Stars that vibrate
In the same way that geologists study earthquakes to find out more about the core of our planet, astronomers study starquakes. Maarten van Hoven investigated the vibrations of magnetars, neutron stars with an extremely strong magnetic field. He discovered that the standard model used by astronomers is not always correct.
First, what is that standard model?
Artist's impression of a magnetar (source: Wikimedia)
‘It is a collection of assumptions existing in modern-day physics regarding the properties of a neutron star: its mass, size, pressure, temperature and composition. These properties cannot be directly observed, nor can they be simulated in a lab, but on the basis of our current theoretical knowledge, we are able to predict them with a certain degree of detail.’
So what is the problem?
‘Astronomic observations show that a number of different vibrations take place on magnetars. There are relatively slow vibrations (starting at 20 per second) up to very fast vibrations (600 per second or more). The slower vibrations can be simulated with our calculations and we seem to be able to explain them using the ‘standard model’. The faster vibrations, on the other hand, cannot be so easily explained using the computational model thar I have developed on the basis of the standard model.’
How come?
‘There are a number of possible explanations. The standard model might include assumptions that do not match reality. For instance, it may be that the inner core of neutron stars is very different from how we think it is.
Another possibility is that the vibrations observed are caused by something other than the vibrations of the star itself. It is possible that some interesting effects are taking place outside in the magnetic atmosphere of the star, the ‘magnetosphere’.
A final possibility is that our computational model is not developed enough. For instance, we might be representing the structure of the magnetic field of a star in a simpler way than it actually is. If we adapt this aspect, our calculations might lead to different results.
So what are we to think of the results of earlier studies?
‘This field of investigation is only six or seven years old, so it’s still in full development. Slowly but surely astrophysicists are starting to make more realistic computational models. From this perspective you can say that the results of earlier studies are incorrect, or less correct than our results.’
How can these statistical models be made more realistic?
‘In our calculations, we take into account complex physics that had previously been omitted. For instance, we explicitly include the strong magnetic link of a magnetar: if the crust moves, then the fluid core moves, too. This effect was not taken into account in earlier publications, but it turns out now that this makes an essential contribution to the vibrations of magnetars.’
'Previous studies on this subject focused primarily on part of the problem. For instance, the question of how it is possible that the crust of a neutron star vibrates without any surrounding magnetic fields. This led to pioneering results which now form the indispensable building blocks of more realistic computations in further research.’
After your PhD defence, what's next?
‘After my defence I will be doing post-doc research at the University of Tel Aviv. I will spend part of my time there working on the problem of vibrating magnetars. I will also focus on other astrophysical problems, such as the physics of supernovas, shock waves, Gamma Ray Bursts (GRBs), etc.’
PhD Defence
Wednesday 15 February 2012, 11.15 hrs
Title: Seismology of Magnetars
Faculty of Science
Supervisor: Prof. K.H. Kuijken
See also
Study in Leiden
Bachelor 's
Astronomy
Master's
Astronomy
(16 February 2012)