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1. MYSTIC: a high angular resolution K-band imager at CHARA

   https://doi.org/10.1117/12.2629437  

   Setterholm, Benjamin R.; Monnier, John D.; Le Bouquin, Jean-Baptiste; Anugu, Narsireddy; Ennis, Jacob; Flores, Becky; Gardner, Tyler; Ibrahim, Nour; Jocou, Laurent; Kraus, Stefan; et al (August 2022, SPIE Astronomical Instrumentation)

   Mérand, Antoine; Sallum, Stephanie; Sanchez-Bermudez, Joel (Ed.)

   The Michigan Young STar Imager at CHARA (MYSTIC) is a K-band interferometric beam combining instrument funded by the United States National Science Foundation, designed primarily for imaging sub-au scale disk structures around nearby young stars and to probe the planet formation process. Installed at the CHARA array in July 2021, with baselines up to 331 meters, MYSTIC provides a maximum angular resolution of λ/2B ∼ 0.7 mas. The instrument injects phase corrected light from the array into inexpensive, single-mode, polarization maintaining silica fibers, which are then passed via a vacuum feedthrough into a cryogenic dewar operating at 220 K for imaging. MYSTIC utilizes a high frame rate, ultra-low read noise SAPHIRA detector, and implements two beam combiners: a 6-telescope image plane beam combiner, based on the MIRC-X design, for targets as faint as 7.7 Kmag, as well as a 4-telescope integrated optic beam-combiner mode using a spare chip leftover from the GRAVITY instrument. MYSTIC is co-phased with the MIRC-X (J+H band) instrument for simultaneous fringe-tracking and imaging, and shares its software suite with the latter to allow a single observer to operate both instruments. Herein, we present the instrument design, review its operational performance, present early commissioning science observations, and propose upgrades to the instrument that could improve its K-band sensitivity to 10th magnitude in the near future. 

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2. The CHARA Array Polarization Model and Prospects for Spectropolarimetry

   https://doi.org/10.3847/1538-3881/ae0648  

   Shuai, Linling; Monnier, John D; Setterholm, Benjamin R; Kraus, Stefan; Anugu, Narsireddy; Gardner, Tyler; Le_Bouquin, Jean-Baptiste; Schaefer, Gail H (November 2025, The Astronomical Journal)

   Abstract Polarimetric data provide key insights into infrared emission mechanisms in the inner disks of young stellar objects (YSOs) and the details of dust formation around asymptotic giant branch (AGB) stars. While polarization measurements are well-established in radio interferometry, they remain challenging at visible and near-infrared wavelengths, due to the significant time-variable birefringence introduced by the complex optical beam train. In this study, we characterize instrumental polarization effects within the optical path of the Center for High Angular Resolution Astronomy (CHARA) Array, focusing on theH-band MIRC-X andK-band MYSTIC beam combiners. Using the Jones matrix formalism, we developed a comprehensive model describing diattenuation and retardance across the array. By applying this model to an unpolarized calibrator, we derived the instrumental parameters for both MIRC-X and MYSTIC. Our results show differential diattenuation consistent with ≥97% reflectivity per aluminum-coated surface at 45° incidence. The differential retardance exhibits small wavelength-dependent variations, in some cases larger than we expected. Notably, telescope W2 exhibits a significantly larger phase shift in the Coudé path, attributable to a fixed aluminum mirror (M4) used in place of deformable mirrors present on the other telescopes during the observing run. We also identify misalignments in the LiNbO₃birefringent compensator plates on S1 (MIRC-X) and W2 (MYSTIC). After correcting for night-to-night offsets, we achieve calibration accuracies of ±3.4% in visibility ratio and $\pm 1\stackrel{°}{.}4$in differential phase for MIRC-X, and ±5.9% and $\pm 2\stackrel{°}{.}4$, respectively, for MYSTIC. Given that the differential intrinsic polarization of spatially resolved sources, such as AGB stars and YSOs, typically greater than these instrumental uncertainties, our results demonstrate that CHARA is now capable of achieving high-accuracy measurements of intrinsic polarization in astrophysical targets. 

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3. Resolving Orbits of Low Mass Companions in the Hyades Cluster

   Schaefer, Gail; Anugu, Narsireddy; Bender, Chad; ten Brummelaar, Theo; Farrington, Christopher; Gies, Douglas; Jones, Jeremy; Kraus, Stefan; Monnier, John; Simon, Michal (June 2022, Bulletin of the American Astronomical Society)

   We are resolving the orbits of spectroscopic binary stars in the Hyades Cluster using the CHARA Array. We obtained positions and flux ratios in the H-band using the MIRC-X combiner and the K-band using the recently commissioned MYSTIC combiner. We present preliminary orbital fits and mass estimates for four binary systems (HD 27691, HD 28033, HD 28294, and HD 28394). The sample consists of binaries where the primary stars have F-G spectral types and the companions are low mass stars with masses in the range of 0.3-0.9 Msun. The results will be used to test evolutionary models for low mass stars. The large mass difference between the components will provide leverage for testing the isochrones and refining the age of the Hyades cluster. 

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4. The ExoGRAVITY survey: A K -band spectral library of giant exoplanet and brown dwarf companions

   https://doi.org/10.1051/0004-6361/202556860  

   Kammerer, J; Winterhalder, T O; Lacour, S; Stolker, T; Marleau, G-D; Balmer, W O; Moore, A F; Piscarreta, L; Toci, C; Mérand, A; et al (December 2025, Astronomy & Astrophysics)

   Context. Direct observations of exoplanet and brown dwarf companions with near-infrared interferometry, first enabled by the dualfield mode of VLTI/GRAVITY, provide unique measurements of the objects’ orbital motions and atmospheric compositions. Aims. Here we compile a homogeneous library of all exoplanet and brown dwarfK-band spectra observed by GRAVITY thus far. This ExoGRAVITY Spectral Library is made publicly available online. Methods. We re-reduced all the available GRAVITY dual-field high-contrast data in a uniform and highly automated way and, where companions were detected, extracted their ~2.0-2.4 μmK-band contrast spectra. We then derived stellar model atmospheres for all the employed flux references (either the host star or the swap calibrator), which we used to convert the companion contrast into companion flux spectra. Solely from the resulting GRAVITYK-band flux spectra, we extracted spectral types, spectral indices, and bulk physical properties for all the companions. Finally, and with the help of age constraints from the literature, we also derived isochronal masses for most of the companions using evolutionary models. Results. The resulting library containsR~ 500 GRAVITYK-band spectra of 39 substellar companions from late M to late T spectral types, including the entire L-T transition. Throughout this transition, a shift from CO-dominated late M- and L-type dwarfs to CH₄-dominated T-type dwarfs can be observed in theK-band. The GRAVITY spectra alone constrain the objects’ bolometric luminosity to typically within ±0.15 dex. The derived isochronal masses agree with dynamical masses from the literature where available, except for HD 4113 c for which we confirm its previously reported potential underluminosity. Conclusions. Medium-resolution spectroscopy of substellar companions with GRAVITY provides insight into the carbon chemistry and the cloudiness of these objects’ atmospheres. It also constrains these objects’ bolometric luminosities, which can yield measurements of their formation entropy if combined with dynamical masses, for instance from Gaia and GRAVITY astrometry. 

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5. Dynamical Masses of the Primary Be Star and Secondary sdB Star in the Single-lined Binary κ Dra (B6 IIIe)

   https://doi.org/10.3847/1538-4357/ac98b8  

   Klement, Robert; Baade, Dietrich; Rivinius, Thomas; Gies, Douglas R.; Wang, Luqian; Labadie-Bartz, Jonathan; dos Santos, Pedro Ticiani; Monnier, John D.; Carciofi, Alex C.; Mérand, Antoine; et al (November 2022, The Astrophysical Journal)

   Abstract Because many classical Be stars may owe their nature to mass and angular-momentum transfer in a close binary, the present masses, temperatures, and radii of their components are of high interest for comparison to stellar evolution models. Object κ Dra is a 61.5 day single-lined binary with a B6 IIIe primary. With the CHARA Array instruments MIRC/MIRC-X and MYSTIC, we detected the secondary at (approximately photospheric) flux ratios of 1.49% ± 0.10% and 1.63% ± 0.09% in the H and K band, respectively. From a large and diverse optical spectroscopic database, only the radial velocity curve of the Be star could be extracted. However, employing the parallaxes from Hipparcos and Gaia, which agree within their nominal 1 σ errors, we could derive the total mass and found component masses of 3.65 ± 0.48 and 0.426 ± 0.043 M ⊙ for the Be star and the companion, respectively. Previous cross-correlation of the observed FUV spectrum with O-type subdwarf (sdO) spectral model templates had not detected a companion belonging to the hot sdO population known from ∼20 earlier-type Be stars. Guided by our full 3D orbital solution, we found a strong cross-correlation signal for a stripped subdwarf B-type companion (FUV flux ratio of 2.3% ± 0.5%), enabling the first firm characterization of such a star and making κ Dra the first mid- to late-type Be star with a directly observed subdwarf companion. 

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