Hypervelocity stars are a class of stars that travel at such
high velocities that the galaxy’s gravitational field cannot contain them.
These stars eventually leave the galaxy and continue on into intergalactic
space. Many hypervelocity stars have trajectories that start out near the
center of the galaxy and were likely yanked out by interactions with the
supermassive black hole residing there.
US 708 is a hypervelocity star estimated to be travelling at
a velocity of about 1,200 km/s. It is currently the fastest unbound star known
in the galaxy. Basically, the observable motion of a star is comprised of two
components - radial motion and proper motion. Radial motion is motion along the
line of sight and proper motion is motion across the plane of the sky.
Measurements of both the radial and proper motions were used
to determine the true velocity of US 708. The radial motion of US 708 was
estimated from measurements of the Doppler shift in the star’s spectral lines
and the measurements show that the star is moving away rapidly with a speed of about
900 km/s. As for the star’s proper motion, it was estimated based on archival
images showing the past positions of US 708.
Spectra of US 708 showing the redshift in the spectral lines
due to the star’s large radial velocity. S. Geier et al. (2015).
Reconstruction of the trajectory of US 708 to trace its
origin show it did not come from anywhere near the center of the galaxy. This
indicates US 708 was not flung out by the galaxy’s supermassive black hole.
Instead, US 708 is believed to have once been part of a tight binary system with
a more massive companion white dwarf star. US 708 is itself a compact sub-dwarf
O-type star, basically the helium core of a former red giant star that was
stripped of its hydrogen envelope due to interactions with its companion.
US 708 was close enough to its companion that its companion
could accrete material from it. As a result, the companion grew in mass until it
reached a critical mass where it exploded in a thermonuclear supernova. This
event is estimated to have occurred about 14 million years ago and it liberated
US 708 from its tight orbit. From there, US 708 shot out in a straight line
with the same high velocity it once orbited its companion and became a
hypervelocity star. The current velocity of US 708 suggests that the tight
binary system it once belonged to had an orbital period of only 10 minutes.
Another indication US 708 was once part of a tight binary
system can be seen from its fast rotation. The projected rotational velocity of
US 708 exceeds ~100 km/s. The fast rotation is due to the fact that in a tight
binary system, the two stars are tidally-locked where the rotational period of
each star is equal to the orbital period. Being so near to a supernova, the
surface of US 708 may be significantly enriched with heavy elements from the supernova.
S. Geier et al. (2015), “The fastest unbound star in our
Galaxy ejected by a thermonuclear supernova”, arXiv:1503.01650 [astro-ph.SR]