Abstract
Abstract We show that because the conditions for producing terrestrial microlens parallax (TPRX; i.e., a nearby disk lens) will also tend to produce a large lens-source relative proper motion (μ rel), source proper motion ( μ S) measurements in general provide a strong test of TPRX signals, which Gould & Yee (2013) showed were an important probe of free-floating planet (FFP) candidates. As a case study, we report a single-lens/single-source microlensing event designated as OGLE-2019-BLG-1058. For this event, the short timescale (∼2.5 days) and very fast μ rel (∼17.6 mas yr−1) suggest that this isolated lens is an FFP candidate located in the disk of our Galaxy. For this event, we find a TPRX signal consistent with a disk FFP, but at low significance. A direct measurement of the μ S shows that the large μ rel is due to an extreme μ S, and thus, the lens is consistent with being a very-low-mass star in the bulge and the TPRX measurement is likely spurious. By contrast, we show how a precise measurement of μ S with the mean properties of the bulge proper motion distribution would have given the opposite result; i.e., provided supporting evidence for an FFP in the disk and the TPRX measurement.
Highlights
Various scenarios predict the existence of free-floating planets
We find that the χ2 improvement comes from the peak part of the light curve covered by KMTC and KMTS. This meets our expectations that two or more observatories located in different places should contribute to the terrestrial microlens parallax (TPRX) detection and that this effect should be most pronounced over the peak of the event
We have demonstrated that a measurement of μS is a powerful test of terrestrial microlens parallax (TPRX) signals
Summary
Various scenarios predict the existence of free-floating planets (hereafter, FFPs). For example, FFPs can be planets that are unbound from their host stars by various dynamical mechanisms such as planet–planet scattering (Rasio & Ford 1996; Weidenschilling & Marzari 1996; Chatterjee et al 2008), ejections from the multiple-star system (Kaib et al 2013) or stellar clusters (Spurzem et al 2009), stellar flyby (Malmberg et al 2011), or post-main-sequence evolution of the host star(s) (Veras et al 2011). Many FFP candidates have bright sources (e.g., see discussion in Ryu et al 2021), so the source proper motion μS can often be directly measured This measurement can be compared to the proper motion distribution of bulge stars to check whether the lens could plausibly be a member of the bulge population given μrel and μS. In the case of their “fast” solution with μrel ∼ 19.6 mas yr−1, the source proper motion measurement gave roughly equal probability to disk lenses as bulge lenses in a Bayesian analysis By contrast, for their “slow” solution with μrel ∼ 11.6 mas yr−1, bulge lenses are strongly favored, as would be expected given the much larger volume for distant lenses and the relative lack of tension with two stars drawn from a distribution with σμ ∼ 3 mas yr−1 in each direction.
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