Physics – Optics
Scientific paper
Apr 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001eso..pres....9.&link_type=abstract
ESO Press Release, 04/2001
Physics
Optics
Scientific paper
Unique Observations of a Microlensing Event
Summary
Like our Sun, stars are large gaseous spheres. However, while we are able to perceive the Sun's disk, all other stars are so far away that they normally appear as points of light . Only specialized observing techniques, like interferometry [1], are able to "resolve" the images of nearby stars and to show them as extended balls of fire.
But opportunities may sometimes arise that allow amazing observational feats in this field . Indeed, an international team of astronomers [2] has just "resolved" a single, normal star some 25,000 light years away , or about 1.6 billion times more distant than the Sun [3], by taking advantage of a multiple microlensing event .
During such a rare event, the light from the remote star is amplified by the gravity of a faint object that passes in front of it, as seen from the Earth . In fact, this gravitational lens acts as a magnifying glass that focusses different parts of the star's image at different times.
Using the FORS1 multi-mode instrument at the 8.2-m VLT ANTU telescope on Paranal during a microlensing event, the team was able to obtain detailed spectra of the different parts of the remote star. In doing so, they managed to probe its gaseous atmosphere at different depths.
This is the first time that it has been possible to obtain detailed, spatially resolved spectra across the full face of a normal star other than the Sun [4].
PR Photo 16a/01 : The light-curve of Microlensing Event EROS-BLG-2000-5 . PR Photo 16b/01 : The sky area of EROS-BLG-2000-5. PR Photo 16c/01 : A VLT spectrum of EROS-BLG-2000-5. PR Photo 16d/01 : The observed change of the H-alpha line strength of EROS-BLG-2000-5. A many-faceted success story
The following story is about a most unusual astronomical observation and also shows how modern astrophysics works .
It combines the study of stellar atmospheres with the intricate optical effects produced by the gravitational field of a binary star in the Milky Way. The successful outcome was dependent on diligent observers in various regions of the world and ultimately on the critical timing of spectral observations with the ESO Very Large Telescope (VLT) at the Paranal Observatory in Chile.
Thanks to the effective collaboration among the scientists and a certain measure of good luck, unique data were obtained that are now providing fundamental new insights into stellar astrophysics. The face of a star
Distant stars appear as small points of light, even to the largest telescopes on Earth. They are simply too far away to be "resolved" by normal telescopes, and no information can therefore be obtained about what the stellar surfaces look like. This is a fundamental obstacle to the detailed study of stars other than the Sun.
We know, however, that the disk of a star does not present itself as a uniform surface. As is the case of the Sun that exhibits variable structures like sunspots (in particular at the time of the present solar maximum), other stars may also have "star-spots" .
Another general feature of solar and stellar disks is that they appear fainter towards the periphery. This phenomenon is known as "limb darkening" and is actually a matter of the viewing angle. When we look towards the middle of the solar disk, we see into rather deep and hot layers of its atmosphere. Contrarily, when we view the very edge of the solar disk, we only see the upper, cooler and dimmer parts.
Thus, by looking at different areas of its disk, we are able to probe different depths of the solar atmosphere. This in turn permits to determine the structure (temperature, pressure, chemical composition, etc.) of the upper layers of the Sun.
For more distant stars, however, their disks appear much too small for this kind of detailed observation. Despite much instrumental progress, therefore, fundamental observational information about stars is still lacking, especially for stars different from the Sun.
This is one of the main reasons why the astronomers are thrilled by a new series of spectra from the FORS1 multi-mode instrument at the 8.2-m VLT ANTU telescope at Paranal. They "resolve" for the first time the surface of a normal star some 25,000 light-years away.
This amazing observational feat has been possible with some help from a natural "magnifying glass". The road leading to this remarkable result is an instructive and interesting one. Gravitational microlensing
ESO PR Photo 16a/01
ESO PR Photo 16a/01
[Preview - JPEG: 361 x 400 pix - 34k] [Normal - JPEG: 721 x 800 pix - 83k] [Hi-Res - JPEG: 2705 x 3000 pix - 536k]
Caption : Schematic representation of the lightcurve of the EROS-BLG-2000-5 microlensing event. It represents the changing brightness of a background star, as its light is being amplified by a binary gravitational lens that passes the line-of-sight from the Earth to the star. The ordinate indicates the factor by which the intensity increases during the various phases of the lensing event, as compared to the normal brightness of the star. The moment of the second "caustic crossing" is indicated, during which the image of the star is substantially brighter. Spectral observations were made with the VLT at the times indicated by arrows. For details, see the text.
The light from a distant star is affected by the gravity of the objects it passes on its way to us. This effect was predicted by Albert Einstein early last century and observationally confirmed in 1919 when a solar eclipse allowed the study of stars close to the line of sight of the Sun. Accurate positional measurements showed that the light from those remote stars was bent by the Sun's gravitational field.
However, the light may not only be deflected, it can also be amplified . In that case, the massive object works like a giant "magnifying lens" that concentrates the light from the distant source.
Effects of gravitational optics in space were first observed in 1979. When produced by extended, very heavy clusters of galaxies, they may take the form of large, spectacular arcs and well-separated multiple images, cf. ESO PR Photos 46d/98 and 46f/98 . Less massive lenses, however, produce images with extensions that are too small to be distinguished directly.
Such "microlensing" effects occur when a compact body (usually a Milky Way star moving in its galactic orbit) passes almost directly between the observer and a luminous background object (usually also a star). One then sees that the brightness of that object rises and falls as the lens passes across the line-of-sight. The observed light intensity is described by a so-called "light curve", cf. PR Photo 16a/01 . Normally, the lensing object is a faint low-mass star, one of the most common objects in the Milky Way. Microlensing events
ESO PR Photo 16b/01
ESO PR Photo 16b/01
[Preview - JPEG: 346 x 400 pix - 44k] [Normal - JPEG: 692 x 800 pix - 112k] [Hi-Res - JPEG: 2596 x 3000 pix - 584k]
Caption : A photo of the sky area around the microlensing event EROS-BLG-2000-5 (indicated, near the centre) that is described in this Press Release. Technical information about this photo is available below.
In most cases, these low-mass stars are too faint to be directly observed. This is especially so in crowded sky fields in which there are many much brighter stars - including the luminous giant stars that are monitored for microlensing effects. However, the gravity of a low-mass star is strong enough to produce a lensing effect if the geometrical alignment is sufficiently precise. This happens rarely, but by looking at a large number of background stars, it has been possible to detect a fair number of microlensing events during the past few years.
International collaborations like Experience pour la Recherche d'Objets Sombres (EROS) , Optical Gravitational Lensing Experiment (OGLE) and Microlensing Observations in Astrophysics (MOA) scan the skies continuously for such microlensing events which typically last from a few weeks to some months. When a star is found to brighten in a way that looks like what is expected from microlensing, they send electronic alerts to other te
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