Convective/Radiative Stellar Cores
Stars on the main sequence produce their energy by converting hydrogen into helium in their cores. Depending on the star's mass and metallicity, fusion proceeds
either via the pp chain or the CNO cycle. The latter one is very temperature sensitive, therefore the core is very hot but its temperature falls off rapidly:
the core region forms a convection zone that mixes the hydrogen fuel with its ashes, helium. As the occurence of the CNO cycle also depends on the presence of its catalysts carbon, nitrogen and oxygen, their abundances influence the strength of the CNO process and thus the occurrence of convection. In stars like our Sun the energy production proceeds mainly via the pp-chain, the CNO cycle accounts only for roughly 1%. This projects aims at understanding the sensitivity of the possible formation of a convective core, focusing on various metallicities in stars within the range of one solar mass. As part of this work, a set of stellar evolution models for different masses and metallicities will be calculated and evaluated. Please contact Dr. Ilka Petermann (ilka-at-astro.uni-bonn.de) for more information.
Determining the Temperature of Stars through Spectral Synthesis
For one of the experiments of the "Spectroscopy of stars" Physics course, held at AIfA, students use the 50-cm telescope
available on the Institute's roof to acquire spectra of several stars of different
spectral type, from K- up to O-type. The students' project requires the knowledge of the effective temperature of the observed stars, which can be very roughly estimated from the spectral type, but the available spectra would allow one to determine a much more precise value. The goal of the Bachelor thesis project is to determine, with modern spectroscopic tools, the effective temperature of all observed stars. The Bachelor student will learn the basics of stellar atmosphere modelling, spectral synthesis and stellar spectroscopy. The Bachelor student will also learn how to calculate and use state-of-the-art model atmosphere and spectral synthesis codes to derive the effective temperature of stars of various nature. Please contact Dr. Luca Fossati (lfossati-at-astro.uni-bonn.de) for more information.
Rotation Rates of Massive Main Sequence Stars
Massive stars are generally rapid rotators. It is generally assumed that the observed projected rotation rates of main sequence stars (i.e. with
stars performing their core hydrogen burning evolution) reflect their initial rotation rates, based on the short life times of these objectsreflect their initial rotation rates, based on the short life times of these objects. However, observed stars are never truly "just born", i.e., they have a non-zero age. And the rotation rate of a massive star, even on the main sequence, where its structure does not change much, can change as function of time due to various effects, such that any observed distribution of (projected) rotational velocities can deviate from the initial distribution. Such effects are in particular stellar winds, which can drag out stellar angular momentum, winds coupled to a magnetic surface field, and internal angular momentum transport. We can model all these effects using a sophisticated stellar evolution computer code, and thereby constrain the distribution function of initial rotation rates based on observed distributions of projected rotational velocities. This project has a strong relation to large recently finished and ongoing observational projects in which we are involved. Please contact Prof. Langer (nlanger-at-astro.uni-bonn.de) for more information.
Binaries in the FLAMES Survey of Massive Stars
The recently finished FLAMES Survey of Massive Stars allowed for the first time, through the ESO multi-object spectrograph FLAMES on the VLT,
to obtain high quality spectra for a large number of hot massive main sequence (core hydrogen burning) stars. Next to rapidly rotating stars with indications for internal rotationally induced mixing (in particular surface nitrogen enhancements), this survey surprisingly found significant fractions of massive stars with unexpected properties: very slow rotators with chemically enriched surfaces, as well as evolved rapid rotators without signs of internal rotationally induced mixing. Binary evolution models have been suggested as an explanation, an our group has produced some models which could potentially explain such properties. Some of the stars with unexpected properties likely are binaries based on observed radial velocity shifts found in the multi epoch spectra. The task here is to investigate whether the constraints on orbital period and mass ratio derived from the observed radial velocity shifts are consistent, or in contradiction, with the binary evolution scenarios which could explain these observations. Furthermore, it should be tested whether the known binaries amongst the enriched rapid rotators in the FLAMES sample are consistent with the idea of rotational mixing within the single star picture. Please contact Prof. Langer (nlanger-at-astro.uni-bonn.de) for more information.
Stability of Mass Transfer Algorithms in Binary Star Simulations
Most stars more massive than our Sun exist not alone but with a companion star in a close orbit. As they age, these stars grow
larger and may interact by transferring mass between each other. Many important stellar astrophysics phenomena only or mostly occur in such systems, for example X-ray binaries, type Ia supernovae, gamma-ray bursts, cataclysmic variable stars, thermonuclear novae, to name just a few. Our models of mass transfer are mostly based on an explicit solution method which is often numerically unstable. An implicit method is often not practical, because it requires too much computer code to be changed, so this project would build on the explicit method and try to make it more numerically stable so that the rate of mass transfer -- and evolution of the stars -- is calculated more accurately. First it will be shown why and when the explicit method is unstable, then the mass transfer algorithm will be modified to improve stability while retaining accuracy and computational speed (in both the explicit and implicit cases). If successful, the results will be implemented in our binary-star population synthesis
code binary_c/nucsyn. Suggested reading :
Buening and Ritter 2005 . Please contact Prof. Izzard (izzard-at-astro.uni-bonn.de) for more information.
Mixing and Nucleosynthesis in Low-Mass Stars
The final phase of the life of a low-mass star is an important time for
the formation of many different elements. The unusual structure of these
stars
opens up many possible pathways for nucleosynthesis as stellar
material can be exposed to proton-, neutron- and alpha-captures,
via a
complex sequence of convective motions. Predictions of state-of-the-art
calculations fail to reproduce the abundance patterns observed in many
stars (as well as the more precise determinations made in pre-solar
grains obtained from meteorites), with some of the lightest elements
being a particular problem. This suggests that stellar theory has missed
something important: something other than convection is at work in these
objects. In this project, the student will compute detailed
nucleosynthesis models to determine the characteristics of this missing
process. They will investigate how deep material needs to be transport
in order to activate the necessary nuclear reactions, and how fast the
transport process needs to be. Ultimately, these characteristics will be
compared to known non-convective processes with the aim of determining
the physical cause of the mixing. Please contact Dr. Stancliff (rjstancl-at-astro.uni-bonn.de) for more information.
Heavy Element Nucleosynthesis
The production of some of the heavy elements beyond iron occurs by what
is called the slow neutron-capture process (the s-process). For the
s-process to occur
the neutron density needs to be low enough that when a nucleus captures a neutron
-- which most likely forms an unstable
nucleus -- the new isotope has time to decay back to stability before
the next neutron is absorbed. In this way, heavy elements from iron to
lead can be produced. The s-process is observed to happen in the final
phase of the life of low-mass stars, with alpha-captures on to carbon-13
nuclei providing the source of neutrons. However, for this to work one
must first produce a pocket of carbon-13 in the star and at present we
do not know how to do this. In this project, the student will use a
nucleosynthesis code to investigate the nature of the carbon-13 pocket
needed to produce the s-process. They will determine how the size and
shape of the pocket affects the production of the s-process nuclei. This
will then allow us to examine what physical processes might be
responsible for the formation of the carbon-13 pocket. Please contact Dr. Stancliffe (rjstancl-at-astro.uni-bonn.de) for more information.
Age Spreads in Star Clusters Caused by Rejuvenation in Close Binaries
Most stars do not live alone but have a binary companion. Alpha Centauri
for example, the closest star to the Sun,
is a binary star. During their life, stars expand such that stars in a
binary can get into physical
contact during which mass is transferred from one star to the other.
Whenever a star
accretes mass, it rejuvenates, i.e. it will appear younger than other
coeval stars which have not accreted mass.
The mysterious blue-straggler stars are thought to have such an origin.
They are observationally found as a blue
extension of the turn-off point in the Hertzsprung-Russell diagram of a
star cluster. The aim of this project is to investigate how large the age spread in a
population of coeval stars can be because of
mass transfer in binary stars - a topic at the forefront of research.
The student will work with a state-of-the-art
binary population synthesis code to create stellar populations and will
thereby learn about single and binary
star evolution, initial distribution functions of stellar masses and
binary orbits, programming and data analysis.
The student has to adjust the output of the population synthesis code,
set up a grid of stars and evaluate the output.
Please contact Fabian Schneider (fschneid-at-astro.uni-bonn.de) for more
information.
Magnetic Fields in Rotating Stars
Some stars contain magnetic fields which we can observe in their spectra via the Zeeman effect. These fields can be very strong: around 1 tesla, which is around
20,000 times stronger than the Earth's magnetic field. These fields are formed as the star forms; whatever magnetic field is left over from the cloud from which the star formed organizes itself into an equilibrium configuration in which it then remains for the whole of the star's lifetime. Very recently, however, it has been discovered that the other 90% of the stellar population displays very much weaker magnetic fields, about 0.0001 tesla, and that no stars have fields between about 0.0003 and 0.03 tesla. This is something of a puzzle: why should there be this bimodal distribution of magnetic field strengths? Moreover, what is the nature of these very weak fields? There is so far only one hypothesis: that the magnetic fields in these stars somehow failed to find their way into an equilibrium because the fast rotation of the star slowed down this process. There is a terrestrial analogy: the air in the atmosphere "wants" to flow from high pressure to low pressure, but is forced by the Earth's rotation to flow instead in circles, giving rise to cyclones and anticyclones. In this project, the student will perform simulations of the magnetic field evolving towards an equilibrium and will investigate the effect of the star's rotation, with the aim of testing this hypothesis. Further reading:
http://arxiv.org/abs/1201.5646. Please contact Dr. Braithwaite (jonathan-at-astro.uni-bonn.de) for more
information.
Magnetic Oscillations in Stars
Some stars contain magnetic fields which we can observe in the spectrum by the Zeeman effect. These fields can be very strong: around 10kG which is around
20,000 times stronger than the Earth's magnetic field. However, since we only observe the surface of the star, it is difficult to say what the structure of the field should be in the interior. Curiously, a few of these stars appear to display periodic oscillations in their rotation periods, i.e. they are spinning up and slowing down over timescales of order a decade. This phenomenon is likely to be magnetic oscillations, that is, standing magnetic waves permeating the entire star. In this project, the student will perform simulations of a magnetic star undergoing oscillations, investigating what kinds of oscillations are possible, how their frequencies are related, and what the observations can tell us about the geometry of these magnetic fields in the stellar interior. In addition, in the growing field of asteroseismology higher frequency modes are observed in some stars; these oscillations are essentially acoustic in nature but the frequencies are affected by magnetic fields. This project will therefore help asteroseismologists probe the internal structure and rotation of stars. Further reading:
http://arxiv.org/abs/1110.1104/. Please contact Dr. Braithwaite (jonathan-at-astro.uni-bonn.de) for more
information.
Magnetars
Neutron stars with extremely high magnetic fields are called magnetars. They are detected at high energies as so-called soft gamma-ray repeaters (SGRs) or anomalous X-ray pulsars
(AXPs). Occasionally, magnetars undergo very strong bursts which are powered by energy stored in their large magnetic fields. As a consequence of their high magnetic fields, magnetars lose rotational energy on a short timescale (~ few 1000 yr) and hence these objects become slow spinning neutron stars shortly after their formation in a supernova explosion, in contrast to ordinary radio pulsars which are observable on much longer timescales (~ 50 Myr). The aim of this project is to understand the possible link between magnetars and other members of the neutron star population. We perform an up-to-date literature search on the topic and construct simple model calculations of the spin evolution of magnetars. Please contact Dr. Thomas Tauris (tauris-at-astro.uni-bonn.de) for more information.
Formation of Galactic Black-Hole Binaries (Soft X-Ray Transients)
There are about 25 black holes detected in our Galaxy. These black holes are, so far, only found in binaries and mainly with
a low-mass companion star. The black holes are detected from the X-rays emitted by the in-falling gas when the black holes virtually consume their companion star in a close orbit binary system. In this project, we try to understand the formation of these black hole systems (also known as soft X-ray transients) via exotic binary evolution involving mass transfer, common envelope evolution and the dynamical effects of supernova explosions. Please contact Dr. Thomas Tauris (tauris-at-astro.uni-bonn.de) for more information.
Each project lasts about three months.
Other Projects for Students
We also offer a number of research projects for master students. Please visit the websites of the senior members of our group to find out available thesis topics.