When a star amplifies the light from another star enter general relativity

With astrometric, velocimetric and transit methods already in use, a new and original technique for finding exoplanets emerged. It is based upon an application of Einstein's theory of general relativity, according to which lightrays are bent in the vicinity of very massive objects. This effect was confirmed during the total solar eclipse of 1919, when astronomers accurately measured

2.10 The gravitational microlensing method 35

The principle of gravitational microlensing. The observer on Earth sees the source (distant) star when the lensing (nearer) star passes across the centre of the image. The framed image shows what may be seen with a ground-based telescope. General relativity tells us that light-rays from distant objects are bent as they pass near (very massive) closer objects. The gravitational field of a star or a galaxy may therefore act as a gravitational lens, amplifying the light from some background objects.

2.10 The gravitational microlensing method 35

observatory

positions of distant stars near the Sun's limb, by which the predicted displacement was observed. We can therefore liken a star's or a galaxy's gravitational field to a 'gravitational lens', able to bend light. Such lenses are able to amplify the light from stars behind them. If a star passes exactly in front of a more distant star, the flux from the latter is, at the exact moment of 'eclipse', much amplified by the gravitational lens as it causes the rays to converge in the direction of the observer.

This effect has been used in the systematic search for very faint stars in our galaxy: the famous brown dwarfs, for example. The method somewhat resembles the transit method. A careful survey of a star-field containing thousands or even millions of stars is undertaken; but instead of watching for a decrease in brightness, astronomers seek, on the contrary, temporary increases in brightness as a star, too faint to be seen otherwise, crosses the line of sight. This is the gravitational microlens effect. What will be seen is a light-curve exhibiting a maximum flux for a period which may last from days to months.

The search for exoplanets by this method is a by-product of the search for low-mass stars. If the microlensing star has a planet, the generated light-curve will show an anomaly as the planet passes in front of the more distant star. It is a productive method, as four planets have already been detected in this way - one in 2003, and the others (one of which is the smallest exoplanet found so far, with a mass six times that of Earth) in 2005. The problem with this method, however, is that the phenomenon cannot be predicted, as it requires a particular configuration to occur; and it is not reproducible, as the passage of the star and its exoplanet in front of the distant object is a singular event. The microlensing method therefore involves the systematic observation of an

The light-curve of a distant star, obtained using the gravitational microlensing method. The passage of an Earth-sized planet across the face of a lensing star may disturb the light-curve due to the lensing star - briefly but spectacularly. The gravitational microlensing method may be used to detect Earth-sized planets from the ground. (After a theoretical calculation by Bennett and Rhie, Ap. ]., 472, 660 (1996).)

The light-curve of a distant star, obtained using the gravitational microlensing method. The passage of an Earth-sized planet across the face of a lensing star may disturb the light-curve due to the lensing star - briefly but spectacularly. The gravitational microlensing method may be used to detect Earth-sized planets from the ground. (After a theoretical calculation by Bennett and Rhie, Ap. ]., 472, 660 (1996).)

especially dense field of stars, as is found, for example, towards the centre of our galaxy. There are several studies in progress, including PLANET (Probing Lensing Anomalies NETwork), OGLE (Optical Gravitational Lensing Experiment), and MOA (Microlensing Observations in Astrophysics). To date, few brown dwarfs have been discovered; but perhaps the search for exoplanets may prove more successful. It must also be admitted that although OGLE has detected several exoplanets, most of them have been revealed by the transit method, and not via microlenses!

Was this article helpful?

0 0

Post a comment