Tesis doctoral de Fermín Moreno Guzmán
In the middle of the 70s, some authors suggested that accretion of hydrogen and helium rich material by a neutron star in a binary system, where the companion star would have low mass, could explain thermonuclear bursts observed by the first satellites launched to the space in order to analyze the x- ray band of the electromagnetic spectrum. This phenomenon, known as type i x-ray bursts (xrb), is a thermonuclear runaway produced by the themonuclear fusion of the accreted material in degenerated conditions. This kind of explosion from stellar source is the most frequent in our galaxy (and the third, after supernovae and classical novae, in terms of total output energy), because his short recurrence period. Due to the extreme gravitational field of a neutron star it is known that produced nucleosynthesis is not ejected to the interstellar medium but forming part of the neutron star crust. in this work we have tried, first, to study the effects of nuclear uncertainties, related to nuclear reaction rates, in the nucleosynthesis produced during a x-ray burst, and second, to simulate physical properties and associated nucleosynthesis to this kind of events, through hydrodynamical models. in order to analyze the impact of uncertainties of the nuclear reaction rates in the nucleosynthesis, and due to the prohibitive calculation time with an hydrodinamical code, we have used a post-processing code which we have coupled, for a given set of 10 temperatures and densities profiles, an extensive nuclear reactions network formed by 606 isotopes and 3551 nuclear reactions, whose reaction rates have been modified using to alternative methods. In the first one, every rate has been individually modified, multiplying it by 0.1 and 10, calculating the final nucleosynthesis. This way, it is possible to evaluate the impact in final nucleosynthesis individual variations in the nuclear reaction rates. Also we have analyzed the effects in final nucleosynthesis by modifying the energy associated to each reaction (q-value). In the second method, nuclear reaction rates have been modified simultaneously, multiplying each one by a random factor which follow a log-normal distribution with a probability of 95.5% of being in the interval [0.1,10]. In order to analyzed this method from an statistically point of view, the nucelosynthesis has been calculated up to 10,000 times, with a monte carlo code specifically built to this thesis, for different set of random numbers. Obtained results with both methods are coincidents and show that for a network formed by 3,500 reactions approximately, only about 60 reactions have an impact in final yields greater than a factor of 2.Finally, we have used an hydrodynamical code, one-dimensional (spherically symmetric), lagrangian and multi-shell, to which we have coupled a nuclear reactions network formed by 324 isotopes and 1392 reactions with the aim of reproduce physical parameters and nucleosynthesis produced during x-ray bursts. T o do that, we have applied this code to different models, analyzing the effect of spatial resolution in the accreted shell, the metallicity of the transferred material as well as the mass of the neutron star, in the final result. For each model, we have simulated different bursts, with energies, luminosities and recurrence times coincidents with observations and, together with the nucleosynthesis, similar to the results obtained by other authors.
Datos académicos de la tesis doctoral «Accretion onto neutron stars: hydrodynamics and nucleosynthesis«
- Título de la tesis: Accretion onto neutron stars: hydrodynamics and nucleosynthesis
- Autor: Fermín Moreno Guzmán
- Universidad: Politécnica de catalunya
- Fecha de lectura de la tesis: 25/11/2009
Dirección y tribunal
- Director de la tesis
- Jordi José Pont
- Tribunal
- Presidente del tribunal: enrique García-berro montilla
- anuj r. Parikh (vocal)
- alain Coc (vocal)
- margarida Hernanz carbó (vocal)