Planets & Stars

We have used the Giant Metrewave Radio Telescope (GMRT; at a frequency of 150 MHz) to undertake observations of known extrasolar planets but have not been able to detect any emission. We are still able to assign an upper limit that allows us (if we assume the emission mechanism to be similar tot hat of Jupiter) to determine an approximate magnetic field strength of the planet (George & Stevens 2007). A number of other searches have also taken place but these have tended to focus on short-period extrasolar planets, which from simple scaling laws product the most promising targets. In contrast, we have chosen to investigate the longer period system, since it is feasible that planets in closer orbits might experience tidal locking and this reduce any dynamo action. It is work noting that though the GMRT offers the best sensitivity of the current range of detectors (at low frequencies), 150 MHz is far from ideal for this work (the of Jupiter emission extends up to around 40 MHz). We would ideally prefer to observe at lower frequencies. The Low Frequency Array (LOFAR) offers a perfect frequency range (with adequate sensitivity), 20-200MHz, and we anticipate that if extrasolar planets do emit at radio wavelength they will be detectable. In the longer term the SKA will provide more sensitive observations. Understanding the magnetic field of these objects will enable us to determine the internal structure and atmosphere.

At higher masses, brown dwarfs offer a good analogue in terms of the emission mechanism. The emission most likely arises from gryoresonance or coherent processes in the magnetic field of the star. Previous, detections at 8.4 GHz require kilo-Gauss magnetic fields. A large number of objects with maximum field strengths below this value probably exist, particularly lower mass L dwarfs. We have undertaken an 100 hour polarization survey of L dwarf stars with the GMRT at 610/1280 MHz. Observing at 610 MHz allow for magnetic field strengths of approximately 200 G. Radio observations (in particular low frequency Stokes V observations) play an important role in our understanding of activity in low mass stars and brown dwarfs. What is emerging is a very complex picture of both quiescent and bursting emission. On occasions the bursts are periodic and are related to the rotation of the star, while other bursts seem to be more sporadic. The bursts are not visible at all wavelengths either.

We have also used the GMRT (at 610 and 1200 MHz) to look at other types of stars to see if we can detect any non-thermal radio emission. In particular we have looked at the magnetic chemically peculiar star, CU Virginis. This work, along with a few other interesting stars is on going.

Away from radio observations I also use optical time series data. This includes using the Solar Mass Ejection Imager (SMEI) to see if Shedir showed any variability of the observing period of the satellite. Over the 3 years of observations the star appears to not show any significant variability.

I’ve also written an article, based on an undergraduate laboratory run at Birmingham, on how to detect Extrasolar Planets in the Classroom: This paper presents a simple laboratory experiment to show how planets are detected using the transit technique.

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