Current research interest

The search for extrasolar planets is a major drive for the development of new astronomical instrumentation. One of the important parts of current exoplanet instruments is the very fine control of light. This is achieved by the use of adaptive optics and specialised filters for coronographs. Recently the use of photonics in astronomy has also caught my attention.

Beyond the limit of the sky

Master major research

Future telescope like the European-Extremely Large Telescope(E-ELT), or the Thirty Meter Telescope(TMT) are going to be used for direct detection and characterization of exoplanets. The main limiting factor on these ground based telescopes to detect exoplanets is the atmosphere[1]. The turbulence in the atmosphere creates random phase perturbations in the incoming star light, which limit the resolution of the telescope. Several strategies can be applied to counter the effects of atmospheric turbulence. One of them is Adaptive Optics where the wavefront is actively modulated to improve the optical quality.

An adaptive optics system consists of three elements as shown in Figure 1.1. The incoming light is reflected from a deformable mirror(DM) and the aberrations are partially compensated by the shape of the mirror. The reflected light is split up and part of the light is used for the science instrument and the other part is directed to the wavefront sensor. The wavefront sensor measures the residual wavefront aberrations which are still present after the compensation by the deformable mirror. From the measurements, the optimal mirror shape is determined by the control system. The new shape is fed back to the deformable mirror. This creates a closed loop feedback system that iteratively compensates the wavefront aberrations.

The method of optical differentiation for wavefront measurements is generalized to nonlinear filters. A new hybrid filter is created based on this principle, which has a high sensitivity and high dynamic range. To characterize the wavefront sensor numerical simulations are done, and a prototype is created in the optical lab.

The curious case of HLX-1

Minor master project

HLX-1 is suspected to be an intermediate mass black hole which emits a periodic signal. This periodic signal is of the order of 10^41 ergs/cm2/s in the X-ray regime. These super luminous signals are thought to originate from a companion star which transfers some of its mass to the intermediate mass black hole at pericenter. One possible way to create such a system is through tidal capture. When a star has a close encounter with another body it will feel a gradient in the gravitational field. This gives rise to a tidal field which distorts the star when it passes the pericenter of the orbit. The distortion of the star requires energy which can only come from the orbit. If the star is initially unbound to the other body, the amount of energy it loses could be enough to make it bound.

Full hydrodynamical simulations were carried out with smoothed particle hydrodynamics to investigate the effects of tidal energy dissipation during a close encounter. The found effects of tidal dissipation are larger by orders of magnitude than previous methods and theories predicted. The tidal capture radius thus has been under-estimated. With the calculated capture rates of this work it would be possible to explain the origin of a companion around intermediate mass black holes in young open clusters like in the case of HLX-1 with a tidal capture.