The effect of dark matter capture on binary stars

Sammanfattning: WIMPs, orWeakly InteractingMassive Particles, are a popular dark matter candidate, but their detection remains elusive. At its core, this project is an effort to bridge the gap between the theory of WIMPs, and astronomical observation. According to the WIMP model, stars in the galaxy travel through a background field of WIMPs that are constantly crossing the star. Some of these WIMPs collide with atomic nuclei inside stars via the weak force interaction. The ones that lose sufficient kinetic energy become bound to the star. Captured WIMPs gradually accumulate inside the core of the star, where they annihilate with each other. This converts the entire WIMP mass into energy. A star that captures a significant number of WIMPs could receive most of its energy from WIMP-WIMP annihilation. This would have dramatic effects in the star’s structure and evolution, greatly prolonging its life. The possibility of directly observing stars powered by WIMP annihilation is the driving force behind this project. In this project I have modelled collisions between WIMPs and atomic nuclei, via the weak interaction. Combined with an N-body simulation, I have been able to model WIMP capture in both single stars and binary systems. This work has led to a number of interesting conclusions: (1) Binary stars, due to their orbital motion, can produce more collisions that result in initially bound orbits. However, the gravitational interaction with two masses quickly scatters nearly all WIMPs out of the system. The few that survive, end up in rosette-shaped orbits that do not pass through either star. As a result, the WIMP capture rate in binaries is essentially zero. (2) These semi-stable rosette orbits mean that stars in a binary are surrounded by WIMP halos, with a much higher density than the background. These halos will be a source of gamma ray radiation as WIMPs collide and annihilate. Unfortunately, the resulting WIMP flux at the Earth is merely 4 × 10−5 photons m−2 yr−1 even in the most optimistic scenario. (3) Lastly, it is highly unlikely that any WIMP-burning star (single or in a binary) can be found anywhere in our galaxy.