Abstract
In recent years free-floating planets (FFPs) have drawn a great interest among astrophysicists. Gravitational microlensing is a unique and exclusive method for their investigation which may allow obtaining precious information about their mass and spatial distribution. The planned Euclid space-based observatory will be able to detect a substantial number of microlensing events caused by FFPs towards the Galactic bulge. Making use of a synthetic population algorithm, we investigate the possibility of detecting finite source effects in simulated microlensing events due to FFPs. We find a significant efficiency for finite source effect detection that turns out to be between 20% and 40% for a FFP power law mass function index in the range [0.9, 1.6]. For many of such events it will also be possible to measure the angular Einstein radius and therefore constrain the lens physical parameters. These kinds of observations will also offer a unique possibility to investigate the photosphere and atmosphere of Galactic bulge stars.
Highlights
Recent years have witnessed a rapid rise in the number of planetary objects discovered in the Milky Way with mass M ≤ 0.01M⊙ that are not bound to a host star [1]
We use a Monte Carlo code to investigate the finite source effects in microlensing events, caused by free-floating planets (FFPs) in the field of the sky towards the Galactic bulge planned to be observed by the Euclid telescope
We assume an observational cadence of 20 min; (2) the source distances DS, based on the bulge spatial distributions, result to be in the range from 7 to 10 kpc; (3) the event impact parameter u0 is assumed to be uniformly distributed in the interval [0, 6.54], not taking into account any selection bias by the experiment [26]; (4) the FFP relative transverse velocity is extracted from a Maxwellian distribution [27, 28]; (5) the lens mass is assumed to follow the mass function defined in [3] with mass function index αPL in the range [0.9, 1.6]
Summary
Recent years have witnessed a rapid rise in the number of planetary objects discovered in the Milky Way with mass M ≤ 0.01M⊙ that are not bound to a host star [1]. The great difference between space-based telescopes like the Euclid and usual ground-based microlensing observations is that the amplification threshold detectable by Euclid telescope is Ath = 1.001 that, in turn, implies that the maximum value of u turns out to be umax = 6.54 (much larger than the corresponding value for ground-based observation umax = 1) In this respect, it is important to emphasize that finite source effects are expected to occur and possibly be observable in a large number of microlensing events due to FFPs. the smaller the lens mass is ( the smaller θE/θ∗), the more likely it is that these events involve source star disk crossing.
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