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

TL;DR

This paper reports two low mass-ratio microlensing planets and analyzes the central-resonant degeneracy, revealing two distinct types that depend on the planet-to-host mass ratio and source size, aiding future planet detection efforts.

Contribution

It introduces a classification of the central-resonant degeneracy into two types based on $q$ and $ ho$, providing guidance for resolving degeneracies in microlensing planet detections.

Findings

Two low mass-ratio microlensing planets discovered.

Identification of two types of central-resonant degeneracy.

Guidance for resolving degeneracies in microlensing events.

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 $\Gamma > 30~{\rm hr}^{-1}$ 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 ($\rho$). Type~I events have similar $q$ but substantially different $\rho$ and are more difficult to resolve from the light curves. For Type~II events, the "resonant" solutions have relatively lower $q$ and larger $\rho$. Our review provides guidance for searching for the alternative solution once one solution has been identified.