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1. The MIRI Excesses Around Degenerates (MEAD) Survey. II. A Probable Planet Detected via IR Excess around WD 0644+025

   https://doi.org/10.3847/1538-3881/ae3c0e  

   Poulsen, Sabrina; Debes, John; Messier, Ashley; Le_Bourdais, Érika; Melis, Carl; Cracraft, Misty; Boucher, Samuel; Kilic, Mukremin; Kenyon, Scott; Wyatt, Mark C; et al (February 2026, The Astronomical Journal)

   Abstract The MIRI Excesses Around Degenerates (MEAD) Survey is a Cycle 2 JWST program designed to image nearby white dwarfs with MIRI at 10 and 15μm. This survey targeted 56 white dwarfs within 25 pc to search for mid-infrared excesses, flux deficits from collision-induced absorption, and resolved substellar companions. In this paper we present our analysis of WD 0644+025, an unusually massive white dwarf (0.95M_⊙) and the MEAD target exhibiting the most significant mid-infrared excess. The observed JWST MIRI photometry shows a 7.3σexcess at 15μm and a 3.6σexcess at 10μm, which may be associated with either a planetary companion or a circumstellar dust disk. This excess corresponds to a companion mass of 6.8M_Jup(T_eff= 261 ±9 K) with orbital distance <11.8 au, although substantially lower masses are possible if we consider a closely orbiting isolated companion. No spatially resolved sources are detected within 200 au, with contrast curve analysis excluding planets more massive than 2M_Jupbeyond ∼12 au. Metal pollution is confirmed in both archival Keck HIRES spectra from 1999 and new observations from 2025, with no evidence suggesting that the accretion rate has substantially changed over the decades. We explore possible dust disk morphologies to describe the observed IR excess and find that traditional debris disks struggle to fit our data. WD 0644+025 thus represents a compelling case study in the growing population of white dwarfs with cold infrared excesses and highlights JWST’s ability to probe planetary system remnants inaccessible to prior infrared observatories. 

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2. KMT-2022-BLG-2397: Brown Dwarf at the Upper Shore of the Einstein Desert

   https://doi.org/10.3847/1538-3881/ace169  

   Gould, Andrew; Ryu, Yoon-Hyun; Yee, Jennifer C; Albrow, Michael D; Chung, Sun-Ju; Han, Cheongho; Hwang, Kyu-Ha; Jung, Youn Kil; Shin, In-Gu; Shvartzvald, Yossi; et al (August 2023, The Astronomical Journal)

   Abstract We measure the Einstein radius of the single-lens microlensing event KMT-2022-BLG-2397 to beθ_E= 24.8 ± 3.6μas, placing it at the upper shore of the Einstein Desert, 9 ≲θ_E/μas ≲ 25, between free-floating planets (FFPs) and bulge brown dwarfs (BDs). In contrast to the six BD (25 ≲θ_E≲ 50) events presented by Gould et al. (2022), which all had giant-star source stars, KMT-2022-BLG-2397 has a dwarf-star source, with angular radiusθ_ast∼ 0.9μas. This prompts us to study the relative utility of dwarf and giant sources for characterizing FFPs and BDs from finite-source point-lens (FSPL) microlensing events. We find “dwarfs” (including main-sequence stars and subgiants) are likely to yield twice as manyθ_Emeasurements for BDs and a comparable (but more difficult to quantify) improvement for FFPs. We show that neither current nor planned experiments will yield complete mass measurements of isolated bulge BDs, nor will any other planned experiment yield as manyθ_Emeasurements for these objects as the Korea Microlensing Telescope (KMT). Thus, the currently anticipated 10 yr KMT survey will remain the best way to study bulge BDs for several decades to come. 

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3. Systematic Search for FFPs in KMTNet Full-frame Images. I. Photometry Pipeline

   https://doi.org/10.1088/1538-3873/add9cf  

   Qian, Qiyue; Yang, Hongjing; Zang, Weicheng; Ryu, Yoon-Hyun; Mao, Shude; Kuang, Renkun; Zhang, Jiyuan; Albrow, Michael D; Chung, Sun-Ju; Gould, Andrew; et al (June 2025, Publications of the Astronomical Society of the Pacific)

   Abstract To exhume the buried signatures of free-floating planets (FFPs) with small angular Einstein radiusθ_E, we build a new full-frame difference image pipeline for the Korean Microlensing Telescope Network (KMTNet) survey based on the newly optimized pySIS package. We introduce the detailed processes of the new pipeline, including frame registration, difference image analysis, and light curve extraction. To test this pipeline, we extract one-year light curves for 483,068 stars withI ≲ 17 and conduct a model-independent search for microlensing events. The search finds 36 microlensing events, including five new events and six events discovered by other collaborations but missed by previous KMTNet searches. We find that the light curves from the new pipeline are precise enough to be sensitive to FFPs withθ_E ∼ 1μas. Using the new pipeline, a complete FFP search on the eight-year KMTNet images can be finished within six months and then yield the FFP mass function. The new pipeline can be used for a new KMTNet AlertFinder system, with significantly reduced false positives. 

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4. A Binary Origin for the First Isolated Stellar-mass Black Hole Detected with Astrometric Microlensing

   https://doi.org/10.3847/2041-8213/acc076  

   Vigna-Gómez, Alejandro; Ramirez-Ruiz, Enrico (March 2023, The Astrophysical Journal Letters)

   Abstract The Milky Way is believed to host hundreds of millions of quiescent stellar-mass black holes (BHs). In the last decade, some of these objects have been potentially uncovered via gravitational microlensing events. All these detections resulted in a degeneracy between the velocity and the mass of the lens. This degeneracy has been lifted, for the first time, with the recent astrometric microlensing detection of OB110462. However, two independent studies reported very different lens masses for this event. Sahu et al. inferred a lens mass of 7.1 ± 1.3M_⊙, consistent with a BH, while Lam et al. inferred 1.6–4.2M_⊙, consistent with either a neutron star or a BH. Here, we study the landscape of isolated BHs formed in the field. In particular, we focus on the mass and center-of-mass speed of four subpopulations: isolated BHs from single-star origin, disrupted BHs of binary-star origin, main-sequence stars with a compact object companion, and double compact object mergers. Our model predicts that most (≳70%) isolated BHs in the Milky Way are of binary origin. However, noninteractions lead to most massive BHs (≳15–20M_⊙) being predominantly of single origin. Under the assumption that OB110462 is a free-floating compact object, we conclude that it is more likely to be a BH originally belonging to a binary system. Our results suggest that low-mass BH microlensing events can be useful to understand binary evolution of massive stars in the Milky Way, while high-mass BH lenses can be useful to probe single stellar evolution. 

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5. KMT-2023-BLG-2669: Ninth Free-floating Planet Candidate with θ _E Measurements

   https://doi.org/10.3847/1538-3881/ad6b12  

   Jung, Youn Kil; Hwang, Kyu-Ha; Yang, Hongjing; Gould, Andrew; Yee, Jennifer C; Han, Cheongho; Albrow, Michael D; Chung, Sun-Ju; Ryu, Yoon-Hyun; Shin, In-Gu; et al (September 2024, The Astronomical Journal)

   Abstract We report a free-floating planet (FFP) candidate identified from the analysis of the microlensing event KMT-2023-BLG-2669. The lensing light curve is characterized by a short duration (≲3 days) and a small amplitude (≲0.7 mag). From the analysis, we find an Einstein timescale oft_E⋍ 0.33 days and an Einstein radius ofθ_E⋍ 4.41μas. These measurements enable us to infer the lens mass as $M=8M_{\oplus }{\left({\pi }_{\mathrm{rel}}/0.1\mathrm{mas}\right)}^{-1}$, whereπ_relis the relative lens–source parallax. The inference implies that the lens is a sub-Neptune- to Saturn-mass object, depending on its unknown distance. This is the ninth isolated planetary mass microlens withθ_E< 10μas, which is a useful threshold for an FFP candidate. We conduct extensive searches for possible signals of a host star in the light curve, but find no strong evidence for the host. We investigate the possibility of using late-time high-resolution imaging to probe for possible hosts. In particular, we discuss the case of finite-source point-lens FFP candidates, for which it would be possible to search for very-wide-separation hosts immediately, although such searches are “high risk, high reward.” 

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