and Cosmology

2. The Milky Way as a Galaxy
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Fig. 2.31. If a binary star acts as a lens, significantly more complicated
light curves can be generated. In the left-hand panel
tracks are plotted for five different relative motions of a background
source; the dashed curve is the so-called critical curve,
formally defined by det(∂β/∂θ) = 0, and the solid line is the
corresponding image of the critical curve in the source plane,
called a caustic. Light curves corresponding to these five
tracks are plotted in the right-hand panel. If the source crosses
the caustic, the magnification μ becomes very large – formally
infinite if the source was point-like. Since it has a finite extent,
μ has to be finite as well; from the maximum μ during caustic
crossing, the radius of the source can be determined, and
sometimes even the variation of the surface brightness across
the stellar disk, an effect known as limb darkening
explain the deviation of the observed light curve from
a simple model light curve. However, the sampling in
time of this particular light curve is not sufficient to
determine the parameters of the binary system.
By now, detailed light curves with very good time
coverage have been measured, which was made possible
with an alarm system. The data from those groups
searching for microlensing events are analyzed immediately
after observations, and potential candidates for
interesting events are published on the Internet. Other
groups (such as the PLANET collaboration, for example)
then follow-up these systems with very good
time coverage by using several telescopes spread over
a large range in geographical longitude. This makes
around-the-clock observations of the event possible.
Using this method light curves of extremely high quality
have been measured. These groups hope to detect
extra-solar planets by characteristic deviations in these
light curves. Indeed, these microlens observations may
be the most realistic (and cheapest) option for finding
low-mass planets. Other methods for finding extra-solar
planets, such as the search for small periodic changes
of the radial velocity of stars which is caused by the
gravitational pull of their orbiting planet, are mostly
sensitive to high-mass planets. Whereas such surveys
Fig. 2.32. Light curve of an event in which the lens was a binary
star. The MACHO group discovered this “binary event”.
Members of the PLANET collaboration obtained this data using
four different telescopes (in Chile, Tasmania, and South
Africa). The second caustic crossing is highly resolved (displayed
in the small diagram) and allows us to draw conclusions
about the size and the brightness distribution of the source star.
The two curves show the fits of a binary lens to the data