The microlensing rate and distribution of free-floating planets towards the Galactic bulge

摘要

Context. Ground-based optical microlensing surveys have provided tantalising, if inconclusive, evidence for a significant population of free-floating planets (FFPs). Both ground- and space-based facilities are being used and developed which will be able to probe the distrubution of FFPs with much better sensitivity. It is also vital to develop a high-precision microlensing simulation framework to evaluate the completeness of such surveys. Aims. We present the first signal-to-noise limited calculations of the FFP microlensing rate using the Besan?on Galactic model. The microlensing distribution towards the Galactic centre is simulated for wide-area ground-based optical surveys ( I -band) such as OGLE or MOA, a wide-area ground-based near-infrared survey ( K -band), and a targeted space-based near-infrared survey ( H -band) which could be undertaken with Euclid or WFIRST. Methods. We present a calculation framework for the computation of the optical and near-infrared microlensing rate and optical depth for simulated stellar catalogues which are signal-to-noise limited, and take account of extinction, unresolved stellar background light, and finite source size effects, which can be significant for FFPs. Results. We find that the global ground-based I -band yield over a central 200 deg ~(2) region covering the Galactic centre ranges from 20 Earth-mass FFPs yr ~(-1) up to 3500 yr ~(-1) for Jupiter FFPs in the limit of 100% detection efficiency, and almost an order of magnitude larger for a K -band survey. For ground-based surveys we find that the inclusion of finite source and the unresolved background reveals a mass-dependent variation in the spatial distribution of FFPs. For a targeted space-based H -band covering 2 deg ~(2) , the yield depends on the target field but maximises close to the Galactic centre with around 76 Earth to 1700 Jupiter FFPs per year. For near-IR space-based surveys like Euclid or WFIRST the spatial distribution of FFPs is found to be largely insensitive to the FFP mass scale.