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).
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.
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.
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.
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
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
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.
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
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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