Douglas R. Gies

Context. AN Col is a shell Be star that has shown transient spectral features caused by a hot, evolved companion on a months-long orbit. If confirmed, this would place it among the growing category of post-mass-transfer spun-up Be and stripped subdwarf OB-type (sdOB) binaries. Characterizing the binary orbit makes it possible to obtain dynamical masses and provides a rare anchor for models of binary evolution and mass transfer. Aims. We aim to present a full three-dimensional orbital solution, dynamical masses, and a dynamical parallax for the newly discovered Be + sdOB binary AN Col. As a prerequisite to obtain a reliable orbital solution, we need to characterize the disk-wide density oscillation cycle that gives rise to the dominant spectroscopic cycles in order to separate the weaker periodic orbital variations. Methods. We used near-infrared interferometry time series from GRAVITY to confirm the companion and to map the relative astrometric orbit. With the knowledge of the orbital period, we identified spectral lines that trace the orbit of each of the two stellar components and measured their radial velocities to obtain a double-lined spectroscopic orbit. Combining interferometry and spectroscopy, we derived the full three-dimensional orbital solution. Results. The disk has completed three full oscillation cycles since 2010, with a cycle length of ∼5.1 yr. The ∼87.7-day orbit is compatible with an edge-on orientation, but the lack of eclipses in TESS light curves limits the orbital inclination to 88.4° or less. The resulting masses are M _Be = 8.39 ± 0.67 M _⊙ and M _sdOB = 0.81 ± 0.22 M _⊙ . As in other Be + sdOB binaries, the estimated efficiency of the past mass transfer is unexpectedly high. Detections of a weak He II λ 4686 absorption and other He I lines from the companion indicate that its effective temperature is close to 30 kK and that it is a slow rotator. Conclusions. This work establishes AN Col as a new benchmark Be + sdOB system, whose fundamental properties can be used to constrain models of binary evolution, as well as the physics of binary mass transfer that probably lead to the formation of the majority of Be stars. AN Col is only the second Be + sdOB binary with a full orbital solution enabling the determination of parallax-independent dynamical masses for both the spun-up Be star and its stripped helium companion.

The nature of the mechanisms that drive mass transfer in symbiotic stars remains an area of active research in stellar astronomy. Constraining the role that both stellar winds and Roche-lobe overflow play in this process is crucial to improving our understanding of these binaries and connecting them to important transient events such as recurrent novae and Type Ia supernovae. The high-resolution capabilities of an optical interferometer can resolve the geometric structure of the red giant in symbiotic stars and help answer this question. This work presents the results of an optical interferometric study using the Center for High Angular Resolution Astronomy (CHARA) Array for the purpose of measuring the angular diameter of and imaging the cool giant in four symbiotic and related systems. Here we report H -band observations collected with MIRC-X. Model fitting and image reconstruction are used to test for Roche-lobe-filling geometries. Near-simultaneous infrared spectroscopy taken using the NASA InfraRed Telescope Facility (IRTF) is used to determine the fundamental stellar parameters of the cool giant in each system. The parametric fits reported here favor circularly symmetric disk models over elongated geometries, while imaging suggests the presence of surface features on three of these stars. We find that the three systems with constrained orbits have inferred time-averaged filling factors below unity.

Yellow hypergiants are rare, evolved massive stars generally interpreted as post-red-supergiant objects evolving blueward through or near the dynamically unstable yellow evolutionary void. V509 Cassiopeiae is a remarkable example: its spectroscopically inferred effective temperature increased from approximately 5000 K in the mid-1980s to about 7900 K by the late 1990s and has since remained near this value during its postheating state. We use simultaneous six-telescope CHARA observations obtained in 2023 with MIRC-X in the H band and MYSTIC in the K band to measure its photospheric diameter, search for a luminous companion, and constrain any spatially extended CO atmosphere. Interferometric data fitting with fixed atmosphere-model center-to-limb variation profiles yield limb-darkened angular diameters of 1.1840–1.1950 mas. Although nominally about 5% smaller than the previous Palomar Testbed Interferometer measurement, the difference is only 1.8 σ , indicating no significant secular change over 17–21 yr. The closure phases are consistent with a centrosymmetric brightness distribution, and no near-infrared-bright companion is detected over the sampled separations. The median 3 σ limits are Δ H = 5.28 mag and Δ K = 6.48 mag. The B1 V companion proposed from ultraviolet spectroscopy is expected to lie below these limits and is not excluded. The CO first-overtone channels give a weighted mean CO-to-continuum diameter ratio of 1.011 ± 0.004, below the adopted 3 σ detection criterion. We find no significant CO-band angular extension. V509 Cas is thus hot and molecularly compact despite retaining spectroscopically visible atomic circumstellar gas.

Limb darkening, the decrease in stellar intensity from the disk center to the limb, encodes the temperature structure and opacity of stellar atmospheres. Direct spatially resolved measurements of this center-to-limb variation remain scarce, especially in the near-infrared. We present interferometric limb-darkening measurements for 31 stars observed simultaneously in the H and K bands with the CHARA Array. The sample spans spectral types F–M and luminosity classes IV–I. The targets are well resolved in H and, for most targets, also in K . This coverage constrains the visibility curvature associated with limb darkening in joint H + K fits. We fit the combined H + K squared visibilities with four analytic limb-darkening laws and compare the resulting coefficients with bandpass-matched predictions from five stellar-atmosphere grids (Kurucz, MPS1, MPS2, Stagger, and spherical SATLAS, with reported coefficients placed on the Rosseland-radius convention) spanning one-dimensional plane-parallel, three-dimensional radiation–hydrodynamic, and spherical low-gravity models. The associated limb-darkened angular diameters are measured with median formal precisions of ≃0.2%–0.3%. The CHARA results show expected weaker limb darkening at longer wavelengths and also with increasing T _eff . The clearest discrepancy with the atmosphere grids is in the wavelength dependence: the median fractional decrease in the power-law coefficient from H to K is ≃39% in the CHARA sample, compared with only ≃17%–22% with model predictions. Relative to the MPS2 comparison, the empirical coefficients are higher by ≃21% in H and lower by ≃5% in K . Synthetic recovery tests indicate that the excess scatter is unlikely to be explained solely by incomplete spatial-frequency coverage or numerical instability in the fitting pipeline. These results provide multiband interferometric limb darkening as a near-infrared benchmark for stellar-atmosphere models.

Context. The suite called Stellar Parameters and Images with a Cophased Array (SPICA) has two interferometric instruments installed at the focus of the CHARA Array located at Mount Wilson, CA. SPICA is made of SPICA-VIS, a fiber-fed six-beam visible spectrograph with three spectral resolutions, and SPICA-FT, a six-beam near-infrared fringe tracker for the fast stabilization of the fringes. Aims. SPICA is opening access to imaging in the visible domain with an unprecedented angular resolution down to 0.2 milliarcseconds. It has been designed around a large survey of fundamental parameters of stars over the Hertzsprung-Russell diagram, aiming at understanding the deviations from the standard empirical relations of stellar physics as a function of activity: limb darkening, multiplicity, rotation, winds, and environments. Methods. SPICA makes use of the advanced technologies in electron multiplying detectors in the visible and electron-avalanche photodiode arrays in the near-infrared. It benefits from the newly commissioned adaptive optics on the one-meter telescopes of the array. The modules of the visible instrument, SPICA-VIS, optimize the injection of light into single-mode fibers for spatial filtering before spectral dispersion in the image plane. The fringe tracker, SPICA-FT, performs group-delay and phase-delay tracking for six beams in the H band. SPICA-FT can use an all-in-one or ABCD encoding of the fringe signals. Results. We present the results of the SPICA commissioning and discuss the initial performance that has been established. In low-resolution mode, SPICA reaches a limiting magnitude of 6.5, consistent with the initial estimate without fringe-tracking stabilization. Conclusions. SPICA is operational on sky and is close to reaching the expected performance in low spectral resolution, in particular, for the Interferometric Survey of Stellar Parameters (ISSP). More work is still needed to achieve the ultimate performance in terms of sensitivity and to allow operations with higher spectral resolutions.

We present the visual orbits and dynamical masses of two longer period spectroscopic binary stars, HD 210763 and HD 221950, using long baseline interferometry with the CHARA Array and high resolution spectroscopy with the APO 3.5 m and CTIO 1.5 m telescopes. By combining the astrometric and radial velocity observations, we solve for the full, three-dimensional orbits and determine the stellar masses to within 0.5% uncertainty and the distance to within 0.2% uncertainty. For HD 210763, we found component masses of M ₁ = 1.748 ± 0.008 M _⊙ , M ₂ = 1.492 ± 0.006 M _⊙ . For HD 221950, we found masses of M ₁ =1.098 ± 0.006 M _⊙ , M ₂ = 1.031 ± 0.005 M _⊙ . We then estimate the effective temperature and radius of each component star through disentangling and spectral energy distribution analyses. We compare the observed stellar parameters to the predictions of the stellar evolution models and estimate the system ages. The primary component of HD 210763 is at the end of the main sequence while the secondary component is on the main sequence, providing a tight age constraint for this system at 1.6 ± 0.1 Gyr. Both components of HD 221950 are still on the main sequence with an age of 3.76 ± 0.38 Gyr. These systems have longer orbital periods, beyond the tidal circularization limit, and therefore are better proxies for single stars and tests of stellar evolution models than short-period, eclipsing systems.

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 ( H band) and MYSTIC ( K band) 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 the H band and 0.72 ± 0.07 mas in the K band. Adopting a distance of 91 4 − 22 + 24 pc, the corresponding stellar radius is 69 ± 5 R _⊙ in the H band and 71 ± 8 R _⊙ in the K band. These values are consistent with the Roche-lobe volume radius of 71 R _⊙ inferred from published orbital solutions. These measurements provide a preeruption angular diameter and support a Roche-lobe-filling donor.

In this paper we present a newly detected companion to the Be star, HD 52244 (B2IVnpe), using the Fine Guidance Sensors (FGS) on the Hubble Space Telescope (HST). In fall 2021, HST became momentarily unavailable to support nominal operations, and we used the operational FGS to carry out a multiplicity survey of six Be stars. We were able to resolve a companion to HD 52244, with a separation of 42.7 ± 1.1 mas (74 au) and a position angle of 144.2 ± 0.3 with a differential magnitude in the F583W filter of 1.91 ± 0.02 mag. This study presents the results of the newly detected companion of HD 52244 and lays the groundwork for future studies looking for wide or third companions to Be stars.

The Galactic Be star binary MWC 656 was long considered the only known Be star+black hole (BH) system, making it a critical benchmark for models of massive binary evolution and for the expected X-ray emission of Be+BH binaries. However, recent dynamical measurements cast doubt on the presence of a BH companion. We present new multi-epoch ultraviolet spectroscopy from the Hubble Space Telescope, combined with high-resolution optical spectra, to reassess the nature of the companion. The far-ultraviolet spectra reveal high-ionisation features – including prominent N V and He II lines – which are absent in the spectra of normal Be stars and are indicative of a hot, luminous companion. Spectral modelling shows that these features cannot originate from the Be star or from an accretion disc around a compact object. Instead, we find that the data are best explained by a hot ( T _eff ≈ 85 kK), compact, hydrogen-deficient star with strong wind signatures, consistent with an intermediate-mass stripped star. Our revised orbital solution and composite spectroscopic modelling yield a companion mass of M ₂ = 1.48 ^+0.55 _−0.46 M _⊙ , definitively ruling out a BH and disfavouring a white dwarf. MWC 656 thus joins the growing class of Be + stripped star binaries. The system’s unusual properties – including a high companion temperature and wind strength – extend the known parameter space of such binaries. The continued absence of confirmed OBe+BH binaries in the Galaxy highlights a growing tension with population synthesis models.

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 triple α Piscium. Using H -band fringe tracking on component A with MIRC-X to stabilize K -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 equal H / 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 of P = 25 days, eccentricity e ≃ 0.6, and inclination i ≃ 65°, yielding precise dynamical masses of 1.668 ± 0.033 M _⊙ and 1.646 ± 0.029 M _⊙ . No additional companion is detected down to Δ H ≈ 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 . ″ 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 establish α Psc 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.