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Abstract We measure the Einstein radius of the single-lens microlensing event KMT-2022-BLG-2397 to beθE= 24.8 ± 3.6μas, placing it at the upper shore of the Einstein Desert, 9 ≲θE/μas ≲ 25, between free-floating planets (FFPs) and bulge brown dwarfs (BDs). In contrast to the six BD (25 ≲θE≲ 50) events presented by Gould et al. (2022), which all had giant-star source stars, KMT-2022-BLG-2397 has a dwarf-star source, with angular radiusθast∼ 0.9μas. This prompts us to study the relative utility of dwarf and giant sources for characterizing FFPs and BDs from finite-source point-lens (FSPL) microlensing events. We find “dwarfs” (including main-sequence stars and subgiants) are likely to yield twice as manyθEmeasurements for BDs and a comparable (but more difficult to quantify) improvement for FFPs. We show that neither current nor planned experiments will yield complete mass measurements of isolated bulge BDs, nor will any other planned experiment yield as manyθEmeasurements for these objects as the Korea Microlensing Telescope (KMT). Thus, the currently anticipated 10 yr KMT survey will remain the best way to study bulge BDs for several decades to come.
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Free-Floating Planets, the Einstein Desert, and 'OUMUAMUA
We complete the survey for finite-source/point-lens (FSPL) giant-source events in 2016--2019 KMTNet microlensing data. The 30 FSPL events show a clear gap in Einstein radius, $$9 \mu{as} < \theta_{E} < 26 \mu{as}$$, which is consistent with the gap in Einstein timescales near $$t_{E} \sim 0.5$$ days found by Mróz et al. (2017) in an independent sample of point-source/point-lens (PSPL) events. We demonstrate that the two surveys are consistent. We estimate that the 4 events below this gap are due to a power-law distribution of free-floating planet candidates (FFPs) $$dN_{FFP}/d log M = (0.4 \pm 0.2)(M/38~M_{\oplus})^{-p}$$/star, with $$0.9 \lesssim p \lesssim 1.2$$. There are substantially more FFPs than known bound planets, implying that the bound planet power-law index $$\gamma = 0.6$$ is likely shaped by the ejection process at least as much as by formation. The mass density per decade of FFPs in the Solar neighborhood is of the same order as that of `Oumuamua-like objects. In particular, if we assume that `Oumuamua is part of the same process that ejected the FFPs to very wide or unbound orbits, the power-law index is $$p = 0.89 \pm 0.06$$. If the Solar System's endowment of Neptune-mass objects in Neptune-like orbits is typical, which is consistent with the results of \citet{wide-orbit}, then these could account for a substantial fraction of the FFPs in the Neptune-mass range.
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- Award ID(s):
- 2108414
- PAR ID:
- 10543466
- Author(s) / Creator(s):
- ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; more »
- Publisher / Repository:
- The Korean Astronomical Society
- Date Published:
- Journal Name:
- Journal of the Korean Astronomical Society
- ISSN:
- 1225-4614
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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