KMT-2018-BLG-0029Lb and OGLE-2019-BLG-0960Lb: Mass Measurements for Two Super-Earth Microlensing Planets

TL;DR

This paper presents precise mass measurements for two super-Earth microlensing planets using combined ground and space-based data, resolving previous inconsistencies and discussing future degeneracy-breaking observations.

Contribution

It provides new mass and orbital data for two low-mass-ratio planets and clarifies degeneracy conventions in microlensing modeling.

Findings

Masses of KMT-2018-BLG-0029Lb and OGLE-2019-BLG-0960Lb determined.

Resolved inconsistencies in degeneracy labeling in microlensing models.

Discussed potential for future observations to break modeling degeneracies.

Abstract

KMT-2018-BLG-0029Lb and OGLE-2019-BLG-0960Lb were the lowest mass-ratio microlensing planets at the time of discovery. For both events, microlensing parallax measurements from the Spitzer Space Telescope implied lens systems that were more distant and massive than those inferred from the ground-based parallax. Here, we report on the detection of excess flux aligned to the event locations using Keck Adaptive Optics imaging, which is consistent with the expected brightness of main-sequence hosts under the ground-based parallax, but inconsistent with that predicted by Spitzer. Based on the excess flux, ground-based parallax, and angular Einstein radius, we determine KMT-2018-BLG-0029Lb to be a $4.2\pm0.5 M_\oplus$ planet orbiting a $0.70\pm0.07 M_\odot$ host at a projected separation of $3.1\pm0.3$ au, and OGLE-2019-BLG-0960Lb to be a $2.0\pm0.2 M_\oplus$ planet orbiting a $0.40\pm0.03 M_\odot$ host at a projected separation of $1.7\pm0.1$ au. We report on additional light-curve models for KMT-2018-BLG-0029 under the generalized inner-outer (offset) degeneracy, which were not reported in the original analysis. We point out inconsistencies in the inner/outer labeling of the degenerate models in the lens and source planes, and advocate for the lens-plane convention, which refers to the planet being closer or further to the host star compared to the image it perturbs. Lastly, we discuss the possibility of breaking this degeneracy via ground concurrent observations with the Roman Space Telescope.