Runaways and hyper-velocity stars

(Napiwotzki)

High mass stars (spectral types O and B) are short-lived and thus normally found close to their birth place in the disk of the Milky Way. However, some of them are found many thousand parsecs away from the disk in the galactic halo. These are called runaway stars. Where do they come from? Two scenarios are known, which can explain how those stars were ejected from the disk.

• The star was initially part of a binary system with an even higher mass companion. When the companion exploded as a supernova the remaining main sequence star received a kick of up to 300 km/s.
• The star "collided" with another star or a binary. "Collision" means a very close fly-by of two stars. The gravitational force of the stars on one another modifies the trajectories. This mechanism is most effective when one or both of the colliding stars are a binary and can lead to ejection velocities of 300 km/s, similar to the supernova mechanism

Although these stars are now far away from their birthplace they are still bound to the Milky Way. This means that after travelling away from the disk for some 50 million years, they will start to "fall back", probably after becoming a white dwarf. Kinematic studies of these stars are a way to trace back their trajectory and identify their birthplace.

However, a small number of stars has measured radial velocities of 500 to 600km/s. They will escape from the Milky Way into intergalactic space and are called hyper-velocity stars. Since the mechanism described above are only able to explain velocities up to 300 km/s it is believed that these stars were ejected after interaction with the super-massive black hole in the centre of the Milky Way (2 million solar masses). The second star of the hyper-velocity class was discovered recently by us. Since the age of the star is much lower than the flight time from the Milky Way, it is more likely that this star was ejected from the Large Magellanic cloud (read the full story). This could point to a previously unknown massive black hole in this companion galaxy of our Milky Way. Projects are under way to verify this hypothesis.