Modelling of the galactic chemical evolution of r-process elements

Wehmeyer, Benjamin. Modelling of the galactic chemical evolution of r-process elements. 2016, Doctoral Thesis, University of Basel, Faculty of Science.


Official URL: http://edoc.unibas.ch/diss/DissB_12839

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Are we made of star dust? This question which already guided me during my
master studies could be a good motivation to justify the efforts taken in this
The Big Bang made space itself and time itself. Also, a lot of energy was
put in the newly formed space. At that time, space was only filled with high
energetic photons, so there was only ”light”. After the inflation phase, parts
of the light transformed to quarks and gluons which formed a plasma. After
10−6 seconds, baryons could form from the quarks and gluons. However,
from that point of time until 10−4 seconds, only matter and anti-matter
pairs formed, which annihilated shortly after their formation. A very tiny
imbalance, an excess of only one matter constituent per 10 billion matteranti
matter pairs permitted that, after 10−4 seconds, matter could become
the dominant species. Since the temperature dropped, the large hadrons
decayed and formed a soup of protons and neutrons. They were evenly
abundant and due to the high temperature, could still transition from one
to the other. In these processes also neutrinos were formed and absorbed.
After one second, neutrons could not transform to protons any more and vice
versa. The neutrinos decoupled and hence stopped interacting with matter.
At that time, the ratio between protons and neutrons was 6:1. Deuterons (2H)
could form, but were almost immediately disintegrated due to high energetic
photons. When a minute had passed, the temperature had become so low,
that not too much of these high energetic photons were around any more. So,
deuterons could not form effectively during that time. Since (free) neutrons
decay very fast, the proton to neutron ratio decreased further to 7:1. From
here, most of the neutrons became bound in a 4He nucleus. Since there is
no stable nucleus with mass number 5 or 8, only traces of 7Li and 3He could
be formed. After five minutes, the density in the Universe had dropped so
far that further nuclear reactions were not possible at that time. From here
on, the primordial nucleosynthesis had come to an end, resulting in a mass
ratio of approx. 75% 1H, and approx. 25% 4He, and traces of 3He and 7Li. Those neutrons
unable to find a reaction partner decayed spontaneously after a couple of
Since only 1H, 4He, and traces of 3He and 7Li were formed, but we observe
and actually consist of heavier elements as carbon and oxygen, and some
heavy elements, we might ask the question:
So, if they were not formed in the beginning, where were they actually formed?
The answer to that is not too easy and still debated. While the situation of
the formation of elements more massive than helium up to iron is relatively
well-known, the situation beyond iron is still under investigation. This thesis
tries to contribute a tiny piece to the big puzzle of solving the riddle where
the heaviest elements were actually formed.
Considering the path from the Big Bang nucleosynthesis up to iron, stars are
the key ingredient. Stars can be seen as giant ”pressure cookers”: They are
formed from clouds of interstellar matter which contracted gravitationally
to form gas balls. Under certain conditions, the inward pull of the selfgravitating
gas is high enough to trigger nuclear fusion reactions in the core
of the gas ball. When this gas ball has started its reaction(s), we call this ball
a ”star”. The mass of a newly born star will (among others) determine the
way how it will burn and ultimately determine its faith. The lowest end of
the mass spectrum of stars (low mass stars, LMS) starts with approx. 0.07 Solar
masses (or the equivalent of approx. 75 Jupiter masses), since at this point, the
requirements to trigger the hydrogen fusion in the core are met. Stars of
this mass only process hydrogen to helium and are unable to perform further
processing, extinguish their nuclear burning and just cool off. In the range
approx. 0.8 Solar masses and approx. 8 Solar masses (intermediate mass stars, IMS,
including our Sun) are also able to burn the produced helium to carbon and
2 Chapter 1
oxygen. After the hydrostatic hydrogen burning (the stage which our Sun
is actually in), these stars start to ”pulsate”, or ignite ”flashes”, and loose
their outer envelopes via stellar winds, forming the beautiful objects called
”planetary nebulae”. When the nuclear ”fuel” is consumed or densities are
not sufficient any more, these stars ultimately end as ”white dwarfs”, being
very hot in the beginning, but radiating their heat away and slowly cool
off until they reach their surroundings temperature. Stars more massive
than approx. 8 Solar masses (high mass / massive stars, HMS) go through all
stellar burning stages including hydrogen-, helium-, carbon-, neon-, oxygen-,
and silicon burning, then collapse under their own weight and die in giant
explosions which are commonly referred to as ”supernovae”. Supernovae
might also be triggered by binary systems of intermediate mass stars when
the conditions are met. However, this mechanism is completely different
than the mechanism of the central collapse of massive stars. What both
”types” of these supernovae have in common is that the progenitor star gets
disrupted (partially leaving a neutron star or black hole behind) and blast
processed elements into the interstellar medium. Stars more massive than
300 Solar masses (the actual limit is depending on many parameters and also
strongly debated) collapse directly to a black hole. Neutron stars (the name
is actually misleading since they are neither purely consisting of neutrons nor
are they ”stars” in the sense that they ”shine” due to nuclear processes) are
very dense and interesting objects which are produced during core collapse
supernovae, and if they are born in a double star system, there is a possibility
that they merge with their companion. These neutron star merger events
are extremely violent processes where the conditions are met for the onset
of mechanisms that are able to synthesize nuclei more massive than iron.
However, since they require two evolved stars being gone through all their
stellar burning stages and having gone supernova, there is one striking open
question about the actual contribution of these systems to the nucleosynthesis
of heavy elements remaining: Since we already observe stars being born
at an early stage of our Galaxy’s history, but neutron star mergers require
1.1 Motivation 3
all prerequisites to be met, which takes lots of time, is the contribution of
neutron star mergers possibly to late? This thesis intends to address a portion
of this open question of the nucleosynthesis of the heaviest elements. In the
following, the (relative or exclusive) nucleosynthesis contribution of neutron
star mergers is tested as well as alternative sites for the formation of the
heaviest elements, with a chemical evolution model of our Galaxy.
This thesis is organized as follows.
• In chapter one, a general introduction and the relevant nuclear/hydro
physics, structure and observational properties of stars and the interstellar
medium is presented. This chapter is loosely oriented on D.
Arnett, ”Supernovae and Nucleosynthesis” (1996), T. Padmanabhan,
”Theoretical Astrophysics, Volume I: Astrophysical Processes” (2000),
T. Padmanabhan, ”Theoretical Astrophysics, Volume II: Stars and Stellar
Systems” (2001), F. Matteucci, ”The chemical evolution of the
Galaxy” (2001), A. Weigert, H. J. Wendker, L. Wisotzki, ”Astronomie
und Astrophysik” (2006), Cowan, Thielemann & Truran, ”The Nuclear
Evolution of the Universe” (in prep.), and the lecture notes of nuclear
astrophysics held by Thomas Rauscher (fall semester 2011 and spring
semester 2012).
• Chapter two consists of a detailed explanation of our chemical evolution
model and an application to the formation scenario of the heaviest
elements. This chapter has been previously published in Monthly Notices
of the Royal Astronomical Society, Volume 452, 1970 ff., with Marco
Pignatari and Friedrich-Karl Thielemann as co-authors.
• Chapter three describes the application of our model to the Draco dwarf
galaxies which orbits the Milky Way.
Advisors:Thielemann, Friedrich-Karl and Matteucci, Francesca
Faculties and Departments:05 Faculty of Science > Departement Physik > Former Organization Units Physics > Theoretische Physik Astrophysik (Thielemann)
UniBasel Contributors:Wehmeyer, Benjamin and Thielemann, Friedrich-Karl
Item Type:Thesis
Thesis Subtype:Doctoral Thesis
Thesis no:12839
Thesis status:Complete
Number of Pages:1 Online-Ressource (xi, 134 Blätter)
Identification Number:
edoc DOI:
Last Modified:05 Dec 2018 05:30
Deposited On:04 Dec 2018 15:10

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