Nucleosynthesis in accretion disks around black holes

Sammanfattning: Nucleosynthesis is the mechanism which produces new elements in nuclear reactions. Nuclear reaction rates are highly temperature dependant, and nuclear reactions take place in very hot environments. Current theories predict that the light elements such as hydrogen and helium were produced during the Big Bang. On the other hand, the core of stars produce heavier elements through nuclear fusion. These new elements are then released to the interstellar medium through stellar winds, enriching the gas which will form later generations of stars. Black hole accretion disks also can contain material of high temperatures generated by high accretion rates, allowing nuclear fusion to take place. Nucleosynthesis products can be expelled in winds driven by a super-Eddington accretion, and enrich the interstellar medium. I wrote a computer program 1 integrating a nuclear burning network in a black hole accretion disk for various ranges of black hole mass and accretion rates. I found that the accretion rate needed for nucleosynthesis to take place increases with the black hole mass. The highest temperatures are located in the inner disk, and the black hole event horizon increases linearly with its mass, preventing the disk from attaining high temperatures. For a stellar mass black hole, highly super-Eddington accretion rates allow nuclear burning and powerful winds. Such accretion rates can be supplied by unstable mass transfer during the disruption of a white dwarf. The alpha chain reactions, involving captures of helium nuclei, structure the disk composition radially, with isotope abundances dominating at specific radii. Assuming a given fraction of the disk material is expelled in winds due to Super-Eddington accretion, and knowing the rate at which such events happen in the Galaxy allowed me to compute upper limits of the contribution of accretion disks to the interstellar medium enrichment. Comparing this production to combined stellar yields from stars, I find that black hole–white dwarf accretion disks produce at most 10 −4 times the amount of the same elements that stars produce. This result shows that such a small contribution can be neglected to the overall content of the Galaxy. But the nucleosynthesis involved in general may perhaps play a role in observing these systems, for example a light curve emitted by radioactive elements produced in these short-lived black hole accretion disks.