Observational studies of low redshift supernovae

Abstract

Supernovae (SNe) are explosive transient events that mark the end stage of stellar evolution. This work presents a study of low redshift, hydrogen deficient (Type I) SNe, with emphasis on SNe of type Ia. The data were primarily obtained from the 2-metre Himalayan Chandra Telescope (HCT). SNe Ia are caused by thermonuclear disruption of accreting White Dwarfs (WDs) that have attained a mass close to the Chandrasekhar limit, rendering them unstable. In general,

SNe Ia follow the width-luminosity relation, making them valuable cosmic standard can- dles. However, the nature of the WD companion, i.e. exact nature of the progenitor, and the

details of the explosion physics remain poorly understood. The two most widely accepted progenitor scenarios include the single degenerate (where the WD has a non-degenerate companion) and double degenerate (involving merger of two WDs). In order to continue using SNe Ia effectively for high precision cosmology, it is essential to understand the explosion mechanism and the nature of the WD companion. The homogeneous nature of SNe Ia as a class of events notwithstanding, a substantial diversity is undeniably present. This diversity can be characterized through variations of light curve width, spectral indicators, bolometric properties and luminosity, and ultimately to different progenitor scenarios and explosion mechanisms. ‘Normal’ Ia events constitute ∼ 70% of all SNe Ia and show minimal scatter in their properties, making them most useful for cosmology. A fraction of SNe Ia, termed as SN 1991T-like events, show slow declining light curves relative to normal events, and are generally overluminous. Another fraction of SNe Ia, the SN 1991bg subclass occupy the other end of the luminosity distribution, with fast declining, narrow light curves, low luminosities and very red intrinsic colours. In

addition, there also exist peculiar SN 2002cx-like events, whose spectra resemble 1991T- like events, but luminosities are low akin to 1991bg-like events. The diversity in SNe Ia,

and in particular the subclass of peculiar events, poses a challenge to theoretical progenitor and explosion models. The normal SNe Ia 2014J, 2014dg and 2011ao are studied in this work. Analytical modelling of their bolometric light curves suggests a total ejected mass of ∼ 1.4 M⊙,

consistent with the Chandrasekhar limit. Spectroscopically, these events occupy the tightly clustered Core Normal (CN) subclass. This is consistent with the notion that normal SNe Ia originate from Chandrasekhar mass WDs, that explains the low scatter in their observed properties.

‘Transitional’ SNe Ia have properties intermediate to normal and extremely fast de- clining, subluminous 1991bg-like events. Transitional events thus signify a link between

normal and subluminous SNe Ia and hold the key to understand the progenitor scenario. Transitional SNe 2015bp, iPTF13ebh and 2003gs are studied in this work. Modelling the bolometric light curves of SNe 2015bp and 2003gs suggests a total ejected mass of ≤ 1 M⊙, suggesting a sub-Chandrasekhar mass WD progenitor. The early nebular spectra (∼ 90 days after B-band maximum) of SNe 2015bp and 2003gs show unusually well developed emission features attributable to [Ni II]. This premature emergence of nebular features also

indicates a small ejecta mass, consistent with the low ejecta mass inferred from the bolo- metric light curves. iPTF13ebh, on the other hand, is consistent with a Chandrasekhar mass

WD progenitor, indicating substantial diversity within the subclass of transitional SNe Ia.

The common properties of SNe Ia as a class of events are explored and various cor- relations are examined using data obtained during the course of this work and publicly

available data from the literature. In particular, the Swift UVOT (uvw1 − V ) colour evolu- tion is related to the decline rate parameter ∆m15(B), and therefore the intrinsic luminosity

for SNe Ia. The relative timing of the peak attained by (uvw1 − V ) colour curve shows a promising correlation with the decline rate parameter. Stripped envelope core collapse SNe (types Ib and Ic) form a relatively rare subclass

of SNe. A study of the type Ib event iPTF13bvn is presented in this work. The photomet- ric and spectroscopic characteristics of iPTF13bvn indicate a small ejecta mass, pointing

towards a low mass progenitor star. Fitting analytic models to the bolometric light curve also yields a small ejecta mass, thus ruling out a single, massive Wolf Rayet star as the progenitor, instead favouring a relatively low mass progenitor in a binary system.

Finally, future prospects in supernova astronomy are discussed in the context of up- coming wide-field, high sensitivity and high cadence surveys, and advanced observational

facilities.