The Velocity Distributions Of Runaway Stars In The Galaxy

When a supernova explosion occurs in a binary system, the system may be disrupted, and the surviving star may became a runaway star. The supernova explosions are divided into two subtypes according to the different explosion mechanisms: core collapse supernova explosions and the thermonuclear supernova explosions. Core collapse supernova explosion is the final great mass loss stage of evolution of massive star which initial mass greater than 8M. The newborn compact object(either black hole or neutron star) receives additional anisotropy kick velocity, after core collapse supernova explosion. If the binary system is disrupted by the asymmetric core collapse supernova explosion, the surving star may become runaway star. In this paper we note them as Core Collapse- Runaway Stars(CC-RASs). Carbon oxygen white dwarfs(CO-WDs) in the binary systems accrete materials from their companions. When the mass of accreting CO-WD reaches the Chandrasekhar mass limit, thermonuclear supernova explosion occurs. CO-WD disappears in a short timescale, its companion becomes a runaway star. In this paper we note such runaway stars as Thermonuclear-Runaway Stars(TN-RASs).Using a method of population synthesis, we investigate the runaway stars produced by disrupted binaries via asymmetric core collapse supernova explosions(CC-RASs) and thermonuclear supernova explosions(TN-RASs). We find that the velocities of CC-RASs mainly are in 30 — 100 km s-1. The runaway stars observed in the Galaxy possibly are CC-RASs.Due to different stellar chemical components and structure, TN-RASs also can be divided into hydrogen-rich TN-RASs and helium-rich TN-RASs. The velocities of the former are in about100 — 500 km s-1, while the velocities of the latter are mainly between about 600 and 1100 km s-1. The hypervelocity stars observed in the Galaxy may originate from thermonuclear supernova explosions. US 708 is the fastest unbound star in the Galaxy ejected by a thermonuclear supernova explosion in binary system. US 708 is a helium rich star, if we consider the impact of type Ia supernova on TN-RASs, our results can cover US 708 which is a compact helium star and travelling with a velocity of 1157±53 km s-1in the Galaxy.