Measuring the stellar atmosphere parameters using follow-up polarimetry microlensing observations

TL;DR

This paper explores how follow-up polarimetry microlensing observations can be used to analyze the atmospheres of distant stars, especially red giant stars, by simulating events and assessing observational capabilities.

Contribution

It introduces a method combining polarimetry and photometry to measure stellar atmosphere parameters using microlensing data, with simulations tailored to VLT and OGLE observations.

Findings

VLT can constrain atmospheres of cool RGB stars

Expected detection of 5-20 polarization microlensing events annually

Method allows measurement of dust opacity and stellar envelope radius

Abstract

We present an analysis of the potential follow-up polarimetry microlensing observation to study the stellar atmospheres of the 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 FOcal Reducer and low dispersion Spectrograph (FORS2) polarimeter at Very Large Telescope (VLT), and then analyze them. We find that the accuracy of the VLT telescope lets us constrain the atmosphere of cool RGB stars. Assuming detection of about $3000$ microlensing events per year by the OGLE telescope, we expect to detect almost $20,~10,~8, $ and $5$ of polarization microlensig events for the four different criteria of being three consecutive polarimetry data points above the baseline with $1\sigma$, $2\sigma$, $3\sigma$, and $4\sigma$, respectively in the polarimetry light curves. We generalize the covariance matrix formulation and present the combination of polarimetry and photometry information that leads us to measure the scattering optical depth of the atmosphere and the inner radius of the stellar envelope of red giant 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.