Abstract
The microlensing technique is a unique method to hunt for cold planets over a range of mass and separation, orbiting all varieties of host stars in the disk of our galaxy. It provides precise mass-ratio and projected separations in units of the Einstein ring radius. In order to obtain the physical parameters (mass, distance, orbital separation) of the system, it is necessary to combine the result of light curve modeling with lens mass-distance relations and/or perform a Bayesian analysis with a galactic model. A first mass-distance relation could be obtained from a constraint on the Einstein ring radius if the crossing time of the source over the caustic is measured. It could then be supplemented by secondary constraints such as parallax measurements, ideally by using coinciding ground and space-born observations. These are still subject to degeneracies, like the orbital motion of the lens. A third mass-distance relation can be obtained thanks to constraints on the lens luminosity using high angular resolution observations with 8 m class telescopes or the Hubble Space Telescope. The latter route, although quite inexpensive in telescope time is very effective. If we have to rely heavily on Bayesian analysis and limited constraints on mass-distance relations, the physical parameters are determined to 30–40% typically. In a handful of cases, ground-space parallax is a powerful route to get stronger constraint on masses. High angular resolution observations will be able to constrain the luminosity of the lenses in the majority of the cases, and in favorable circumstances it is possible to derive physical parameters to 10% or better. Moreover, these constraints will be obtained in most of the planets to be discovered by the Euclid and WFIRST satellites. We describe here the state-of-the-art approaches to measure lens masses and distances with an emphasis on high angular resolution observations. We will discuss the challenges, recent results and perspectives.
Highlights
Gravitational microlensing is unique in its sensitivity to exoplanets in the Earth-Saturn mass range beyond the snow line [1,2,3], where the core accretion theory predicts that the most massive planets will form [4,5]
It gives us the opportunity to detect exoplanets orbiting host stars ranging from just over a solar mass down to the brown dwarf regime, and it can find them at distances ranging from a few hundred pc all the way to the central Galactic Bulge
High angular resolution observations have been obtained with KECK, SUBARU, VLT or MAGELLAN on about 40 planetary microlensing events and 15 free-floating planets, while the Hubble
Summary
Gravitational microlensing is unique in its sensitivity to exoplanets in the Earth-Saturn mass range beyond the snow line [1,2,3], where the core accretion theory predicts that the most massive planets will form [4,5]. The number of detected planets is relatively low compared to that discovered by the radial velocity and transit methods, microlensing probes a part of the parameter space (host separation vs planet mass), which is mostly not accessible in the medium term to any other technique (Figure 1). Microlensing complements these detections because it is most sensitive to planets beyond the distance where water ice forms (the snow line), and down to earth mass planets.
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