Production of Neutron Stars (2) ...
Observations concerning a peculiar kind of star called
an OB runaway may shed some light on the ejection of
neutron stars by supernova explosions.
OB Runaways
It has been known for some time that there are
anomalously high space velocity stars that appear to have been ejected from OB stellar
associations (see the right figure).
These stars are called OB runaways. Two mechanisms have been
proposed to account for them. (1) Close gravitational encounters between members of an
OB association may eject a star with high velocity. (2) Many O and B stars are probably
destined to become core-collapse supernovae.
If an OB star is a member of a binary and the other
star becomes a supernova, it may be ejected by the explosion at high velocity.
Binary Stars and Supernova Explosions
Under normal conditions, a core-collapse
supernova explosion in a binary is likely to disrupt the binary.
Prior to the explosion, the stars of the binary are bound in orbit around each other by their
mutual gravitational attraction. But when one of the stars produces a neutron star in
a supernova explosion,
it ejects much
of its mass in the explosion (a neutron star has a maximum theoretical
mass of 2-3 solar masses, but the
star that became a supernova probably had a mass of at least 8-10 solar masses).
The gravitational force between the two stars, which is
proportional to the product of their masses, is suddenly reduced and the system becomes unbound
gravitationally, allowing the neutron star and the other star to fly off separately into
space. The following figure illustrates this scenario.
