Microlensing, which employs the principles of gravitational lensing, is the only current technique able to discover cool low-mass (below 30 Earth mass) exoplanets that lie beyond a few AU from their host. Planet formation theories suggests that low mass planets in this region may not have migrated far since their formation. Therefore, understanding the demographics of these system can allow us to directly test planet formation theories.
Previous evidence from near-IR observations of open clusters and tentative data from ground-based microlensing surveys hint at a potentially large population of planets isolated from any host star (so-called free floating planets). Small planets (eg similar to Earth) could become unbound as a result of ejection during the planet formation stage. On the other hand a large abundance of massive free-floating planets (ie Jupiter-like) would suggest that they must form in isolation from a stellar host. Current data tentatively hints at the latter scenario but a larger dataset is badly needed.
The following projects involve the use of the newly develped Manchester-Besancon Microlensing Simulator (MaBulS), which is available online at www.mabuls.net. The simulator is the most advanced of its kind in the World and the projects below will extend its capabilities to allow its use for real-time modeling of bound exoplanet candidates and for assessing the abundance of free-floating planets.
A) Real time microlensing modeling
Manchester has developed a real-time microlensing simulator (MaBulS - www.mabuls.net) which we would like to extend to provide real-time modelling constraints for ongoing microlensing events. We participate in the MiNDSTEp microlensing follow-up team which performs lucky imaging photometry using the Danish 1.54m Telescope at La Silla, Chile. One available project is to extend MaBulS to provide real-time information on the microlensing lens and source properties based upon the observed location, timescale and brightness of ongoing events. Additional information such as the source angular size and lens-source proper motion can also be modeled and can be used to provide more detailed mass modeling of the lens system. At any given time there are always many ongoing events so the observations need to prioritise the most promising systems. We intend to use the MaBulS real-time system to inform the ongoing observation strategy, leading to improved observing efficiency across all ongoing event. This project will also include opportunities to contribute to the MiNDSTEp observing effort through both remote observing and on-site observing at the Danish Telescope at the ESO La Silla Observatory, which is located at the edge of the Atacama desert in Chile.
B) Precision planet mass measurements from Near-IR observations of Kepler2 microlensing events
The second phase of the NASA Kepler mission (dubbed K2) will be making high time-resolution observations of the Galactic bulge from April-July 2016, and also towards the end of 2016. The principal aim of these observations will be to look for free-floating planets. Several ground-based observatories will also monitor the same events, including the near-infrared VVV survey in which Manchester has strong involvement. The baseline between Kepler and Earth should give rise to observable parallax effects in the microlensing signatures which, coupled with near-IR observations, should allow, for the first time, direct mass measurements of free floating planets. Comparison of the observations with detailed simulations will allow limits to be placed on their abundance. This PhD project involves developing the MaBulS simulator to enable its use for statistics of free-floating planets, and to apply it to the K2 sample to make an assessment of the free-floating planet abundance.