article source:
http://universeathome.pl/universe/forum_thread.php?id=145
The plot above (credits to Wojciech Gładysz) presents the evolution of a
particular system which emits strong gravitational on the way. The leftmost
column provides the system age. In the centre we have the simplified plots of
the binary with evolutionary phase and mass (in solar units) of the primary (on
the left) and the secondary (on the right). The rightmost column presents the
separation (distance between the stars) in the solar radius units and the
eccentricity, which describes how elliptic is the orbit (0.00 means a circular
orbit). Below you can find the short description of the system's evolution.
The description of a binary evolution leading to the merger of two black holes
within the age of the Universe
The Zero Age Main Sequence (ZAMS) may be perceived as a birth time of a binary.
At this moment the stars start the helium synthesis inside their cores. This
marks also the beginning of the longest phase in their lifespan, the so-called
Main Sequence (MS). The star on the main sequence shines nearly invariably.
The situation changes abruptly when the hydrogen fuel becomes depleted in the
core. The heavier the star the earlier it occurs. For a massive primary the main
sequence ends after a few million years. Afterwards, the core starts to collapse
and heats considerably, whereas the star's envelope swells and becomes even 100
times larger! These very short but extremely dynamic phase is called the
Hertzsprung Gap (HG).
In such a large star it is easy for the outermost layers to become unbound with
the core. Then, the free-floating matter may be caught by the gravitational
field of the secondary and fall on it enlarging its weight. This mass transfer
is known as a Roche lobe overflow (RLOF). Lasting only a few hundred years, it
is strong enough to reverse the mass ratio, i.e., the primary, which was nearly
two times heavier on ZAMS, now is nearly two times lighter.
As the outcome of the RLOF the primary is totally stripped of its outer hydrogen
envelope and becomes a bare helium star of a mass of about 25 solar masses. Such
a star, after a brief episode of strong mass loss due to stellar wind, forms a
black hole (BH) through a direct collapse. 'direct' here means that no
supernova explosion is observed as the gravitational field of the new-formed
object is to large to allow any matter to be ejected from the system.
The secondary, which was lighter on ZAMS, evolves less rapidly. However, after
about 6 million years of evolution, it also ends the hydrogen burning phase (the
main sequence) and commences the expansion. But, in contrast to the RLOF phase
explained above, currently no stable mass transfer is possible. The primary is
unable to accrete all the mass, which is lost by the secondary and the two stars
become engulfed by this matter. We obtain the phase known as the Common Envelope
(CE). Although being extremely short (less than 1000 years), the phase alters
the system significantly. Firstly, the secondary loses the hydrogen envelope and
becomes a helium star more than 20 times heavier than the Sun. As it occurred to
the primary, such a star cannot live long and soon collapses to form a black
hole. Secondly, the common envelope takes away a lot of angular momentum from
the binary, which results in a large orbit shrinkage (from ~2000 solar radii to
~25 solar radii). Finally, all the common envelope matter becomes repelled from
the system.
We reached the double black hole system on a relatively close orbit (~10% of the
distance between the Earth and the Sun!). Such a configuration will allow for
the further tightening of the orbit due to gravitational waves emission (they
take away angular momentum and orbital energy). In the end, after 5.4 billion
years (nearly half the age of the Universe) the merger will occur accompanied
with a strong burst of gravitational emission, which may be observable on the
Earth.
____________
Grzegorz Wiktorowicz
Astronomical Observatory
University of Warsaw
