Production of Neutron Stars (3) ...
However, it is possible for a binary to survive a supernova explosion if it is a close binary
and there is significant mass transfer or sharing of the envelope mass between the two stars
before the supernova explosion.
Then, enough of the envelope could be retained in the explosion
so that the
gravitational force between the two stars remains large enough to keep them bound.
Surviving the Supernova
Calculations show that if less than half of the total mass of the binary is ejected by the
supernova the system remains bound, and further is given a high kick velocity.
If this
happens in an OB association, we may expect that a binary consisting of a neutron star
(or possibly a black hole) and an
OB star is ejected from the association at high velocity. This scenario for a binary
to survive a supernova explosion of one of its stars and receive a large kick velocity is
illustrated in the following figure.
The High-Mass X-Ray Binary HD77581 (Vela X-1)
An observation with the 1.54 meter Danish telescope of the European Southern Observatory
lends support to this idea. The bright B0Ia (that is, spectral class B0, luminosity
class Ia) blue supergiant
star HD77581 is about 6000 light years away
in the southern constellation Vela. It has a neutron star companion called
Vela X-1 that is an X-ray pulsar.
(Pulsars are rapidly spinning neutron stars that will be discussed in the next
module.)
The mass of the blue supergiant is estimated to be about 23.5 solar masses and that of the
neutron star about 1.9 solar masses, with a binary orbital period of 8.9 days.
The X-ray emission is thought to originate
in a strong wind from the supergiant that accretes onto
the neutron star. Systems such as this
in which a massive OB supergiant is in close orbit with a neutron
star (or black hole) and accretion from the supergiant onto the neutron star produces X-rays
are called high mass X-ray binaries (HMXB).
Bow Shock for HD77581
The adjacent image of the region near
HD77581 was taken through an H-alpha filter passing a narrow range of wavelengths, with the
light of the star then subtracted using data taken through a different filter to allow
seeing the surrounding region more clearly.
It indicates
that HD77581 and its companion neutron star are
generating a parabola-shaped
bow shock as they move through the interstellar medium.
(Click on the "Show Labels" button to annotate the image; the spikes around the star position
are
imaging system artifacts.)
A bow shock is an indication that the star is moving faster than the speed of sound in the
interstellar medium. You can simulate formation of such a shock wave by using this
Java applet and setting the speed control to a value greater than one
(implying that the speed of the source is greater than the local speed of sound).
Runaway OB Binary
In fact, the space velocity of HD77581 is very high,
about 90 km/s, which is well above the speed of sound in the region through which the
binary
is moving. When the motion of the binary suggested by the shape of the bow shock is
extrapolated backwards, the path
crosses the outskirts of an OB association called Vel OB1 about 2.5
million years ago. The detailed properties of the supergiant and the neutron star
indicate that the supernova explosion happened also about 2.5 million years ago. These pieces
of
evidence taken together
suggest that the high mass X-ray binary HD77581 is in fact a runaway OB binary
ejected from the OB association Vel OB1 about 2.5 million years ago in a
Type II supernova explosion.
The Ejected Mass
Stellar structure calculations suggest that the star that went supernova in the binary
(progenitor of the neutron star) had an
original mass of 25 solar masses, while its companion (the present blue supergiant) had a mass
of 22.5 solar masses (these are masses
before the mass transfer preceding the supernova explosion). The
present total estimated mass of the HD77581 binary (23.5 + 1.9 = 25.4 solar masses) is more
than half the original mass before the supernova explosion (25 + 22.5 = 47.5 solar masses).
If these mass estimates are correct (all have some uncertainty associated with them),
less than half the total binary mass was ejected in the
supernova explosion. Thus, the theoretical condition for the binary being
able to survive the supernova explosion was apparently satisfied.
