Originally thought of as a constant and unchanging place, the Universe is full of dramas of stars emerging, dying, eating each other, colliding, etc. One of the first transient phenomena noticed were called novae (the name means "new" in Latin). Years later, supernovae were discovered. Despite their names, both novae and supernovae are events in relatively old stars, with supernovae marking the point of stellar death. Known for thousands of years, supernovae and novae remain among the most studied events in our Universe. Supernovae strongly influence the circumstellar medium, enriching it with heavy elements and shocking it, facilitating star formation. Cosmic rays are believed to be accelerated in shocks from supernovae, with small contribution possibly coming from novae. Even though the basic physics of novae is understood, many questions remain unanswered. These include the geometry of the ejecta, why some novae are luminous radio or gamma-ray sources and others are not, what is the ultimate fate of recurrent novae, etc.
Supernova explosions are the primary sources of elements heavier than hydrogen and helium. The elements up to nuclear masses A around 100 can form through successive nuclear fusion in the cores of stars starting with hydrogen. Beyond iron, the fusion becomes endothermic instead of exothermic. In addition, for these nuclear masses the temperatures required to overcome the Coulomb barriers are so high that the nuclei are dissociated into alpha particles and free nucleons. Hence all elements heavier than A around 100 should have formed by some other means. These heavier nuclear species are formed by neutron capture on seed nuclei close to or heavier than iron-group nuclei. Depending on the ratio between neutron-capture timescale and beta-decay timescale, neutron-capture processes are called rapid or slow (r- and s-processes, respectively). The s-process, which occurs near the valley of stable isotopes, terminates at Bi (Z=83), because after Bi there is a gap of four elements with no stable isotopes (Po, At, Rn, Ac) until we come to stable Th. The significant abundance of Th and U in our Universe therefore implies the presence of a robust source of r-process. The astrophysical site of r-process is still under debate. Here we present a study of a candidate site for r-process, neutrino-heated winds from newly-formed strongly magnetized, rapidly rotating neutron stars ("proto-magnetars"). Even though we find such winds are incapable of synthesizing the heaviest r-process elements like U and Th, they produce substantial amounts of weak r-process (38<Z<47) elements. This may lead to a unique imprint of rotation and magnetic fields compared to such yields from otherwise analogous slowly rotating non-magnetized proto-neutron stars.
Novae explosions are not as powerful as those of supernovae, but they occur much more frequently. The standard model of novae assumes a one-stage ejection of mass from the white dwarf following thermonuclear runaway. The discovery by the Fermi space telescope of gamma-rays from classical novae made the existence of shocks in novae outflows evident. The presence of shocks in novae was considered well before the discovery of gamma-ray emission; however, little previous theoretical work acknowledged the overwhelming effect of shocks on observed emission and ejecta geometry. Here we present the calculations of synchrotron radio emission from the shocks as they propagate down the density gradient and peak at the timescale of a few months. The model satisfactory fits observations and has several implications for the physics of novae.
| Identifer | oai:union.ndltd.org:columbia.edu/oai:academiccommons.columbia.edu:10.7916/D8KK9Q25 |
| Date | January 2017 |
| Creators | Vlasov, Andrey Dmitrievich |
| Source Sets | Columbia University |
| Language | English |
| Detected Language | English |
| Type | Theses |
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