An investigation of spectroscopic diversity in the pre-peak regime of thermonuclear supernovae

Abstract

As the most commonly found transient in the night sky due to their high intrinsic brightness, Type Ia supernovae (SNe Ia) have shown themselves to be powerful cosmological tools due to their standardisable nature. There still exist however many open questions regarding their progenitor channels and explosion mechanisms. Modern day photometric and spectroscopic surveys give us the chance to capture these events closer to the time that light first escapes the ejecta, and with a higher cadence than ever before. Pairing this with radiative transfer codes such as TARDIS, we can explore the properties of the material ejected in these exotic events. The work reported upon here is a culmination of three projects, the first of which focuses upon the spectroscopic modelling of SN 2021rhu. This target resembles the sub-luminous SN Ia SN 1986G and stands as a transitional object between the standardisable 'normal' population and the 91bg-like subclass. With spectroscopic observations commencing some 9 days before those of SN 1986G this data allows for tighter constraints upon the outer ejecta. I find the preferred density profile from the literature modelling work of SN 1986G insufficient in the context SN 2021rhu at the earliest phases. I place lower and upper limits upon the elemental abundances and find SN 2021rhu to be more consistent with models of the Chandrasekhar mass delayed-detonation mechanism than those of the sub-Chandrasekhar mass double-detonation mechanism. The second project focused on a search for, and subsequent analysis of, high-velocity components in the Si II 6355 feature in the pre-peak spectra of SNe Ia from the Zwicky Transient Facility Cosmology Data Release 2. These features appear as secondary absorption components some several thousand km/s above the photosphere. Through the fitting of simulated single- and double-component features, I characterised the detection efficiency of the classification method as a function of spectral signal-to-noise, resolution, and the separation between the components in velocity space. I calculate these features to occur in around three quarters of SN Ia spectra before -11 d, with this fraction dropping to about one third of spectra in the six days before maximum light. In all parameters tested - peak magnitude, lightcurve decline rate, host galaxy mass, host galaxy colour - I observe no statistical difference between the SNe with high-velocity components and those without. This work was followed up with a project to model the evolution of high-velocity Si II 6355 features in six well studied literature objects with simplistic Gaussian density enhancements at high velocities. High-velocity components detached further from the photosphere are found to require more extreme density enhancements, explaining why they appear so rarely in nature. While I find solutions for the six objects for the silicon, these derived density enhancements fail to simultaneously produce the calcium high-velocity components which lie several thousand km/s higher up, leaving the door open for potential density enhancements higher up in the ejecta. While the double-detonation models exhibit bumps in their density profiles, these lie at velocities far too low to produce these high-velocity features, and as such we suggest that neither the delayed-detonation nor the double-detonation models alone are capable of reproducing the high-velocity components we see in nature.