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1. The Small Separation A-star Companion Population: Tentative Signatures of Enhanced Multiplicity with Primary Mass

   https://doi.org/10.3847/1538-4357/adf3b1  

   De_Furio, Matthew; Gardner, Tyler; Monnier, John D; Meyer, Michael R; Kratter, Kaitlin M; Lanthermann, Cyprien; Anugu, Narsireddy; Kraus, Stefan; Setterholm, Benjamin R (August 2025, The Astrophysical Journal)

   Abstract We present updated results from our near-infrared long-baseline interferometry (LBI) survey to constrain the multiplicity properties of intermediate-mass A-type stars within 80 pc. Previous adaptive optics surveys of A-type stars are incomplete at separations <20 au. Therefore, an LBI survey allows us to explore separations previously unexplored. Our sample consists of 54 A-type primaries with estimated masses between 1.44 and 2.93M_⊙and ages 10–790 Myr, which we observed with the Michigan Infra-Red Combiner-eXeter and Michigan Young Star Imager at Center for High Angular Resolution Astronomy instruments at the Center for High Angular Resolution Astronomy Array. We use the open source software CANDID to detect two new companions, seven in total, and we performed a Bayesian demographic analysis to characterize the companion population. We find the separation distribution consistent with being flat, and we estimate a power-law fit to the mass ratio distribution with index –0.13 ${}_{-0.95}^{+0.92}$and a companion frequency of 0.25 ${}_{-0.11}^{+0.17}$over mass ratios 0.1–1.0 and projected separations 0.01–27.54 au. We find a posterior probability of 0.53 and 0.04 that our results are consistent with extrapolations based on previous models of the solar-type and B-type companion population, respectively. Our results suggest that the close companion population to A-type stars is comparable to that of solar-type stars and that close companions to B-type stars are potentially more frequent, which may be indicative of increased disk fragmentation for stars ≳3M_⊙. 

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2. The VLA/ALMA Nascent Disk And Multiplicity (VANDAM) Survey of Orion Protostars. V. A Characterization of Protostellar Multiplicity

   https://doi.org/10.3847/1538-4357/ac36d2  

   Tobin, John J.; Offner, Stella S. R.; Kratter, Kaitlin M.; Megeath, S. Thomas; Sheehan, Patrick D.; Looney, Leslie W.; Diaz-Rodriguez, Ana Karla; Osorio, Mayra; Anglada, Guillem; Sadavoy, Sarah I.; et al (January 2022, The Astrophysical Journal)

   Abstract We characterize protostellar multiplicity in²⁰Current address: Niels Bohr Institute, University of Copenhagen, Øster Voldgade 5–7, DK-1350, Copenhagen K, Denmark.the Orion molecular clouds using Atacama Large Millimeter/submillimeter Array 0.87 mm and Very Large Array 9 mm continuum surveys toward 328 protostars. These observations are sensitive to projected spatial separations as small as ∼20 au, and we consider source separations up to 10⁴au as potential companions. The overall multiplicity fraction (MF) and companion fraction (CF) for the Orion protostars are 0.30 ± 0.03 and 0.44 ± 0.03, respectively, considering separations from 20 to 10⁴au. The MFs and CFs are corrected for potential contamination by unassociated young stars using a probabilistic scheme based on the surface density of young stars around each protostar. The companion separation distribution as a whole is double peaked and inconsistent with the separation distribution of solar-type field stars, while the separation distribution of Flat Spectrum protostars is consistent solar-type field stars. The multiplicity statistics and companion separation distributions of the Perseus star-forming region are consistent with those of Orion. Based on the observed peaks in the Class 0 separations at ∼100 au and ∼10³au, we argue that multiples with separations <500 au are likely produced by both disk fragmentation and turbulent fragmentation with migration, and those at ≳10³au result primarily from turbulent fragmentation. We also find that MFs/CFs may rise from Class 0 to Flat Spectrum protostars between 100 and 10³au in regions of high young stellar object density. This finding may be evidence for the migration of companions from >10³au to <10³au, and that some companions between 10³and 10⁴au must be (or become) unbound. 

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3. Multiplicity of northern bright O-type stars with optical long baseline interferometry: Results of the pilot survey

   https://doi.org/10.1051/0004-6361/202245364  

   Lanthermann, C.; Le Bouquin, J.-B.; Sana, H.; Mérand, A.; Monnier, J. D.; Perraut, K.; Frost, A. J.; Mahy, L.; Gosset, E.; De Becker, M.; et al (April 2023, Astronomy & Astrophysics)

   Context . The study of the multiplicity of massive stars gives hints on their formation processes and their evolutionary paths, which are still not fully understood. Large separation binaries (>50 milliseconds of arc, mas) can be probed by adaptive-optics-assisted direct imaging and sparse aperture masking, while close binaries can be resolved by photometry and spectroscopy. However, optical long baseline interferometry is mandatory to establish the multiplicity of Galactic massive stars at the separation gap between 1 and 50 mas. Aims . In this paper, we aim to demonstrate the capability of the new interferometric instrument MIRC-X, located at the CHARA Array, to study the multiplicity of O-type stars and therefore probe the full range of separation for more than 120 massive stars ( H < 7 . 5 mag). Methods . We initiated a pilot survey of bright O-type stars ( H < 6.5 mag) observable with MIRC-X. We observed 29 O-type stars, including two systems in average atmospheric conditions around a magnitude of H = 7.5 mag. We systematically reduced the obtained data with the public reduction pipeline of the instrument. We analyzed the reduced data using the dedicated python software CANDID to detect companions. Results . Out of these 29 systems, we resolved 19 companions in 17 different systems with angular separations between ~0.5 and 50 mas. This results in a multiplicity fraction ƒ m = 17/29 = 0.59 ± 0.09, and an average number of companions ƒ c = 19/29 = 0.66 ± 0.13. Those results are in agreement with the results of the SMASH+ survey in the Southern Hemisphere. Thirteen of these companions have been resolved for the first time, including the companion responsible for the nonthermal emission in Cyg OB2-5 A and the confirmation of the candidate companion of HD 47129 suggested by SMASH+. Conclusions . A large survey on more than 120 northern O-type stars ( H < 7.5) is possible with MIRC-X and will be fruitful. 

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4. 89 New Ultracool Dwarf Comoving Companions Identified with the Backyard Worlds: Planet 9 Citizen Science Project

   https://doi.org/10.3847/1538-3881/ad324e  

   Rothermich, Austin; Faherty, Jacqueline K; Bardalez-Gagliuffi, Daniella; Schneider, Adam C; Kirkpatrick, J Davy; Meisner, Aaron M; Burgasser, Adam J; Kuchner, Marc; Allers, Katelyn; Gagné, Jonathan; et al (May 2024, The Astronomical Journal)

   Abstract We report the identification of 89 new systems containing ultracool dwarf companions to main-sequence stars and white dwarfs, using the citizen science project Backyard Worlds: Planet 9 and cross-reference between Gaia and CatWISE2020. 32 of these companions and 33 host stars were followed up with spectroscopic observations, with companion spectral types ranging from M7–T9 and host spectral types ranging from G2–M9. These systems exhibit diverse characteristics, from young to old ages, blue to very red spectral morphologies, potential membership to known young moving groups, and evidence of spectral binarity in nine companions. 20 of the host stars in our sample show evidence for higher-order multiplicity, with an additional 11 host stars being resolved binaries themselves. We compare this sample’s characteristics with those of the known stellar binary and exoplanet populations, and find our sample begins to fill in the gap between directly imaged exoplanets and stellar binaries on mass ratio–binding energy plots. With this study, we increase the population of ultracool dwarf companions to FGK stars by ∼42%, and more than triple the known population of ultracool dwarf companions with separations larger than 1000 au, providing excellent targets for future atmospheric retrievals. 

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5. Discovery of 34 Low-mass Comoving Systems Using NOIRLab Source Catalog DR2

   https://doi.org/10.3847/1538-3881/ac68e7  

   Kiwy, Frank; Faherty, Jacqueline K.; Meisner, Aaron; Schneider, Adam C.; Kirkpatrick, J. Davy; Kuchner, Marc J.; Burgasser, Adam J.; Casewell, Sarah; Kiman, Rocio; Calamari, Emily; et al (June 2022, The Astronomical Journal)

   Abstract We present the discovery of 34 comoving systems containing an ultracool dwarf found by means of the NOIRLab Source Catalog (NSC) DR2. NSC’s angular resolution of ∼ 1″ allows for the detection of small separation binaries with significant proper motions. We used the catalog’s accurate proper motion measurements to identify the companions by cross-matching a previously compiled list of brown dwarf candidates with NSC DR2. The comoving pairs consist of either a very low-mass star and an ultracool companion, or a white dwarf and an ultracool companion. The estimated spectral types of the primaries are in the K and M dwarf regimes, those of the secondaries in the M, L, and T dwarf regimes. We calculated angular separations between ∼2″ and ∼ 56″, parallactic distances between ∼43 and ∼261 pc, and projected physical separations between ∼169 and ∼8487 au. The lowest measured total proper motion is 97 mas yr⁻¹, with the highest 314 mas yr⁻¹. Tangential velocities range from ∼23 to ∼187 km s⁻¹. We also determined comoving probabilities, estimated mass ratios, and calculated binding energies for each system. We found no indication of possible binarity for any component of the 34 systems in the published literature. The discovered systems can contribute to the further study of the formation and evolution of low-mass systems as well as to the characterization of cool substellar objects. 

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