When a foreground star passes in front of a background (source) star the observed intensity increases, sometimes by over 100 times. The foreground star is called the lens because its gravitational field bends the light from the source focussing more of it on the observer. Because stars move relative to each other this leads to a bell-shaped light curve with a duration of weeks to months. The PLANET collaboration uses telescopes in Chile, South Africa, Western Australia and Tasmania to monitor the light curve of selected microlensing events more or less continuously every 1-2 hours. If the lensing object has planets there will be short duration anomalies (deviations from the "normal" bell- shape) in the light curve. Detailed mathematical modelling of the light curve can provide information about the mass and orbital radius of the planet.
The principal advantage of microlensing is that it is most sensitive to planets at a few AU from their parent star. Hence, unlike the radial velocity and transit methods of exo-planet searches (which are most sensitive to "hot Jupiters") it probes for planets similar to those in our own solar system. Recently we contributed to the discovery of a Jupiter-mass planet in the OGLE 2005-BLG-071 event. This discovery demonstrated that even Earth-mass planets can be detected by microlensing.
When the lenshas a binary stellar companion the light curve becomes very complex with sudden episodes of very rapid change in apparent brightness. Study of these rapid changes (called "caustic crossings") provides a powerful tool for studying limb darkening in the source star i.e. the distribution of light across the stellar disc. This important property is poorly known for stars other than the sun. Many of the physical parameters of the binary system can also be deduced. During some caustic crossing events we use one of the ESO VLT 10m telescopes to conduct spatially resolved spectroscopy of source stars. This has enabled us to measure the atmospheric structure of stars in the Galactic Bulge - something which has not previously been done for stars other than the sun. The huge magnification seen in some events will make it possible to do medium and sometimes even high resolution spectroscopy of stars in the Bulge with our own spectrograph - making it equivalent, for a few hours, to a 5 or 10 m telescope! For more details of our microlensing observations see the PLANET homepage
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