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Abstract The evolution of starspots of the giant primaries of RS CVn systems is typically detected indirectly with photometric and spectroscopic monitoring. These observations suggest slowly-evolving stellar surfaces and can constrain differential rotation as starspots move with respect to one another. However, starspot latitudes are difficult to constrain without resolved images of the stellar surfaces from which the unambiguous locations of starspots are determined. We imaged the active RS CVn primaryζ And with the 330-m-baseline Center for High Angular Resolution Astronomy Array for three epochs over approximately six rotations of the star. The resultant images show a more complicated picture of stellar activity than expected from the contemporaneous photometry and earlier Doppler images. The spot structures change on the timescale of rotation, making differential rotation difficult to study. Our observations show changes in the polar spot, growing over time. We do not detect the secondary star in the interferometric data, though the observations are sensitive to the predicted 0.75 M_⊙main-sequence star, and we suggest the companion may be a white dwarf. 

ABSTRACT We present a detailed study of the secondary red clump star, $$\kappa$$ Cyg, by combining long-baseline visible interferometry using the PAVO beam combiner at the CHARA Array with high-precision asteroseismology from TESS. This dual approach allowed for a stringent test of stellar evolutionary models in the core helium-burning phase, which remains a regime of significant theoretical uncertainty. Using the PAVO interferometric data and fitting the limb-darkened intensity profile directly, we measured $$R = 8.65\pm 0.10~\rm R_\odot$$. We fitted the spectral energy distribution using Phoenix model atmospheres and calculated $$L = 44.46 \pm 1.09~\rm L_\odot$$ and $$T_{\rm eff} = 5066^{+47}_{-50}~\mathrm{K}$$. Using 16 sectors of TESS photometry, we detected clear solar-like oscillations in $$\kappa$$ Cyg. Through comparison of oscillation frequencies with mesa grids using either predictive mixing or exponential overshooting (OS), we found that models reproducing the oscillation frequencies systematically overestimate the stellar radius, with OS models performing only marginally better. The same models also under-predict the observed dipole-mode period spacing ($$\Delta \Pi _1$$). By inspecting the phase offset ($$\epsilon _\mathrm{ p}$$), we conclude that models misrepresent the interior structure of the star. Our results demonstrate that matching envelope-dominated asteroseismic observables alone is insufficient to ensure a correct core or even global structure, and highlight the need for improved treatments of convective boundary mixing in the models of core helium-burning stars. 

Abstract T Coronae Borealis (T CrB, HD 143454) is a symbiotic recurrent nova consisting of a red giant and white dwarf with recent eruptions in 1866 and 1946 and an anticipated eruption in the mid-2020s. We report Center for High Angular Resolution Astronomy (or CHARA) Array observations collected with MIRC-X (Hband) and MYSTIC (Kband) in 2022–2025. We fit limb-darkened disk models constrained with literature limb-darkening coefficients to the squared visibilities as only the first visibility lobe is sampled. The average limb-darkened diameter of the star across these epochs is 0.70 ± 0.04 mas in theHband and 0.72 ± 0.07 mas in theKband. Adopting a distance of $914_{-22}^{+24}$pc, the corresponding stellar radius is 69 ± 5R_⊙in theHband and 71 ± 8R_⊙in theKband. These values are consistent with the Roche-lobe volume radius of 71R_⊙inferred from published orbital solutions. These measurements provide a preeruption angular diameter and support a Roche-lobe-filling donor. 

Abstract It is unclear how directly imaged substellar companions with masses near the deuterium burning limit form, because these objects are rare and their bulk properties are not diagnostic of their formation. In this Letter, we revisit this problem using JWST/NIRCam coronagraphic images of the 29 Cygni (=HIP 99770) system that reveal the recently discovered super-Jovian companion 29 Cyg b at wavelengths covering 4–5μm for the first time. This object has an uncertain mass that straddles the deuterium burning limit (M_b ≃ 15 ± 5M_J) and a low mass ratio with its early-type host star (M_b/M_⋆ ∼ 0.01). Absorption from CO₂and CO is apparent at 4.3 and 4.6μm in our images. The strength of the CO₂feature relative to CO provides strong evidence, based on empirical comparison with literature observations at these wavelengths and atmospheric modeling, that the companion is enriched in heavier elements compared to the roughly solar abundances of the host (Z_b/Z_⋆ = 3 ± 2). In addition, we measure the stellar inclination angle with CHARA/PAVO interferometry: the system is consistent with spin–orbit alignment at the 2σlevel, with Δi = 12° ± 6°. This ensemble of evidence is suggestive of formation within the protoplanetary disk and the rapid accretion of metal-rich material versus disk fragmentation or capture, like higher-mass-ratio companions. 29 Cyg b shows that planet formation around early-type stars can occur on scales at or exceeding the deuterium burning limit, in agreement with the recently revised planetary mass/metallicity trend that predictsZ_pl/Z_⋆ = 3.3 ± 0.5 at high masses from transiting planet densities. 

Abstract We present the first on-sky demonstration of dual-field interferometry at the Center for High Angular Resolution Astronomy (CHARA) Array and the first direct resolution of the inner Ba–Bb subsystem in the bright hierarchical tripleαPiscium. UsingH-band fringe tracking on component A with MIRC-X to stabilizeK-band science fringes on component B with MYSTIC, we detected a companion at a projected separation of 7 mas, confirming a long-suspected but previously unresolved short-period subsystem within the B component. The nearly equalH/K-band flux ratio indicates that Ba and Bb are near-twin F-type stars, consistent with the two narrow-lined components seen in optical spectra of B. By combining CHARA interferometry with archival VLTI/GRAVITY astrometry and radial velocities from archival and new spectroscopy (NARVAL and ARCES), we derive a well-constrained orbit with a period ofP= 25 days, eccentricitye ≃ 0.6, and inclinationi ≃ 65°, yielding precise dynamical masses of 1.668 ± 0.033M_⊙and 1.646 ± 0.029M_⊙. No additional companion is detected down to ΔH ≈ 5 at separations of 0.2–2 au. We also obtained dual-field differential astrometry of the wide A–B pair with a precision of ∼0.234 mas at a separation of 1 $\stackrel{\text{″}}{.}$85, with an error budget dominated by internal delay-line actuators, fringe-tracking performance, and chromatic dispersion. While the long-period outer orbit is not refined by these measurements, their agreement with the published astrometric orbit provides an on-sky validation of the CHARA dual-field mode. These results establishαPsc as a well-characterized hierarchical system suitable for future benchmark studies and demonstrate CHARA’s new capability for off-axis interferometry and submilliarcsecond astrometry on arcsecond-scale binaries. 

Abstract We present the discovery of a young, close-in companion to the early B-type Herbig Be star MWC 340 (V1685 Cyg), a rare system known to host a protoplanetary disk. Herbig Be stars often show complex circumstellar environments shaped by accretion and multiplicity. Using near-infrared (NIR) interferometric observations with the Center for High Angular Resolution Astronomy (CHARA) array over 5 yr in theHband (λ = 1.5–1.7μm) andKband (λ= 2.1–2.4μm), we resolve the binary and constrain its orbital and stellar properties through image reconstruction, model fitting, and spectral energy distribution analysis. Orbital modeling suggests a 16 yr minimum period and a 27.5 mas projected separation (≈25 au). Assuming anM_A+B = 14.65M_⊙total mass, we find a best-fit semimajor axis of 21 mas and eccentricitye= 0.69. The reconstructed images reveal a slightly elongated primary that is roughly 3 times brighter than the secondary (flux B/A = 0.28 in theHband, 0.3 in theKband), contributing 65% of the NIR flux. The secondary contributes 21%, with the remainder from extended emission. Stellar photospheres account for only ∼20% of totalH- andK-band emission. Isochrone fitting yields masses and ages of 7.5–9M_⊙, 0.13–0.19 Myr for the primary, and 5.8–7M_⊙, 0.28–0.45 Myr for the secondary, suggesting non-coevality unless the secondary is more extinguished. The apparent age discrepancy and high eccentricity ruled out by mass constraints may reflect differential reddening or point to dynamical influence from an unresolved third component. Continued CHARA monitoring and Gaia astrometry will help clarify the system’s architecture. 

Abstract Ground-based long baseline interferometry is a powerful tool for characterizing exoplanets that are too close to their host star to be imaged with single-dish telescopes. The CHARA Array can resolve companions down to 0.5 mas, allowing us in principle to directly measure the near-infrared spectra of nontransiting “hot Jupiter” exoplanets. We present data taken with the Michigan InfraRed Combiner-Exeter (MIRC-X) and MYSTIC instruments at the CHARA Array on the hot Jupiter Upsilon Andromedae b. By resolving the star–planet system, we attempt to directly detect the flux from the planet. We describe our self-calibration methods for modeling systematics in the closure phase data, which allows us to reach subdegree precision. Through combining multiple nights of data across two MIRC-X runs in 2019 and 2021, we achieved a very tentative detection of Ups And b in theHband at a planet/star contrast of 2–3 × 10⁻⁴. Unfortunately, we cannot confirm this detection with 2021 MYSTIC data in theKband, or in a 2023 joint MIRC-X and MYSTIC data set. We run updated global circulation models and create post-processed spectra for this planet, and report the resulting model spectra inH- andKbands as a function of orbital phase. We then run planetary injection tests to exploreH/K-band contrast limits, and find that we can confidently recover planets down to a planet/star contrast of 1–2 × 10⁻⁴. We show that we are probing contrasts fainter than predicted by the model, making our nondetection surprising. We discuss prospects for the future in using this method to characterize companions with interferometry. 

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. 

Abstract We present a study of the double-lined spectroscopic binary HD 21278 that contains one of the brightest main-sequence stars in the youngαPersei open cluster. We analyzed new spectra and reanalyzed archived spectra to measure precise new radial velocity curves for the binary. We also obtained interferometric data using the CHARA Array at Mount Wilson to measure the sky positions of the two stars and the inclination of the ∼2 mas orbit. We determine that the two stars have masses of 5.381 ± 0.084M_⊙and 3.353 ± 0.064M_⊙. From isochrone fits, we find the cluster’s age to be 49  ±  7 Myr (using PARSEC models) or 49.5 ± 6 Myr (MIST models). Finally, we revisit the massive white dwarfs that are candidate escapees from theαPersei cluster to try to better characterize the massive end of the white dwarf initial–final mass relation. The implied progenitor masses challenge the idea that Chandrasekhar-mass white dwarfs are made by single stars with masses near 8M_⊙. 

Abstract We report new spectroscopic and interferometric observations of the Pleiades binary star Atlas, which played an important role nearly 3 decades ago in settling the debate over the distance to the cluster from ground-based and space-based determinations. We use the new measurements, together with other published and archival astrometric observations, to improve the determination of the 291 day orbit and the distance to Atlas (136.2 ± 1.4 pc). We also derive the main properties of the components, including their absolute masses (5.04 ± 0.17M_⊙and 3.64 ± 0.12M_⊙), sizes, effective temperatures, projected rotational velocities, and chemical compositions. We find that the more evolved primary star is rotationally distorted, and we are able to estimate its oblateness and the approximate orientation of its spin axis from the interferometric observations. The spin axis may well be aligned with the orbital axis. Models of stellar evolution from the Modules for Experiments in Stellar Astrophysics (or MESA) that account for rotation provide a good match to all of the primary’s global properties, and point to an initial angular rotation rate on the zero-age main sequence of about 55% of the breakup velocity. The current location of the star in the Hertzsprung–Russell diagram is near the very end of the hydrogen-burning main sequence, at an age of about 105 Myr, according to these models. Our spectroscopic analysis of the more slowly rotating secondary indicates that it is a helium-weak star, with other chemical anomalies.