Brown dwarf companions in binaries detected in the 2021 season high-cadence microlensing surveys

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 Korea University of Science and Technology (UST), 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea
⁴ Institute of Natural and Mathematical Science, Massey University, Auckland 0745, New Zealand
⁵ Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA
⁶ University of Canterbury, Department of Physics and Astronomy, Private Bag 4800, Christchurch 8020, New Zealand
⁷ Max-Planck-Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁸ Department of Astronomy, Ohio State University, 140 W. 18th Ave., Columbus, OH 43210, USA
⁹ Department of Particle Physics and Astrophysics, Weizmann Institute of Science, Rehovot 76100, Israel
¹⁰ Department of Astronomy, Tsinghua University, Beijing 100084, PR China
¹¹ School of Space Research, Kyung Hee University, Yongin, Kyeonggi 17104, Republic of Korea
¹² Institute for Space-Earth Environmental Research, Nagoya University, Nagoya 464-8601, Japan
¹³ Code 667, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹⁴ Department of Astronomy, University of Maryland, College Park, MD 20742, USA
¹⁵ Komaba Institute for Science, The University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo 153-8902, Japan
¹⁶ Instituto de Astrofísica de Canarias, Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
¹⁷ Department of Earth and Space Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
¹⁸ Institute of Astronomy, Graduate School of Science, The University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan
¹⁹ Department of Physics, The Catholic University of America, Washington, DC 20064, USA
²⁰ Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo, Sagamihara, Kanagawa 252-5210, Japan
²¹ Sorbonne Université, CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98 bis bd Arago, 75014 Paris, France
²² Department of Physics, University of Auckland, Private Bag 92019, Auckland, New Zealand
²³ University of Canterbury Mt. John Observatory, PO Box 56 Lake Tekapo 8770, New Zealand

Received: 26 May 2023
Accepted: 10 July 2023

Abstract

Aims. As a part of the project aiming to build a homogeneous sample of binary-lens (2L1S) events containing brown dwarf (BD) companions, we investigate the 2021 season microlensing data collected by the Korea Microlensing Telescope Network (KMTNet) survey.

Methods. For this purpose, we first identified 2L1S events by conducting systematic analyses of anomalous lensing events. We then selected candidate BD companion events by applying the criterion that the mass ratio of the lens components is lower than q_th ∼ 0.1.

Results. From this procedure, we find four events including KMT-2021-BLG-0588, KMT-2021-BLG-1110, KMT-2021-BLG-1643, and KMT-2021-BLG-1770, for which the estimated mass ratios are q ∼ 0.10, 0.07, 0.08, and 0.15, respectively. Event KMT-2021-BLG-1770 was selected as a candidate even though the mass ratio is slightly greater than q_th because the lens mass expected from the measured short timescale of the event, t_E ∼ 7.6 days, is low. From the Bayesian analyses, we estimate that the primary and companion masses are (M₁/M_⊙,M₂/M_⊙) = (0.54_−0.24^+0.31, 0.053_−0.023^+0.031) for KMT-2021-BLG-0588L, (0.74_−0.35^+0.27, 0.055_−0.026^+0.020) for KMT-2021-BLG-1110L, (0.73_−0.17^+0.24,0.061_−0.014^+0.020) for KMT-2021-BLG-1643L, and (0.13_−0.07^+0.18, 0.020_−0.011^+0.028) for KMT-2021-BLG-1770L. It is estimated that the probabilities that the lens companions are in the BD mass range are 82%, 85%, 91%, and 59% for the individual events. To confirm the BD nature of the lens companions found in this and previous works by directly imaging the lenses from future high-resolution adaptive-optics (AO) followup observations, we provide the lens-source separations expected in 2030, which is the approximate year of the first AO light on 30 m class telescopes.