Bulk properties and physical characteristics of stripped-envelope supernovae

Abstract

Stripped-envelope supernovae (SE-SNe) are a subset of core-collapse supernovae; the explosive death of a massive star. Their defining characteristic is the lack of promi- nent He and/or H envelope suggesting significant mass loss prior to explosion. Their progenitors may be high mass single stars (> 30 M⊙) or lower mass stars that are stripped via binary interaction. Since their discovery as a separate population in 1983, and until recently, the data on these objects steadily increased. SN discoveries have increased year on year since the early 2000s with the advent of targeted and untargeted surveys looking at the skies for transient objects. As a result, some of these surveys have amassed photometric and spectroscopic data on a large number of SE-SNe. The last few years has seen this data made available, dramatically increasing the number of objects with data. I present an investigation into the bulk properties of SE-SNe, using a large database accumulated from public sources, the Palomar Transient Factory, the Public ESO Spectroscopic Survey of Transient Objects, and my own observations. I begin the investigation by constructing and analysing the largest set of bolometric light curves of SE-SNe to date – 85 objects. The light curves are analysed to derive temporal characteristics and peak luminosity Lp, enabling the construction of a lumi- nosity function. Subsequently, the mass of 56Ni synthesized in the explosion, along with the ratio of ejecta mass to ejecta kinetic energy, are calculated. It is found that broad-lined SNe Ic (SNe Ic-BL) and gamma-ray burst SNe are the most luminous sub- types with a combined median Lp, in erg/s, of log10 (Lp)= 43.00 compared to 42.51 for SNe Ic, 42.50 for SNe Ib, and 42.36 for SNe IIb. It is also found that SNe Ic-BL synthesize approximately twice the amount of 56Ni compared with SNe Ic, Ib, and IIb, with median MNi = 0.34, 0.16, 0.14, and 0.11 M⊙, respectively. SNe Ic-BL, and to a lesser extent SNe Ic, typically rise quicker than SNe Ib/IIb; consequently, their light curves are not as broad. Next I examine the spectroscopic properties of these SNe using analytical methods. For He-rich SNe, the presence of H becomes the focus. The strength, velocity, and ratio between absorption and emission of H are measured, along with additional analysis of He I lines, in order to categorize the SNe. The He-poor SNe are ordered according to the number of absorption features N present in the spectra, which is a measure of the degree of line blending. The kinetic energy per unit mass Ek/Mej is strongly affected by mass at high velocity, and such situations principally occur when the outer density profile of the ejecta is shallow, leading to the blending of lines. Using the results, the existing SE-SN taxonomic scheme is adapted I then present the data and analysis of 19 SE-SNe observed since 2012. These SNe are analysed within the context of the earlier findings in this work, as well as exam- ining the ejecta mass distributions as derived from an analytical light curve model. The results support the assertion that SE-SNe reside in a parameter space which is still under-sampled as approximately 20 – 25 percent of these objects have properties that deviate significantly from that of the bulk population. The statistics of the ejecta mass distributions also provide evidence that these SNe arise from relatively low mass progenitors (< 25 M⊙) as the mean ejecta mass for all SN types is 2 – 4 M⊙. Furthermore, distribution of ejecta mass appears unimodal, which suggests that SE-SNe are preferentially arising from one channel; stars that undergo binary interaction. Understanding SE-SNe is important as their stripped pre-explosion progenitor stars are hot, making them sources of ionizing radiation. Their explosions influence their local environment by injecting energy, both radiative and kinetic, and seeding the ISM with the ashes of nucleosynthesis. Finally, they are a source of neutron stars and stellar mass black holes in the universe, which gives rise to other astrophysical events such as X-ray binaries, pulsars, and strong gravitational wave events.