Cosmic ray acceleration in star forming regions



Cosmic rays are energetic particles (mainly protons) that constantly bombard the Earth’s atmosphere from outer space. They were discovered more than a century ago, but their origin is still debated. This is because cosmic rays are strongly deflected by interstellar magnetic fields, which makes their particle distribution function almost perfectly isotropic. This implies that the observed arrival direction of a cosmic ray particle does not point to the site of its production, i.e. cosmic ray astronomy is not feasible. Luckily, cosmic rays can produce gamma rays in their interactions with interstellar matter. The study of such gamma rays is an indirect way to search for the acceleration sites of these particles.

Gamma-ray observations revealed that cosmic rays fill the entire Galaxy and provide a sizeable fraction of the energy stored in the interstellar medium. It follows that cosmic ray sources must be very powerful. For this reason, the remnants of powerful Galactic supernova explosions have been identified as the most likely candidate to accelerate cosmic rays.

However, a number of recent observations suggest that other classes of objects could compete with supernova remnants. In particular: 1) gamma rays have been detected from the surroundings of clusters of young massive stars, and 2) the chemical composition of cosmic rays suggests that at least a fraction of them should come from the winds of young massive stars. This points towards star forming regions, where young massive stars are formed, as possible sites for cosmic ray acceleration. Remarkably, star forming regions have also been observed in gamma rays in galaxies different than the Milky Way (starbursts galaxies).

The goal of the thesis will be to develop theoretical models describing the acceleration of particles in star forming regions, and use them to interpret the growing number of high-energy observations of such astrophysical objects. The main question to be answered is: do star forming regions provide a major contribution to the observed flux of cosmic rays?

Answering this question is mandatory in order to have a solid understanding of the origin of cosmic rays.


Stefano Gabici






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