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Abstract

The importance of nuclear reactions in low-density astrophysical plasmas with ion temperatures kT>1 MeV has been recognized for more than thirty years. However, the lack of comprehensive data banks of relevant nuclear reactions and the limited computational power did not allow detailed theoretical studies. In this thesis, using the publicly available code TALYS, I have built a large nuclear reaction network relevant for temperatures exceeding 1MeV. It contains about 270 nuclear species and include the calculation of gamma-ray emissivity due to different nuclear reactions. The pi0-mesons production are also included. An approach to calculate the gamma-ray spectra through p + p to pi0 channel for an arbitrary proton distribution is also proposed. The nuclear network is applied to two-temperature accretion disk models the so-called Advection Dominated Accretion Flows (ADAF) and Shapiro-Lightman-Eardly (SLE). The gamma-rays emissivity are calculated for a wide parameter space including initial chemical composition. For a 10 solar mass black hole, both models can produce nuclear gamma-ray lines luminosities as large as L_N ~ 10^{34} erg/s. SLE is not an effective source of pi0 photons, whereas ADAF luminosity can be as large as L_pi ~ 10^{35} erg/s. ADAF regime is hot enough to evaporate neutrons. They can reach the companion star atmosphere and initiate secondary nuclear reactions.