The retention of black holes in globular clusters

Abstract (Swedish)

We simulate the velocities of formed Black Holes to determine the fraction
of retained Black Holes in 152 Globular Clusters. The model we used for the
escape velocity is from Plummer. The scenarios of the natal kick are Neutronstar like kicks and momentum conserving kicks. We also consider the binary
fraction which affect the retention of Black Holes. A typical Globular Cluster
retains 0 or 102 Black Hole with NS-like kicks or momentum conserving kicks
respectively in the simulation which only consider Black Holes as single stars. If
the binary fraction fb is three quarters, a typical Globular Cluster retains 102.5
Black Holes with two scenarios.

Popular Abstract

The Black Hole are popular objects in astronomy, a lot of scientists want to
study their formation and evolution. Most astronomers agree with that there
is a super massive black hole in the center of our galaxy. Also, some previous
observations show that at least some clusters have black holes. As we all known,
a typical black hole is formed by a core-collapse supernova of a massive star.
The core-collapse supernova is a explosion that blow out material surrounding
the core, so the star always looses the majority of its mass and only leave one
core. The mass of this massive stars has a upper limit, extremely massive stars
will not become black hole but disappear during their evolution. Here comes
a conclusion that the mass of a typical black hole is smaller than stellar black
holes in some clusters or the super massive black hole in the center of our galaxy.
This means, those massive black holes may be produced by the merger of some
smaller black holes.
One possible pathway of formation of the surrounding material of super
massive black holes is the merger of globular clusters that spiralled into the
center. If we want to produced such super massive black holes, the globular
cluster must have some black holes. The core-collapse supernova is a asymmetric
explosion, the force from different directions are not the same. The black hole
may get a natal kick from this explosion. Because of the natal kick, the velocity
of black holes may be enough to escape from the host cluster. The magnitude
of the black hole natal kick is very unclear, unlike neutron stars, which emit
pulses of radio waves as they rotate, black holes can not be observed directly. In
this project, I simulate two kinds of kick in 152 globular clusters(from Harris’
database). If the natal kicks of black holes are exactly the same as neutron
stars, only a handful of black holes can be retained in the cluster. If the natal
kicks of black holes are reduced by the mass relations of BH-NS, hundreds to
thousands of black holes remain.
However, most of massive stars are in binary systems. The companion of
the black holes in binaries also affects the retention of the black hole. The
companion may slow down the black hole after the explosion. If the massive
star interact with the companion before become a black hole, the merger and
common-envelope evolution of binary will affect the velocity of the black hole. I
simulate those globular clusters with three quarters of massive stars in binaries.
Both kinds of kick could let a typical globular cluster retain hundreds of black
holes.
In conclusion, a typical globular cluster can retain hundreds of black holes
in this simulation. The case of the kick is the same as neutron stars should be
ruled out if most massive stars are not in binaries.

• author: Liu, Wuxueman
• supervisor: Ross Church
• organization: Lund Observatory - Has been reorganised; Department of Astronomy and Theoretical Physics - Has been reorganised
• course: ASTM31 20211
• year: 2021
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• publication/series: Lund Observatory Examensarbeten
• report number: 2021-EXA185
• language: English
• id: 9067131
• date added to LUP: 2021-10-25 16:40:15
• date last changed: 2022-07-05 16:49:09