KMT-2018-BLG-0748Lb: sub-Saturn microlensing planet orbiting an ultracool host

¹ Department of Physics, Chungbuk National University, Cheongju 28644, Republic of Korea
e-mail: cheongho@astroph.chungbuk.ac.kr
² Korea Astronomy and Space Science Institute, Daejon 34055, Republic of Korea
³ Center for Astrophysics | Harvard & Smithsonian 60 Garden St., Cambridge, MA 02138, USA
⁴ University of Canterbury, Department of Physics and Astronomy, Private Bag 4800, Christchurch 8020, New Zealand
⁵ Korea University of Science and Technology, 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea
⁶ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁷ Department of Astronomy, Ohio State University, 140 W. 18th Ave., Columbus, OH 43210, USA
⁸ Department of Particle Physics and Astrophysics, Weizmann Institute of Science, Rehovot 76100, Israel
⁹ Department of Astronomy and Tsinghua Centre for Astrophysics, Tsinghua University, Beijing 100084, PR China
¹⁰ School of Space Research, Kyung Hee University, Yongin, Kyeonggi 17104, Republic of Korea

Received: 15 April 2020
Accepted: 10 July 2020

Abstract

Aims. We announce the discovery of a microlensing planetary system, in which a sub-Saturn planet is orbiting an ultracool dwarf host.

Methods. We detected the planetary system by analyzing the short-timescale (t_E ~ 4.4 days) lensing event KMT-2018-BLG-0748. The central part of the light curve exhibits asymmetry due to negative deviations in the rising side and positive deviations in the falling side.

Results. We find that the deviations are explained by a binary-lens model with a mass ratio between the lens components of q ~ 2 × 10⁻³. The short event timescale, together with the small angular Einstein radius, θ_E ~ 0.11 mas, indicate that the mass of the planet host is very small. The Bayesian analysis conducted under the assumption that the planet frequency is independent of the host mass indicates that the mass of the planet is M_p = 0.18_−0.10^+0.29 M_J, and the mass of the host, M_h = 0.087_−0.047^+0.138 M_⊙, is near the star–brown dwarf boundary, but the estimated host mass is sensitive to this assumption about the planet hosting probability. High-resolution follow-up observations would lead to revealing the nature of the planet host.