KMT-2024-BLG-0404L: A triple microlensing system consisting of a star, a brown dwarf, and a planet

¹ Department of Physics, Chungbuk National University, Cheongju 28644, Republic of Korea
² Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478 Warszawa, Poland
³ Korea Astronomy and Space Science Institute, Daejon 34055, Republic of Korea
⁴ University of Canterbury, Department of Physics and Astronomy, Private Bag 4800, Christchurch 8020, New Zealand
⁵ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁶ Department of Astronomy, The Ohio State University, 140 W. 18th Ave., Columbus, OH 43210, USA
⁷ Center for Astrophysics | Harvard & Smithsonian 60 Garden St., Cambridge, MA 02138, USA
⁸ Department of Particle Physics and Astrophysics, Weizmann Institute of Science, Rehovot 76100, Israel
⁹ Department of Astronomy, Tsinghua University, Beijing 100084, China
¹⁰ Center for Cosmology and AstroParticle Physics, Ohio State University, 191 West Woodruff Ave., Columbus, OH 43210, USA
¹¹ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry, CV4 7AL, UK
¹² Villanova University, Department of Astrophysics and Planetary Sciences, 800 Lancaster Ave., Villanova, PA 19085, USA

^★ Corresponding author: leecu@kasi.re.kr

Received: 26 October 2024
Accepted: 7 July 2025

Abstract

Aims. We have investigated the lensing event KMT-2024-BLG-0404. The light curve of the event exhibited a complex structure with multiple distinct features, including two prominent caustic spikes, two cusp bumps, and a brief discontinuous feature between the caustic spikes. While a binary-lens model captured the general anomaly pattern, it could not account for a discontinuous anomaly feature between the two caustic spikes.

Methods. To explore the origin of the unexplained feature, we conducted more advanced modeling beyond the standard binary-lens framework. This investigation demonstrated that the previously unexplained anomaly was resolved by introducing an additional lens component with planetary mass.

Results. The estimated masses of the lens components are M_p = 17.3_−8.8^+25.5 M_E for the planet, and M_h,A = 0.090_−0.046^+0.133 M_⊙ and M_h,B = 0.026_−0.013^+0.038 M_⊙ for the binary host stars. Based on these mass estimates, the lens system is identified as a planetary system where a Uranus-mass planet orbits a binary consisting of a late M dwarf and a brown dwarf. The distance to the planetary system is estimated to be D_L = 7.21_−0.97^+0.93 kpc, with an 82% probability that it resides in the Galactic bulge. This discovery represents the ninth planetary system found through microlensing with a planet orbiting a binary host. Notably, it is the first case in which the host consists of both a star and a brown dwarf.