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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 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 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_⊙. 

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 Planets are a natural byproduct of the stellar formation process, resulting from local aggregations of material within the disks surrounding young stars. Whereas signatures of gas-giant planets at large orbital separations have been observed and successfully modeled within protoplanetary disks, the formation pathways of planets within their host star’s future habitable zones remain poorly understood. Analyzing multiple nights of observations conducted over a short, 2 month span with the MIRC-X and PIONIER instruments at the CHARA Array and VLTI, respectively, we uncover a highly active environment at the inner-edge of the planet formation region in the disk of HD 163296. In particular, we localize and track the motion of a disk feature near the dust-sublimation radius with a pattern speed of less than half the local Keplerian velocity, providing a potential glimpse at the planet formation process in action within the inner astronomical unit. We emphasize that this result is at the edge of what is currently possible with available optical interferometric techniques and behooves confirmation with a temporally dense followup observing campaign. 

Abstract Classical Wolf–Rayet (WR) stars are descendants of massive OB-type stars that have lost their hydrogen-rich envelopes and are in the final stages of stellar evolution, possibly exploding as Type Ib/c supernovae. It is understood that the mechanisms driving this mass loss are either strong stellar winds and or binary interactions, so intense studies of these binaries including their evolution can tell us about the importance of the two pathways in WR formation. WR 138 (HD 193077) has a period of just over 4 yr and was previously reported to be resolved through interferometry. We report on new interferometric data combined with spectroscopic radial velocities in order to provide a three-dimensional orbit of the system. The precision on our parameters tend to be about an order of magnitude better than previous spectroscopic techniques. These measurements provide masses of the stars, namely,M_WR= 13.93 ± 1.49M_⊙andM_O= 26.28 ± 1.71M_⊙. The derived orbital parallax agrees with the parallax from Gaia, namely, with a distance of 2.13 kpc. We compare the system’s orbit to models from BPASS, showing that the system likely may have been formed with little interaction but could have formed through some binary interactions either following or at the start of a red supergiant phase but with the most likely scenario occurring as the red supergiant phase starts for a ∼40M_⊙star. 

Abstract Classical Wolf–Rayet (W-R) stars are the descendants of massive OB stars that have lost their hydrogen envelopes and are burning helium in their cores prior to exploding as Type Ib/c supernovae. The mechanisms for losing their hydrogen envelopes are either through binary interactions or through strong stellar winds potentially coupled with episodic mass loss. Among the bright classical W-R stars, the binary system WR 137 (HD 192641; WC7d + O9e) is the subject of this paper. This binary is known to have a 13 yr period and produces dust near periastron. Here we report on interferometry with the Center for High Angular Resolution Astronomy Array collected over a decade of time and providing the first visual orbit for the system. We combine these astrometric measurements with archival radial velocities to measure masses of the stars ofM_WR= 9.5 ± 3.4M_⊙andM_O= 17.3 ± 1.9M_⊙when we use the most recent Gaia distance. These results are then compared to predicted dust distribution using these orbital elements, which match the observed imaging from JWST as discussed recently by Lau et al. Furthermore, we compare the system to the Binary Population And Spectral Synthesis models, finding that the W-R star likely formed through stellar winds and not through binary interactions. However, the companion O star did likely accrete some material from the W-R star’s mass loss to provide the rotation seen today that drives its status as an Oe star. 

Abstract Rapid rotation and nonradial pulsations enable Be stars to build decretion disks, where the characteristic line emission forms. A major but unconstrained fraction of Be stars owe their rapid rotation to mass and angular momentum transfer in a binary. The faint, stripped companions can be helium-burning subdwarf OB-type stars (sdOBs), white dwarfs (WDs), or neutron stars. We present optical/near-infrared Center for High Angular Resolution Astronomy (CHARA) interferometry of 37 Be stars selected for spectroscopic indications of low-mass companions. From multiepochH- and/orK-band interferometry plus radial velocities and parallaxes collected elsewhere, we constructed 3D orbits and derived flux ratios and absolute dynamical masses of both components for six objects, quadrupling the number of anchor points for evolutionary models. In addition, a new wider companion was identified for the known Be + sdO binary 59 Cyg, while auxiliary Very Large Telescope Interferometer/GRAVITY spectrointerferometry confirmed circumstellar matter around the sdO companion to HR 2142. On the other hand, we failed to detect any companion to the six Be stars withγCas–like X-ray emission, with sdOB and main-sequence companions of the expected spectroscopic mass being ruled out for the X-ray-prototypical starsγCas andπAqr, leaving elusive WDs as the most likely companions, as well as a likely explanation of the X-rays. No low-mass main-sequence close companions were identified for the other stars. 

Abstract Stars with initial masses larger than 8M_⊙undergo substantial mass loss through mechanisms that remain elusive. Unraveling the origins of this mass loss is important for comprehending the evolutionary path of these stars, the type of supernova explosion, and whether they become neutron stars or black hole remnants. In 2022 December, RW Cep experienced the Great Dimming in its visible brightness, presenting a unique opportunity to understand mass-loss mechanisms. Our previous observations of RW Cep from the CHARA Array, taken during the dimming phase, show a compelling asymmetry in the star images, with a darker zone on the west side of the star indicating the presence of dust in front of the star in our line of sight. Here, we present multiepoch observations from CHARA while the star rebrightened in 2023. We created images using three image reconstruction methods and an analytical model fit. Comparisons of images acquired during the dimming and rebrightening phases reveal remarkable differences. Specifically, the west side of RW Cep, initially obscured during the dimming phase, reappeared during the subsequent rebrightening phase, and the measured angular diameter became larger by 8%. We also observed image changes from epoch to epoch while the star is brightening, indicating the time evolution of dust in front of the star. We suggest that the dimming of RW Cep was a result of a recent surface mass ejection event, generating a dust cloud that partially obstructed the stellar photosphere.