Making Hypervelocity Stars

Abstract: In this thesis, I investigate if the binaries that are being tidally disrupted by means of the Hills mechanism, producing hypervelocity stars, could have been transported to the galactic centre by two-body scattering. To survive a scattering event, the binary must be hard, meaning that the binding energy of the binary is higher than the kinetic energy of the collider. If the binary is soft, the binary is disrupted at the scattering event and cannot contribute to the production of hypervelocity stars. From 43 observed hypervelocity stars, I calculate a distribution of the binary separation for each binary prior to disruption. Assuming that the mass of the collider is 1M', the fraction of binaries considered hard in the distribution is calculated for three binary mass ratios, 2:1, 1:1, 1:2, between the ejected and captured star and distances from the galactic centre, 0.1, 0.3, 1 pc. No binaries within 0.1 pc could be considered hard since the limiting binary separation at that distance from the galactic centre is less than the physical size of both binary components. For binaries with a mass ratio of 1:2, scattered 1 pc from the galactic centre, 10^−2 − 10^−1 of all binaries would be considered hard. Equivalently, to sustain the expected disruption rate of 10−5 yr^−1, 100-1000 binaries needs to two-body scatter every 1 Myr. The observations of B-stars on highly eccentric orbits around Sgr A', so called S-stars, indicate that hypervelocity stars might be produced by higher mass binaries, corresponding to a mass ratio of 1:4 for the observed hyper velocity stars. This increase in mass increases the fraction of hard binaries by magnitude of 10. This is still too inefficient to solely sustain the expected disruption rate without depleting the galactic centre of stars within 1 pc. Although, the possibility of a 1:4 mass ratio S-star binary surviving a scattering event is large enough to not rule out as possible origin of individual hypervelocity stars.