The Impact of the Free-floating Planet Mass Function on the Event Rate for Accurate Microlensing Parallax Determination: Application to Euclid and Roman Parallax Observation

Abstract

A microlensing event is mainly used to search for free-floating planets (FFPs). To estimate the FFP mass and distance via the microlensing effect, a microlensing parallax is one of the key parameters. A short duration of FFP microlensing makes it difficult to yield a parallax by the observer’s motion at a recognizable level, so the FFP microlensing parallax is expected on the simultaneous observation by multiple telescopes. Here, we approach the FFP detection by considering a variation in the FFP mass functions and the event rate of accurately measured microlensing parallaxes. We used our FFP microlensing simulator assuming a parallax observation between upcoming space-based missions (Euclid and Roman) with full kinematics. As a result, we confirmed that the event rate of accurately measured microlensing parallaxes (i.e., within a factor of 2 uncertainty) does not simply follow the number of FFPs at a given mass but the ratio of the FFP population per star. This is because the population ratio determines the optical depth for a given mass and potential sources. In addition, we found that the probability of the event that can estimate the FFP mass and distance within a factor of 2 is not so high: ∼40% of Earth-mass, ∼16% of Neptune-mass, and ∼4% of Jupiter-mass FFP events under our criteria. The probability can be improved by some technical approach such as using high cadence and observation in parallax of more than two observers.