Core-collapse supernovae and their progenitors

Overview & Synopsis

Dedicated SN searches started over 100 years ago. Over that time, astronomers have collected large sets of observational data. They have developed detailed classification and achieved general agreement on the nature of these events. Nevertheless, a lot of questions remain unanswered. In short, most Type II SNe (hydrogen-rich SNe) are terminal explosions of red supergiant (RSG) stars. The main source of knowledge about these objects are the way their luminosity changes with time (`light curves') and how their radiation is distributed in wavelength. One of the widely used methods to extract the information from the observational data is computer modelling. The largest part of our work lays in the numerical simulations. In Chapter 1 (Introduction), we present succinctly the necessary theory which includes stellar evolution, explosion physics and ejecta evolution. We discuss different types of SNe; the modern knowledge on CCSNe and its problems. We discuss stellar evolution theory. We describe the nucleosynthesis that takes place in the cores of massive stars and gives rise to their final chemical stratification. We describe our numerical approach in Chapter 2 (Supernova modelling). It includes three major steps: stellar evolution modelling, explosion of the resulting RSG star, and ejecta evolution modelling. We present modelled structure of RSG stars; these models and computational techniques are similar to models used in subsequent chapters. We then discuss our numerical methods of exploding a SN once its degenerate core starts collapsing. We discuss explosive nucleosynthesis and its impact on the progenitor composition, production of unstable isotopes and the basic physics of radioactive decay. In Chapter 3 (Observational properties), we discuss the photometric and spectral observational properties of core-collapse SNe. We extract statistical properties of the existing sample. Using the presented technique, we performed a detailed study of the well observed object SN 2008bk (Chapter 4). We are able to constrain its progenitor and explosion properties. Our modelling allows us to compare not only the basic properties such as luminosity, but also to analyze in detail the spectral features, such as line identification and morphology. We show that a 12 M⊙ star on the main sequence is a good candidate for the progenitor of SN 2008bk. Also we discuss the asymmetric shape of the Hα line and conclude that it stems from the overlap with the strong Ba II 6596.9 Å line. SN 2008bk, together with about 20 objects, form a subclass of low-luminosity CCSNe Type II. We devoted a particular attention to this class of objects, whose uniformity and observational data allows us to draw important conclusions. In Chapter 5 (Low-luminosity Type II-P SNe), we study the sample of 17 low-luminosity SNe and analyze the possibility that these events are the result of explosions of low- and high-mass progenitors. There is no solid agreement in the astronomical community on the possible progenitors of the low-luminosity explosions of Type II SNe. Our analysis shows that lower-mass massive stars (~12 M⊙) are good candidates for the progenitors of this subclass of SNe. Moreover, our simulations of high-mass stars (25 and 27 M⊙) show that such low brightness of the explosion of such a massive object would have notable observational properties which are not present in the data. In Chapter 6 (Kinetic energy variation), we extend our study further on the whole class of hydrogen-rich core-collapse SN, using more energetic models than in Chapters 4 and 5. We provide evidences that what differentiates the diversity of SNe II is the explosion energy rather than the ejecta mass (or H-rich envelope mass of the progenitor).