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1. A Reanalysis of the Isolated Black Hole Candidate OGLE-2011-BLG-0462/MOA-2011-BLG-191

   https://doi.org/10.3847/1538-4357/aced4a  

   Lam, Casey Y. ; Lu, Jessica R. ( September 2023 , The Astrophysical Journal)

   There are expected to be ∼10⁸isolated black holes (BHs) in the Milky Way. OGLE-2011-BLG-0462/MOA-2011-BLG-191 (OB110462) is the only such BH with a mass measurement to date. However, its mass is disputed: Lam et al. measured a lower mass of 1.6–4.4M_⊙, while Sahu et al. and Mróz et al. measured a higher mass of 5.8–8.7M_⊙. We reanalyze OB110462, including new data from the Hubble Space Telescope (HST) and rereduced Optical Gravitational Lensing Experiment (OGLE) photometry. We also rereduce and reanalyze the HST data set with newly available software. We find significantly different (∼1 mas) HST astrometry than Lam et al. in the unmagnified epochs due to the amount of positional bias induced by a bright star ∼0.″4 from OB110462. After modeling the updated photometric and astrometric data sets, we find the lens of OB110462 is a${6.0}_{-1.0}^{+1.2}{M}_{\odot }$BH. Future observations with the Nancy Grace Roman Space Telescope, which will have an astrometric precision comparable or better to HST but a field of view 100× larger, will be able to measure hundreds of isolated BH masses via microlensing. This will enable the measurement of the BH mass distribution and improve understanding of massive stellar evolution and BH formation channels.

    

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2. OGLE-2016-BLG-1195 AO: Lens, Companion to Lens or Source, or None of the Above?

   https://doi.org/10.3847/1538-3881/aced3c  

   Gould, Andrew ; Shvartzvald, Yossi ; Zhang, Jiyuan ; Yee, Jennifer C. ; Calchi Novati, Sebastiano ; Zang, Weicheng ; Ofek, Eran O. ( September 2023 , The Astronomical Journal)

   We systematically investigate Vandorou et al.’s claim to have detected the host star of the low-mass-ratio (q< 10⁻⁴) microlensing planet OGLE-2016-BLG-1195Lb, via Keck adaptive optics (AO) measurements Δt= 4.12 yr after the event’s peak (t₀). If correct, this measurement would contradict the microlens-parallax measurement derived from Spitzer observations taken neart₀. We show that this host identification would be in 4σconflict with the original ground-based relative lens–source proper-motion measurements. By contrast, Gould estimated a probabilityp= 10% that the “other star” resolved by single-epoch late-time AO would be a companion to the host or the microlensed source, which is much more probable than a 4σstatistical fluctuation. Independent of this proper-motion discrepancy, the kinematics of this host identification are substantially less probable than those of the Spitzer solution. Hence, this identification should not be accepted, pending additional observations that would either confirm or contradict it, which could be taken in 2023. Motivated by this tension, we present two additional investigations. We explore the possibility that Vandorou et al. identified the wrong “star” for their analysis. Astrometry of KMT and Keck images favors a star (or asterism) lying about 175 mas northwest of Vandorou et al.’s star. We also present event parameters from a combined fit to all survey data, which yields a more precise mass ratio,q= (4.6 ± 0.4) × 10⁻⁵. Finally, we discuss the broader implications of minimizing such false positives for the first measurement of the planet mass function, which will become possible when AO on next-generation telescopes are applied to microlensing planets.

    

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3. MOA-2007-BLG-400 A Super-Jupiter-mass Planet Orbiting a Galactic Bulge K-dwarf Revealed by Keck Adaptive Optics Imaging

   https://doi.org/10.3847/1538-3881/abfec5  

   Bhattacharya, Aparna ; Bennett, David P. ; Beaulieu, Jean Philippe ; Bond, Ian A. ; Koshimoto, Naoki ; Lu, Jessica R. ; Blackman, Joshua W. ; Vandorou, Aikaterini ; Terry, Sean K. ; Batista, Virginie ; et al ( July 2021 , The Astronomical Journal)

   Abstract We present Keck/NIRC2 adaptive optics imaging of planetary microlensing event MOA-2007-BLG-400 that resolves the lens star system from the source. We find that the MOA-2007-BLG-400L planetary system consists of a 1.71 ± 0.27 M Jup planet orbiting a 0.69 ± 0.04 M ⊙ K-dwarf host star at a distance of 6.89 ± 0.77 kpc from the Sun. So, this planetary system probably resides in the Galactic bulge. The planet–host star projected separation is only weakly constrained due to the close-wide light-curve degeneracy; the 2 σ projected separation ranges are 0.6–1.0 au and 4.7–7.7 au for close and wide solutions, respectively. This host mass is at the top end of the range of masses predicted by a standard Bayesian analysis. Our Keck follow-up program has now measured lens-source separations for six planetary microlensing events, and five of these six events have host star masses above the median prediction under the assumption that assumes that all stars have an equal chance of hosting planets detectable by microlensing. This suggests that more massive stars may be more likely to host planets of a fixed mass ratio that orbit near or beyond the snow line. These results also indicate the importance of host star mass measurements for exoplanets found by microlensing. The microlensing survey imaging data from NASA’s Nancy Grace Roman Space Telescope (formerly WFIRST) mission will be doing mass measurements like this for a huge number of planetary events. 

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4. OGLE-2019-BLG-1180Lb: Discovery of a Wide-orbit Jupiter-mass Planet around a Late-type Star

   https://doi.org/10.3847/1538-3881/ad0723  

   Chung, Sun-Ju ; Udalski, Andrzej ; Yee, Jennifer C. ; Gould, Andrew ; Leading Authors ; Albrow, Michael D. ; Jung, Youn Kil ; Hwang, Kyu-Ha ; Han, Cheongho ; Ryu, Yoon-Hyun ; et al ( November 2023 , The Astronomical Journal)

   We report on the discovery and analysis of the planetary microlensing event OGLE-2019-BLG-1180 with a planet-to-star mass ratioq∼ 0.003. The event OGLE-2019-BLG-1180 has unambiguous cusp-passing and caustic-crossing anomalies, which were caused by a wide planetary caustic withs≃ 2, wheresis the star–planet separation in units of the angular Einstein radiusθ_E. Thanks to well-covered anomalies by the Korea Micorolensing Telescope Network (KMTNet), we measure both the angular Einstein radius and the microlens parallax in spite of a relatively short event timescale oft_E= 28 days. However, because of a weak constraint on the parallax, we conduct a Bayesian analysis to estimate the physical lens parameters. We find that the lens system is a super-Jupiter-mass planet of$M_{\mathrm{p}}={1.75}_{-0.51}^{+0.53}{M}_{\mathrm{J}}$orbiting a late-type star of$M_{\mathrm{h}}={0.55}_{-0.26}^{+0.27}{M}_{\odot }$at a distance$D_{\mathrm{L}}={6.1}_{-1.3}^{+0.9}\mathrm{kpc}$. The projected star–planet separation is$a_{\perp }={5.19}_{-1.23}^{+0.90}\mathrm{au}$, which means that the planet orbits at about four times the snow line of the host star. Considering the relative lens–source proper motion ofμ_rel= 6 mas yr⁻¹, the lens will be separated from the source by 60 mas in 2029. At that time one can measure the lens flux from adaptive optics imaging of Keck or a next-generation 30 m class telescope. OGLE-2019-BLG-1180Lb represents a growing population of wide-orbit planets detected by KMTNet, so we also present a general investigation into prospects for further expanding the sample of such planets.

    

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5. Precision measurement of a brown dwarf mass in a binary system in the microlensing event: OGLE-2019-BLG-0033/MOA-2019-BLG-035

   https://doi.org/10.1051/0004-6361/202243490  

   Herald, A. ; Udalski, A. ; Bozza, V. ; Rota, P. ; Bond, I. A. ; Yee, J. C. ; Sajadian, S. ; Mróz, P. ; Poleski, R. ; Skowron, J. ; et al ( July 2022 , Astronomy & Astrophysics)

   Context. Brown dwarfs are transition objects between stars and planets that are still poorly understood, for which several competing mechanisms have been proposed to describe their formation. Mass measurements are generally difficult to carry out for isolated objects as well as for brown dwarfs orbiting low-mass stars, which are often too faint for a spectroscopic follow-up. Aims. Microlensing provides an alternative tool for the discovery and investigation of such faint systems. Here, we present an analysis of the microlensing event OGLE-2019-BLG-0033/MOA-2019-BLG-035, which is caused by a binary system composed of a brown dwarf orbiting a red dwarf. Methods. Thanks to extensive ground observations and the availability of space observations from Spitzer, it has been possible to obtain accurate estimates of all microlensing parameters, including the parallax, source radius, and orbital motion of the binary lens. Results. Following an accurate modeling process, we found that the lens is composed of a red dwarf with a mass of M 1 = 0.149 ± 0.010 M ⊙ and a brown dwarf with a mass of M 2 = 0.0463 ± 0.0031 M ⊙ at a projected separation of a ⊥ = 0.585 au. The system has a peculiar velocity that is typical of old metal-poor populations in the thick disk. A percent-level precision in the mass measurement of brown dwarfs has been achieved only in a few microlensing events up to now, but will likely become more common in the future thanks to the Roman space telescope. 

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