Two Low Mass-Ratio Microlensing Planets and Two Types of Central-Resonant Degeneracy

Abstract

We present observations and analysis of two low planet/host mass-ratio ($q$) microlensing planets discovered in high-magnification events. KMT-2025-BLG-0811Lb has $q\sim 4.5\times 10^{-5}$, and a Bayesian analysis favors a super-Earth/mini-Neptune orbiting an M- or K-dwarf host at a projected separation of $\sim 3$ au. KMT-2025-BLG-0912Lb has $q=2.6\times 10^{-4}$ and likely hosts a super-Earth/mini-Neptune around either a low-mass M dwarf or a brown dwarf at $\sim 1$ au. Even with an observing cadence of $\hat{I}“>30{\mathrm{hr}}^{-1}$ during the planetary signal, KMT-2025-BLG-0811 still exhibits the "central-resonant" degeneracy. Reviewing nine such events, we find that the "central-resonant" degeneracy can be divided into two distinct types that occupy separate regions in the plane of $q$ and normalized source radius ($\ddot{I}$). Type~I events have similar $q$ but substantially different $\ddot{I}$ and are more difficult to resolve from the light curves. For Type~II events, the "resonant" solutions have relatively lower $q$ and larger $\ddot{I}$. Our review provides guidance for searching for the alternative solution once one solution has been identified.

14 pages, 8 figures, 6 tables