Polarization in microlensing towards the Galactic bulge
  Gravitational microlensing, when finite size source effects are relevant,
provides an unique tool for the study of source star stellar atmospheres
through an enhancement of a characteristic polarization signal. This is due to
the differential magnification induced during the crossing of the source star.
In this paper we consider a specific set of reported highly magnified, both
single and binary exoplanetary systems, microlensing events towards the
Galactic bulge and evaluate the expected polarization signal. To this purpose,
we consider several polarization models which apply to different types of
source stars: hot, late type main sequence and cool giants. As a result we
compute the polarization signal P,which goes up to P=0.04% for late type stars
and up to a few percent for cool giants, depending on the underlying physical
polarization processes and atmosphere model parameters. Given a I band
magnitude at maximum magnification of about 12, and a typical duration of the
polarization signal up to 1 day, we conclude that the currently available
technology, in particular the polarimeter in FORS2 on the VLT, potentially may
allow the detection of such signals. This observational programme may take
advantage of the currently available surveys plus follow up strategy already
routinely used for microlensing monitoring towards the Galactic bulge (aimed at
the detection of exoplanets). In particular, this allows one to predict in
advance for which events and at which exact time the observing resources may be
focused to make intensive polarization measurements.
