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1. Searching for GEMS: TOI-6383Ab, a Giant Planet Transiting an M3-dwarf Star in a Binary System*

   https://doi.org/10.3847/1538-3881/ad7fe8  

   Bernabò, Lia_Marta; Kanodia, Shubham; Cañas, Caleb_I; Cochran, William_D; Csizmadia, Szilárd; Mahadevan, Suvrath; Stefánsson, Gudhmundur; Gupta, Arvind_F; Monson, Andrew; Kobulnicky, Henry_A; et al (November 2024, The Astronomical Journal)

   Abstract We report on the discovery of a transiting giant planet around the 3500 K M3-dwarf star TOI-6383A located 172 pc from Earth. It was detected by the Transiting Exoplanet Survey Satellite and confirmed by a combination of ground-based follow-up photometry and precise radial velocity measurements. This planet has an orbital period of ∼1.791 days, a mass of 1.040 ± 0.094M_J, and a radius of ${1.008}_{-0.033}^{+0.036}{R}_J$, resulting in a mean bulk density of ${1.26}_{-0.17}^{+0.18}$g cm⁻³. TOI-6383A has an M dwarf companion star, TOI-6383B, which has a stellar effective temperature ofT_eff∼ 3100 K and a projected orbital separation of 3126 au. TOI-6383A is a low-mass dwarf star hosting a giant planet and is an intriguing object for planetary evolution studies due to its high planet-to-star mass ratio. This discovery is part of the Searching for Giant Exoplanets around M-dwarf Stars (GEMS) Survey, intending to provide robust and accurate estimates of the occurrence of GEMS and the statistics on their physical and orbital parameters. This paper presents an interesting addition to the small number of confirmed GEMS, particularly notable since its formation necessitates massive, dust-rich protoplanetary discs and high accretion efficiency (>10%). 

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2. Searching for GEMS: Discovery and Characterization of Two Brown Dwarfs Around M Dwarfs*

   https://doi.org/10.3847/1538-3881/adbb54  

   Larsen, Alexander; Swaby, Tera N; Kobulnicky, Henry A; Cañas, Caleb I; Kanodia, Shubham; Libby-Roberts, Jessica; Monson, Andrew; Gupta, Arvind F; Cochran, William; Mahadevan, Suvrath; et al (April 2025, The Astronomical Journal)

   Abstract Brown dwarfs bridge the gap between stars and planets, providing valuable insight into both planetary and stellar-formation mechanisms. Yet the census of transiting brown-dwarf companions, in particular around M-dwarf stars, remains incomplete. We report the discovery of two transiting brown dwarfs around low-mass hosts using a combination of space- and ground-based photometry along with near-infrared radial velocities. We characterize TOI-5389Ab ( $68.0_{-2.2}^{+2.2}{M}_{\mathrm{J}}$) and TOI-5610b ( $40.4_{-1.0}^{+1.0}{M}_{\mathrm{J}}$), two moderately massive brown dwarfs orbiting early M-dwarf hosts (T_eff = 3569 ± 59 K and 3618 ± 59 K, respectively). For TOI-5389Ab, the best fitting parameters are periodP = 10.40046 ± 0.00002 days, radius $R_{\mathrm{BD}}=0.824_{-0.031}^{+0.033}$R_J, and low eccentricity $e=0.0962_{-0.0046}^{+0.0027}$. In particular, this constitutes one of the most extreme substellar-stellar companion-to-host mass ratios ofq= 0.150. For TOI-5610b, the best-fitting parameters are periodP = 7.95346 ± 0.00002 days, radius $R_{\mathrm{BD}}=0.887_{-0.031}^{+0.031}$R_J, and moderate eccentricity $e=0.354_{-0.012}^{+0.011}$. Both targets are expected to have shallow, but potentially observable, occultations: ≲500 ppm in the JohnsonKband. A statistical analysis of M-dwarf/BD systems reveals for the first time that those at short orbital periods (P < 13 days) exhibit a dearth of 13M_J < M_BD < 40M_Jcompanions (q < 0.1) compared to those at slightly wider separations. 

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3. Gaia22dkvLb: A Microlensing Planet Potentially Accessible to Radial-velocity Characterization

   https://doi.org/10.3847/1538-3881/ad5203  

   Wu, Zexuan; Dong, Subo; Yi, Tuan; Liu, Zhuokai; El-Badry, Kareem; Gould, Andrew; Wyrzykowski, L; Rybicki, K A; Bachelet, Etienne; Christie, Grant W; et al (July 2024, The Astronomical Journal)

   Abstract We report discovering an exoplanet from following up a microlensing event alerted by Gaia. The event Gaia22dkv is toward a disk source rather than the traditional bulge microlensing fields. Our primary analysis yields a Jovian planet with $M_{\mathrm{p}}={0.59}_{-0.05}^{+0.15}{M}_{\mathrm{J}}$at a projected orbital separation $r_{\perp }={1.4}_{-0.3}^{+0.8}$au, and the host is a ∼1.1M_⊙turnoff star at ∼1.3 kpc. At $r\prime \approx 14$, the host is far brighter than any previously discovered microlensing planet host, opening up the opportunity to test the microlensing model with radial velocity (RV) observations. RV data can be used to measure the planet’s orbital period and eccentricity, and they also enable searching for inner planets of the microlensing cold Jupiter, as expected from the “inner–outer correlation” inferred from Kepler and RV discoveries. Furthermore, we show that Gaia astrometric microlensing will not only allow precise measurements of its angular Einstein radiusθ_Ebut also directly measure the microlens parallax vector and unambiguously break a geometric light-curve degeneracy, leading to the definitive characterization of the lens system. 

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4. TOI-5375 B: A Very Low Mass Star at the Hydrogen-burning Limit Orbiting an Early M-type Star* †

   https://doi.org/10.3847/1538-3881/acc651  

   Lambert, Mika; Bender, Chad F.; Kanodia, Shubham; Cañas, Caleb I.; Monson, Andrew; Stefánsson, Gudmundur; Cochran, William D.; Everett, Mark E.; Gupta, Arvind F.; Hearty, Fred; et al (April 2023, The Astronomical Journal)

   Abstract The Transiting Exoplanet Survey Satellite (TESS) mission detected a companion orbiting TIC 71268730, categorized it as a planet candidate, and designated the system TOI-5375. Our follow-up analysis using radial-velocity data from the Habitable-zone Planet Finder, photometric data from Red Buttes Observatory, and speckle imaging with NN-EXPLORE Exoplanet Stellar Speckle Imager determined that the companion is a very low mass star near the hydrogen-burning mass limit with a mass of 0.080 ± 0.002M_☉(83.81 ± 2.10M_J), a radius of ${0.1114}_{-0.0050}^{+0.0048}{R}_{☉}$(1.0841 ${}_{0.0487}^{0.0467}{R}_J$), and brightness temperature of 2600 ± 70 K. This object orbits with a period of 1.721553 ± 0.000001 days around an early M dwarf star (0.62 ± 0.016M_☉). TESS photometry shows regular variations in the host star’s TESS light curve, which we interpreted as an activity-induced variation of ∼2%, and used this variability to measure the host star’s stellar rotation period of ${1.9716}_{-0.0083}^{+0.0080}$days. The TOI-5375 system provides tight constraints on stellar models of low-mass stars at the hydrogen-burning limit and adds to the population in this important region. 

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5. Analysis of the Full Spitzer Microlensing Sample. I. Dark Remnant Candidates and Gaia Predictions

   https://doi.org/10.3847/1538-4357/ad7bb1  

   Rybicki, Krzysztof A; Shvartzvald, Yossi; Yee, Jennifer C; Calchi_Novati, Sebastiano; Ofek, Eran O; Bond, Ian A; Beichman, Charles; Bryden, Geoff; Carey, Sean; Henderson, Calen; et al (November 2024, The Astrophysical Journal)

   Abstract In the pursuit of understanding the population of stellar remnants within the Milky Way, we analyze the sample of ∼950 microlensing events observed by the Spitzer Space Telescope between 2014 and 2019. In this study we focus on a subsample of nine microlensing events, selected based on their long timescales, small microlensing parallaxes, and joint observations by the Gaia mission, to increase the probability that the chosen lenses are massive and the mass is measurable. Among the selected events we identify lensing black holes and neutron star candidates, with potential confirmation through forthcoming release of the Gaia time-series astrometry in 2026. Utilizing Bayesian analysis and Galactic models, along with the Gaia Data Release 3 proper-motion data, four good candidates for dark remnants were identified: OGLE-2016-BLG-0293, OGLE-2018-BLG-0483, OGLE-2018-BLG-0662, and OGLE-2015-BLG-0149, with lens masses of ${3.0}_{-1.3}^{+1.8}{M}_{\odot }$, ${4.7}_{-2.1}^{+3.2}{M}_{\odot }$, ${3.15}_{-0.64}^{+0.66}{M}_{\odot }$and ${1.40}_{-0.55}^{+0.75}{M}_{\odot }$, respectively. Notably, the first two candidates are expected to exhibit astrometric microlensing signals detectable by Gaia, offering the prospect of validating the lens masses. The methodologies developed in this work will be applied to the full Spitzer microlensing sample, populating and analyzing the timescale (t_E) versus parallax (π_E) diagram to derive constraints on the population of lenses in general and massive remnants in particular. 

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