Presentation of EROS



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EROS stands for "Expérience pour la Recherche d'Objets Sombres". Its main objective is the search and the study of dark stellar bodies, so-called "brown dwarfs" or "MACHOs" which belong gravitationally to our Galaxy. This is made possible by their gravitational microlensing effects on the light from stars in the Magellanic Clouds (two dwarf galaxies orbiting our own Milky Way).

Dark matter

Various observations on the dynamics of spiral galaxies (a category to which our Galaxy belongs) indicate that the observed stars, dust and gases cannot be their only constituents. About 80% (or more) of their mass is in fact "hidden", although it can be "seen" indirectly through the way it affects the rotation velocity distribution of the visible components. This dark component seems to form an approximately spherical halo, which extends much further outside the galaxy. Similar conclusions are reached from the study of the dynamics of satellites of our Galaxy, i.e. dwarf galaxies such as the Magellanic Clouds, or distant globular clusters. This hidden matter, the so-called "dark matter", may be more or less exotic and it is actively researched in several experiments.

Brown dwarfs

One the most conservative model for dark matter is to imagine that the dark mass (or at least some of it) is under the form of stellar bodies, with a mass lying below the thermonuclear ignition limit (below about 0.1 solar mass). Such bodies that do not emit light have been called "brown dwarfs". In 1986, B. Paczynski showed that these brown dwarfs, if orbiting in our galaxy's halo, can modify the apparent magnitude of stars from the Magellanic Clouds through gravitational microlensing.

Gravitational lensing and micro-lensing

Einstein demonstrated that light rays are bent in a gravitational field. This was first evidenced in 1919 by observing small displacements in the apparent positions of stars that happened to lie near the Sun during the total solar eclipse of May 29th of that year.

Gravitational lensing

This curvature of light rays is accompanied by a distortion of the image of objects located in the background of the gravitational field source. Moreover, there may frequently be more than one image of a given source. Recently, in the seventies, luminous arcs have been observed in dense regions of the sky, for example near the center of galaxy clusters. These have been interpreted as the images, very distorted, of quasars from the background. This is called gravitational lensing.

Microlensing and brown dwarfs

If the light source and the deflector are both quasi point-like, the two distorted images (the "arc") are also quasi point-like. (This means that their real apparent size is much smaller than the resolving power of the telescope used to observe them.) This effect would thus seem to be undetectable, were it not for an increase in the apparent luminosity of the source. The deflected light rays are in fact concentrated by the massive deflecting body, and thus the source luminosity is magnifified. (This phenomenon is also frequently, and loosely called amplification.) Because the light deflection is independent of its wavelength, the same is true of the magnification. Paczynski showed that one can use this phenomenon to discover dark compact bodies within our galactic halo. As the source star, the deflecting body and the earthly observer all orbit the Galaxy, the magnification is a transient phenomenon. To visualize the phenomenon, click here (in french).
The EROS collaboration (and others, such as for example the MACHO and OGLE groups) has built an apparatus to survey the luminosity of Magellanic Cloud stars to look for such magnifications. In the case of the Large Magellanic Cloud (LMC) and a galactic halo full of solar mass dark objects, one expects about 1.5 magnification per million stars monitored and per year for an ideal, 100% efficient observing program : this is indeed a rare phenomenon ! It can only be detected when the monitored source star and the dark object have an angular separation smaller than a milli-arcsecond (1 mas). The phenomenon duration is called the Einstein timescale, as Einstein was the first one to publish (in 1936) his computation of the magnification during such a lensing event. The timescale depends on the speed, position and mass of the deflector. (In contrast, the maximum magnification is largely independent of these; in the simplest case, it is only a function of the minimum angular distance between the source star and the dark object.) For dark objects of our galactic halo, the Einstein timescale varies between a half-hour to about two months for objects in the mass range between that of the Moon and that of the Sun (7 orders of magnitude in mass, 3.5 in timescale). Assuming all dark objects have similar masses, one may conclude that short duration events will be more frequent than longer ones. To separate a microlensing effect from other transient phenomena encountered in stellar evolution, one uses the already mentionned achromaticity : the phenomenon should be identical in two distinct wavelength ranges.


EROS started in 1990. In its first phase the collaboration developped two complementary programs both operated at the La Silla observatory (ESO). One used a 40 cm telescope equipped with a 3.5 million pixel CCD camera and looked for short timescale microlensing phenomena (one hour to a few days). No candidate were found, in this timescale range. On the other hand, 380 photographic plates were obtained using the 1 meter Schmidt ESO telescope, alternately with a blue and a red filter. These plates have been digitized using the MAMA and then analysed. Two candidates were found in 1993, that could be interpreted as microlensing events, out of about 8 million stars surveyed. (since that time, the second candidate was observed by EROS II to vary again, in 1999, thus effectively removing it from the list of microlensing candidates.)


To continue more economically the same program, and get rid of the difficulties inherent to photographic plates calibration, and increase its sensivity, the collaboration is currently building a new apparatus. It is based on a 1.5m telescope recuperated from french observatories (so called the MARLY telescope). This telescope will be equiped with a new CCD camera and will be guided using another smaller CCD camera. Different components are under construction ; first tests should take place this (1995) spring in the OHP observatory (in France). Installation in ESO in place of the existsing 40cm telescope is scheduled this (1995) summer.

Other fields covered by EROS

The EROS experiment monitors the luminosities of a few million stars as precisely as possible. The different categories of variable stars may be naturally studied in our data. On the other hand, the apparatus is also sensitive to the apparition of new stars and thus well adapted to the search for supernovae (in other sky region than in LMC).

Variable stars

Variable stars in LMC are still not well sampled. Among the astrophysically interesting categories one could mention the Cepheids, RR-Lyrae, eclipsing binaries or red giants. The first are used as "standard candles" for intergalactic distance measurements. EROS has localized abouut 40 eclipsing binaries in LMC. A study of Cepheids we have identified is also in publication.


Studies of suspernovae could in fine lead to a measurement of the density of the Universe. In a first stage, it is also a way of determining the Hubble constant H0. Type Ia supernovae emit in first approximation a constant amount of energy. Measuring their apparent luminosity and their spectra will thus enable to measure H0. Comparing different measurement for nearby and distant supernovae could then lead to a measurement of the variation of H0, linked to the Universe's density. These two fundamental objectives are among those of EROS. To reach them a search for apparitions of supernovae in dense region of the sky is foreseen, and should be possible thanks to the fast sampling of the experiment (the rising time of supernovae is of the order of one week).

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