KMT-2022-BLG-2397: Brown Dwarf at the Upper Shore of the Einstein Desert

We measure the Einstein radius of the single-lens microlensing event KMT-2022-BLG-2397 to be θ E = 24.8 ± 3.6 μ as, placing it at the upper shore of the Einstein Desert, 9 ≲ θ E / μ as ≲ 25, between free-floating planets (FFPs) and bulge brown dwarfs (BDs). In contrast to the six BD (25 ≲ θ E ≲ 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 radius θ ast ∼ 0.9 μ as. 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 many θ E measurements 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 many θ E measurements 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.