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

Yuchen Tang, Weicheng Zang, Yoon-Hyun Ryu, Andrzej Udalski, Hongjing Yang, Michael D. Albrow, Sun-Ju Chung, Andrew Gould, Cheongho Han, Kyu-Ha Hwang, Youn Kil Jung, In-Gu Shin, Yossi Shvartzvald, Jennifer C. Yee, Dong-Jin Kim, Chung-Uk Lee, Byeong-Gon Park, Leandro de Almeida, Yunyi Tang, Zhixing Li, Jiyuan Zhang, Hongyu Li, Shude Mao, Qiyue Qian, Dan Maoz, Christian Elias Borges, Fabrício Santos Kalaki, Altair Ramos Gomes Júnior, Wei Zhu, Przemek Mróz, Michał K. Szymański, Jan Skowron, Radosław Poleski, Igor Soszyński, Paweł Pietrukowicz, Szymon Kozłowski, Krzysztof A. Rybicki, Patryk Iwanek, Krzysztof Ulaczyk, Marcin Wrona, Mariusz Gromadzki, Mateusz J. Mróz

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

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 () microlensing planets discovered in high-magnification events. KMT-2025-BLG-0811Lb has , and a Bayesian analysis favors a super-Earth/mini-Neptune orbiting an M- or K-dwarf host at a projected separation of au. KMT-2025-BLG-0912Lb has and likely hosts a super-Earth/mini-Neptune around either a low-mass M dwarf or a brown dwarf at au. Even with an observing cadence of 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 and normalized source radius (). Type~I events have similar but substantially different and are more difficult to resolve from the light curves. For Type~II events, the "resonant" solutions have relatively lower and larger . Our review provides guidance for searching for the alternative solution once one solution has been identified.
Paper Structure (5 sections, 2 figures)

This paper contains 5 sections, 2 figures.

Table of Contents

  1. Introduction
  2. Observations
  3. Survey Observations
  4. Follow-up Observations
  5. Data Reduction and Error Analysis

Figures (2)

  • Figure 1: Light curve of the microlensing event, KMT-2025-BLG-0811, with the 2L1S models (solid black and orange lines), 1L2S model (magenta line), and the underlying 1L1S model (dashed grey line). Different data sets are plotted in different colors. The upper panels show the 8-day time interval around the peak. The lower panels present a close-up of the planetary anomaly and the residuals relative to the 2L1S and 1L2S models.
  • Figure 2: Light curve of the microlensing event, KMT-2025-BLG-0912. The symbols are similar to those in Figure \ref{['fig:250811-lc']}, with the 2L1S models (solid black lines) and the underlying 1L1S model (dashed grey line).