Microlens effect
The micro-lens effect ( English micro lensing ) referred to in the astronomy the case of gravitational lenses , where the distance between the various generated by the gravitational lens images of the background object is so small that it of today's telescopes can not be observed separately and also the deflection of light can not be measured can.
The effect of the gravitational lens is then shown by the fact that the total light of the unresolved images of the background object appears brighter than it would be without the lens. Such a gain would not yet be easily recognizable as the actual brightness and distance of the background object are normally not known. If, however, the lens and background object move very close to one another in the sky, then the brightness increases and decreases again in a characteristic manner during such a microlens event, while the area of high gain given by the Einstein radius is traversed.
Microlenses in the Milky Way and nearby galaxies
In the best-studied case of the microlens effect , background stars are observed in the bulge of the Milky Way or in the Magellanic Clouds . If an object crosses from planets to stellar mass in front of such a star, the background star is noticeably amplified as long as it is close to or within the Einstein radius. For stellar masses and distances of several kilo parsecs, this radius is on the order of a thousandth of an arc second . Depending on the mass of the lens object and the relative movement of the lens and background object, the microlens event lasts from a few days to months. Since the lens effect only depends on the mass, it also occurs for weak lenses such as old white dwarfs , neutron stars and brown dwarfs , which are too weak for direct observation at great distances.
The probability that a certain background star will show such an event at a certain point in time is very low, on the order of 10 −6 . So millions of background stars have to be monitored simultaneously to find microlens events. However, among these stars there are also about 1% variable stars of various types, from which the microlens events must be distinguished. Criteria for this are the shape and symmetry of the increase and decrease in brightness , the uniqueness of the microlens event in contrast to the repetition of many changes in the brightness of stars and the fact that gravitational lenses amplify equally for all wavelengths , while internal changes in brightness of stars are often associated with color changes.
Microlens events can also be observed towards more distant local group galaxies such as the Andromeda Nebula. Because of the greater distance, they appear as an increase in brightness of a picture element (' pixel ') in which the light of the actually amplified star is mixed with that of other stars.
Search for the constituents of dark matter
The first search for the microlens effect emerged as a result of a work by Bohdan Paczyński from 1986. If a large part of the dark matter in the halo of our Milky Way consisted of weakly or non-luminous objects with a mass of planets or stars, such massive astronomical compacts could be Halo Objects ( MACHO S) can be discovered by searching for microlens events towards background stars in the Magellanic Clouds . Several search programs such as OGLE discovered the first events around 1993. However, they can essentially be explained by known star populations ; there are no indications that a predominant part of dark matter consists of MACHOS. A previously popular model, in which the dark matter consists of objects with only 10 −6 to 10 −2 solar masses , similar to planets, could thus be ruled out, since then many more short microlens events should occur.
Search for exoplanets
When a star causing a microlens event has a planet and that exoplanet wanders very close to the line of sight to the background star, the light curve of the event is modified. In addition to the slow increase and decrease in brightness caused by the lens effect of the foreground star, there is a brief peak in brightness that lasts only about one day. Such events have actually been observed.
Microlenses at high redshift
Gravitation lenses, in which a galaxy as a lens depicts a compact, distant object such as a quasar , show another form of the microlens effect: the variable microlens effect of the individual stars in this galaxy contributes to the fluctuations in brightness of the images of the background object . Kyongae Chang described this effect as early as 1980 in her dissertation. Macro-gravitational lenses with an additional micro-lens effect through a single star that lies in the line of sight of one of the images of a background object are also called Chang-Refsdal lenses after her and Sjur Refsdal .
literature
- Chris Kitchin: Exoplanets: finding, exploring, and understanding alien worlds Springer 2012, ISBN 978-1-4614-0643-3