Measuring stellar atmosphere parameters using follow-up polarimetric microlensing observations

Abstract

ABSTRACT We present an analysis of potential follow-up polarimetric microlensing observations to study the stellar atmospheres of distant stars. First, we produce synthetic microlensing events using the Galactic model, stellar population and interstellar dust toward the Galactic bulge. We simulate the polarization microlensing light curves and pass them through the instrument specifications of the FOcal Reducer and low dispersion Spectrograph (FORS2) polarimeter at the Very Large Telescope (VLT), and then analyse them. We find that the accuracy of the VLT allows us to constrain the atmospheres of cool red giant branch (RGB) stars. Assuming that about 3000 microlensing events are detected per year by the Optical Gravitational Lensing Experiment (OGLE) telescope, we expect to detect almost 20, 10, 8 and 5 polarization microlensing events for the following four criteria: being three consecutive polarimetric data points above the baseline with 1σ, 2σ, 3σ and 4σ, respectively, in the polarimetric light curves. We generalize the covariance matrix formulation and present the combination of polarimetric and icinformation that leads us to measure the scattering optical depth of the atmosphere and the inner radius of the stellar envelope of RGB stars. These two parameters could determine the dust opacity of the atmosphere of cool RGB source stars and the radius where dust can be formed.