טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentLevanon Naveh
SubjectEvolution, Merging and Explosion of Degenerate Stars as
Type Ia Supernovae
DepartmentDepartment of Physics
Supervisor Professor Noam Soker
Full Thesis textFull thesis text - English Version


Abstract

Type Ia Supernovae (SNe Ia) are the highly energetic explosions of compact and inert stars called white dwarfs (WDs). A WD explodes due to interaction with a binary star, but the nature of this binary companion is an open question. The WD is originally made up of carbon and oxygen, and it explodes due to a thermonuclear detonation that burns these elements into heavier elements. The burning releases enough energy to send the WD material flying in all directions at velocities of ≈10000 km s-1 and above. The WD is destroyed completely and its material becomes SN ejecta. A major product of the nuclear burning is radioactive 56Ni. The decay of 56Ni powers the SN Ia light curves that we observe. Such light curves typically rise for about 18 days, outshining entire galaxies at their peak, and then fade away over several months. In this thesis I explore specific signatures in SN Ia light curves that might constrain the possible systems that explode as SNe Ia. One such signature is the behavior of the light curve in the first few days after explosion, where in some SNe Ia we observe a unique rise. This early emission excess indicates there is an additional source of radiation in these first few days.

One possible explanation is that the SN ejecta collides with the companion star and the shock energy from this collision provides the excess light. This means the companion star is a Sun-like star, which is incompatible with the absence of hydrogen in SNe Ia observations. I offer an alternative where the ejecta collides with material blown away from the merger of two WDs. This means the companion is another WD and also sets a constraint on the time of the explosion relative to the merger.

Another signature I study is the behavior of the late light curve. The decline of the light curve is encoded in a parameter named the gamma-ray escape time scale, which measures the typical time at which gamma-rays from nuclear decay can escape the SN ejecta rather than deposit their energy in it to power the light curve. The values of the gamma-ray escape time scale are set by the total ejecta mass, and of the density profile and 56Ni distribution within it. The mass of the ejecta is important as different scenarios require the total mass to be either near the critical WD mass called the Chandrasekhar mass or below it. In my research I find both Chandrasekhar mass and sub-Chandrasekhar mass explosions can explain the observed range of gamma-ray escape times, and quantify how asymmetry in the 56Ni distribution affects these times. The emerging picture from the work in this thesis is of WD mergers being central to explain both early and late properties of SN Ia light curves, while leaving room for other scenarios so that more than one type of system can explode as a SN Ia.