Date: 27.05.2025

Efstathia Natalia Rektsini (IAP,FR)

Entering the new era of microlensing observations with high angular resolution techniques

Abstract:
Gravitational microlensing has proven to be a very useful technique to explore the populations of bound planets, free-floating planets and faint or dark stellar remnants and their companions. The detection depends on the chance alignment between two distant stars and the observer. Through the years our modelling and observing skills have improved significantly offering a constantly expanding number of new discoveries. Since microlensing is probing a parameter space that is not accessible to any other exoplanet detection technique, these discoveries are fundamental for our understanding of planet formation and demographics. For this reason, microlensing has been attributed a dedicated survey with the Nancy Grace Roman Space Telescope, a NASA mission to be launched at the end of next year. Roman is expected to detect more than 1400 bound planets and hundred FFP candidates via microlensing in addition to more than 200.000 transits, promising a real breakthrough in our understanding of our Galaxy’s planetary population. But observing the microlensing event alone cannot guarantee a precise mass-measurement of the planet and its host star. Furthermore, sometimes light curve fitting models conclude in degenerate solutions and as a consequence, the detections cannot be used as constraints for the construction of exoplanet mass-functions. Thankfully, there are methods to face these issues. Simultaneous observations from ground and space, interferometric microlensing and detection of the finite source effects are some of them. Here I will focus on another method, the use of high-angular resolution observations conducted in the decade that follows the microlensing event. By resolving the source and lens we can derive their relative proper motion and fluxes, which creates additional constraints to the lens mass and distance measurements. I will introduce the mass and distance relations and their connection to the high angular resolution images, present examples of constrained events that used Keck and/or HST follow-up observations and finally, I will provide an update on the Euclid observations of the upcoming Roman fields, conducted in March 2025.