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We measure the Einstein radius of the single-lens microlensing eventKMT-2022-BLG-2397 to be theta_E=24.8 +- 3.6 uas, placing it at the upper shoreof the Einstein Desert, 9 < theta_E / uas < 25, between free-floating planets(FFPs) and bulge brown dwarfs (BDs). In contrast to the six BD (25 < theta_E <50) events presented by Gould+22, which all had giant-star source stars,KMT-2022-BLG-2397 has a dwarf-star source, with angular radius theta_* ~ 0.9uas. This prompts us to study the relative utility of dwarf and giant sourcesfor characterizing FFPs and BDs from finite-source point-lens (FSPL)microlensing events. We find `dwarfs' (including main-sequence stars andsubgiants) are likely to yield twice as many theta_E measurements for BDs and acomparable (but more difficult to quantify) improvement for FFPs. We show thatneither current nor planned experiments will yield complete mass measurementsof isolated bulge BDs, nor will any other planned experiment yield as manytheta_E measurements for these objects as KMT. Thus, the currently anticipated10-year KMT survey will remain the best way to study bulge BDs for severaldecades to come.

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