Measuring Polarization in microlensing events

We re-consider the polarization of the star light that may arise during microlensing events due to the high gradient of magnification across the atmosphere of the source star, by exploring the full range of microlensing and stellar physical parameters. Since it is already known that only cool evolved giant stars give rise to the highest polarization signals, we follow the model by Simmons et al. (2002) to compute the polarization as due to the photon scattering on dust grains in the stellar wind. Motivated by the possibility to perform a polarization measurement during an ongoing microlensing event, we consider the recently reported event catalog by the OGLE collaboration covering the 2001-2009 campaigns (OGLE-III events), that makes available the largest and more comprehensive set of single lens microlensing events towards the Galactic bulge. The study of these events, integrated by a Monte Carlo analysis, allows us to estimate the expected polarization profiles and to predict for which source stars and at which time is most convenient to perform a polarization measurement in an ongoing event. We find that about two dozens of OGLE-III events (about 1 percent of the total) have maximum polarization degree in the range 0.1 < P_{rm max} <1 percent, corresponding to source stars with apparent magnitude I < 14.5, being very cool red giants.This signal is measurable by using the FORS2 polarimeter at VLT telescope with about 1 hour integration time.

Exoplanetary searches with gravitational microlensing: polarization issues

There are different methods for finding exoplanets such as radial spectral shifts, astrometrical measurements, transits, timing etc. Gravitational microlensing (including pixel-lensing) is among the most promising techniques with the potentiality of detecting Earth-like planets at distances about a few astronomical units from their host star or near the so-called snow line with a temperature in the range $0-100^0$ C on a solid surface of an exoplanet. We emphasize the importance of polarization measurements which can help to resolve degeneracies in theoretical models. In particular, the polarization angle could give additional information about the relative position of the lens with respect to the source.

Polarization in binary microlensing events

The light received by source stars in microlensing events may be significantly polarized if both an efficient photon scattering mechanism is active in the source stellar atmosphere and a differential magnification is therein induced by the lensing system. The best candidate events for observing polarization are highly magnified events with source stars belonging to the class of cool, giant stars {in which the stellar light is polarized by photon scattering on dust grains contained in their envelopes. The presence in the stellar atmosphere of an internal cavity devoid of dust produces polarization profiles with a two peaks structure. Hence, the time interval between them gives an important observable quantity directly related to the size of the internal cavity and to the model parameters of the lens system.} We show that {during a microlensing event} the expected polarization variability can solve an ambiguity, that arises in some cases, related to the binary or planetary lensing interpretation of the perturbations observed near the maximum of the event light-curve. We consider a specific event case for which the parameter values corresponding to the two solutions are given. Then, assuming a polarization model for the source star, we compute the two expected polarization profiles. The position of the two peaks appearing in the polarization curves and the characteristic time interval between them allow us to distinguish between the binary and planetary lens solutions.