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Abstract We present preliminary results from our long-baseline interferometry (LBI) survey to constrain the multiplicity properties of intermediate-mass A-type stars within 80 pc. Previous multiplicity studies of nearby stars exhibit orbital separation distributions well fitted with a lognormal with peaks >15 au, increasing with primary mass. The A-star multiplicity survey of De Rosa et al., sensitive beyond 30 au but incomplete below 100 au, found a lognormal peak around 390 au. Radial velocity surveys of slowly rotating, chemically peculiar Am stars identified a significant number of very close companions with periods ≤5 days, ∼0.1 au, a result similar to surveys of O- and B-type primaries. With the improved performance of LBI techniques, we can probe these close separations for normal A-type stars where other surveys are incomplete. Our initial sample consists of 27 A-type primaries with estimated masses between 1.44 and 2.49 M ⊙ and ages 10–790 Myr, which we observed with the MIRC-X instrument at the CHARA Array. We use the open-source software CANDID to detect five companions, three of which are new, and derive a companion frequency of 0.19 − 0.06 + 0.11 over mass ratios of 0.25–1.0 and projected separations of 0.288–5.481 au. We find a probabilitymore »of 10 −6 that our results are consistent with extrapolations based on previous models of the A-star companion population over the mass ratios and separations sampled. Our results show the need to explore these very close separations to inform our understanding of stellar formation and evolution processes.« less

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, andmore »propose upgrades to the instrument that could improve its K-band sensitivity to 10th magnitude in the near future.« less

Context. Stellar evolution models are highly dependent on accurate mass estimates, especially for highly massive stars in the early stages of stellar evolution. The most direct method for obtaining model-independent stellar masses is derivation from the orbit of close binaries. Aims. Our aim was to derive the first astrometric plus radial velocity orbit solution for the single-lined spectroscopic binary star MWC 166 A, based on near-infrared interferometry over multiple epochs and ∼100 archival radial velocity measurements, and to derive fundamental stellar parameters from this orbit. A supplementary aim was to model the circumstellar activity in the system from K band spectral lines. Methods. The data used include interferometric observations from the VLTI instruments GRAVITY and PIONIER, as well as the MIRC-X instrument at the CHARA Array. We geometrically modelled the dust continuum to derive relative astrometry at 13 epochs, determine the orbital elements, and constrain individual stellar parameters at five different age estimates. We used the continuum models as a base to examine differential phases, visibilities, and closure phases over the Br γ and He  I emission lines in order to characterise the nature of the circumstellar emission. Results. Our orbit solution suggests a period of P  = 367.7 ± 0.1 d, approximatelymore »twice as long as found with previous radial velocity orbit fits. We derive a semi-major axis of 2.61 ± 0.04 au at d  = 990 ± 50 pc, an eccentricity of 0.498 ± 0.001, and an orbital inclination of 53.6 ± 0.3°. This allowed the component masses to be constrained to M 1  = 12.2 ± 2.2  M ⊙ and M 2  = 4.9 ± 0.5  M ⊙ . The line-emitting gas was found to be localised around the primary and is spatially resolved on scales of ∼11 stellar radii, where the spatial displacement between the line wings is consistent with a rotating disc. Conclusions. The large spatial extent and stable rotation axis orientation measured for the Br γ and He  I line emission are inconsistent with an origin in magnetospheric accretion or boundary-layer accretion, but indicate an ionised inner gas disc around this Herbig Be star. We observe line variability that could be explained either with generic line variability in a Herbig star disc or V/R variations in a decretion disc scenario. We have also constrained the age of the system, with relative flux ratios suggesting an age of ∼(7 ± 2)×10 5 yr, consistent with the system being composed of a main-sequence primary and a secondary still contracting towards the main-sequence stage.« less

Castor is a system of six stars in which the two brighter objects, Castor A and B, revolve around each other every ∼450 yr and are both short-period spectroscopic binaries. They are attended by the more distant Castor C, which is also a binary. Here we report interferometric observations with the Center for High Angular Resolution Astronomy (CHARA) array that spatially resolve the companions in Castor A and B for the first time. We complement these observations with new radial velocity measurements of A and B spanning 30 yr, with the Hipparcos intermediate data, and with existing astrometric observations of the visual AB pair obtained over the past three centuries. We perform a joint orbital solution to solve simultaneously for the three-dimensional orbits of Castor A and B as well as the AB orbit. We find that they are far from being coplanar: the orbit of A is nearly at right angles (92°) relative to the wide orbit, and that of B is inclined about 59° compared to AB. We determine the dynamical masses of the four stars in Castor A and B to a precision better than 1%. We also determine the radii of the primary stars ofmore »both subsystems from their angular diameters measured with the CHARA array, and use them together with stellar evolution models to infer an age for the system of 290 Myr. The new knowledge of the orbits enables us to measure the slow motion of Castor C as well, which may assist future studies of the dynamical evolution of this remarkable sextuple system.

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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. ObjectκDra 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 theHandKband, 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 1σerrors, we could derive the total mass and found component masses of 3.65 ± 0.48 and 0.426 ± 0.043M_⊙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 ofmore »such a star and makingκDra the first mid- to late-type Be star with a directly observed subdwarf companion.

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We observe the brightest member of the Praesepe cluster,ϵCnc, to precisely measure the characteristics of the stars in this binary system, en route to a new measurement of the cluster’s age. We present spectroscopic radial velocity measurements and interferometric observations of the sky-projected orbit to derive the masses, which we find to beM₁/M_⊙= 2.420 ± 0.008 andM₂/M_⊙= 2.226 ± 0.004. We place limits on the color–magnitude positions of the stars by using spectroscopic and interferometric luminosity ratios while trying to reproduce the spectral energy distribution ofϵCnc. We reexamine the cluster membership of stars at the bright end of the color–magnitude diagram using Gaia data and literature radial velocity information. The binary star data are consistent with an age of 637 ± 19 Myr, as determined from MIST model isochrones. The masses and luminosities of the stars appear to select models with the most commonly used amount of convective core overshooting.

We present multi-instrument observations of the disc around the Herbig Ae star, HD 145718, employing geometric and Monte Carlo radiative transfer models to explore the disc orientation, the vertical and radial extent of the near-infrared (NIR) scattering surface, and the properties of the dust in the disc surface and sublimation rim. The disc appears inclined at 67–71°, with position angle, PA = −1.0 to 0.6°, consistent with previous estimates. The NIR scattering surface extends out to ${\sim}75\,$ au and we infer an aspect ratio, hscat(r)/r ∼ 0.24 in J band; ∼0.22 in H band. Our Gemini Planet Imager images and VLTI + CHARA NIR interferometry suggest that the disc surface layers are populated by grains ≳λ/2π in size, indicating these grains are aerodynamically supported against settling and/or the density of smaller grains is relatively low. We demonstrate that our geometric analysis provides a reasonable assessment of the height of the NIR scattering surface at the outer edge of the disc and, if the inclination can be independently constrained, has the potential to probe the flaring exponent of the scattering surface in similarly inclined (i ≳ 70°) discs. In re-evaluating HD 145718’s stellar properties, we found that the object’s dimming events – previously characterized as UX Ormore »and dipper variability – are consistent with dust occultation by grains larger, on average, than found in the ISM. This occulting dust likely originates close to the inferred dust sublimation radius at $0.17\,$ au.

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We present a new polarimetric mode for the MIRC-X 6-telescope beam combiner at CHARA. Utilizing the extensive u - v coverage afforded by CHARA this mode will be able to resolve and constrain scattered light in environs at milliarcsecond separations of target stars, a largely unexplored parameter space to-date in astronomy. Notably, this upgrade will allow for the investigation of the scattering properties of the inner dust wall at the sublimation radius of Herbig Ae/Be star disks, dust shells surrounding evolved stars, and gas-rich disks around Be stars. Our design adds a series of rotating half-wave plates, achromatic across J- and H-bands, and a polarizing beamsplitter into the MIRC-X beam path. In this work, we also preview on-sky observations, discussing ongoing work calibrating instrumental polarization effects in the CHARA beam path as well as upgrades to the MIRC-X data reduction pipeline.

Classical Be stars are possible products of close binary evolution, in which the mass donor becomes a hot, stripped O- or B-type subdwarf (sdO/sdB), and the mass gainer spins up and grows a disk to become a Be star. While several Be+sdO binaries have been identified, dynamical masses and other fundamental parameters are available only for a single Be+sdO system, limiting the confrontation with binary evolution models. In this work, we present direct interferometric detections of the sdO companions of three Be stars—28 Cyg, V2119 Cyg, and 60 Cyg—all of which were previously found in UV spectra. For two of the three Be+sdO systems, we present first orbits and preliminary dynamical masses of the components, revealing that one of them could be the first identified progenitor of a Be/X-ray binary with a neutron star companion. These results provide new sets of fundamental parameters that are crucially needed to establish the evolutionary status and origin of Be stars.