early phase spectra (∼11 – 25 d) of SN 2016gfy indicating an interaction of the ejecta with a nearby CSM. Numerical modeling of the early phase light curves inferred a presence of 0.15 M⊙ CSM spread to a radius of∼70 AU. The CSM is likely a result of a mass-loss episode 30-80 yrs before the explosion assuming a typical RSG wind speed of 10 km/s. A bump is seen in the late-plateau light curve (∼50–95 d) which is explained as a result of interaction with the swooped up CSM and/or partial mixing of 56Ni in the SN ejecta. The host galaxy NGC 2276 is a starburst with a star formation rate of∼8.5 M⊙ / yr and a sub-solar metallicity. The above is consistent with the spectral evolution of SN 2016gfy which features a metal-poor spectrum in comparison to other Type II SNe and the theoretical models of Dessart et al. (2013). Chapter 6 describes the photometric and spectroscopic evolution of the peculiar Type II SN 2018hna. SN 2018hna resembles SN 1987A but is relatively bluer and brighter in comparison. It is only the 2nd 1987A-like SN in which the early phase cooling envelope emission is seen. Numerical modeling of the light curve dominated by the cooling emission suggested a BSG progenitor for SN 2018hna, similar to that of SN 1987A. The sub-solar metallicity for the host of SN 2018hna is in coherence with the low-metallicity of 1987A-like SNe. Chapter 7 summarises the work during the course of this thesis and highlights the future plan of work.