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Abstract of presentation: https://eas2025programme.kuoni-congress.info/pdf/presentation/the-new-mobile-telescope-at-the-chara-array 

Abstract: https://eas2025programme.kuoni-congress.info/pdf/presentation/the-chara-array-building-towards-the-michelson-array 

Abstract: https://baas.aas.org/pub/2025n2i302p27/release/1 

Abstract: https://horizons-olbin.sciencesconf.org/file/1181666 

Context.Many classical Be stars acquire their very rapid rotation by mass- and angular-momentum transfer in massive binaries, marking the first phase of the evolutionary chain. Later-stage products, such as Be+subdwarf- and Be+neutron-star binaries (Be X-ray binaries), are also well known, although the search for definitive proof of Be+white dwarf companions is ongoing. Short-lived intermediate-phase objects, that is, binaries past the interaction stage but with a donor star that has not yet reached the end of its evolution or contraction, have only recently been discovered. Aims.The main hallmark of this kind of binary is a system of absorption lines with low width, significant radial-velocity variations, and peculiar relative line strengths. Data archives and the literature can be searched for additional candidates exhibiting this pattern, and follow-up observations can be obtained in order to increase the number of these systems with quantitatively known orbits, providing a basis for an initial statistical investigation and to develop observational strategies for abundance analyses. Methods.We identified 13 candidates at various confidence levels. To verify their nature, we derived orbital elements from new high-quality spectra and interferometric observations where possible. We also performed qualitative analyses of other basic parameters, and preliminarily evaluated indicators of advanced stages of nucleosynthesis. Results.Adding to the two known systems identified as classical Be star+pre-subdwarf binaries (LB-1 andHR 6819), we confirm two more (V742 Cas,HD 44637) with interferometry, with V742 Cas setting a new record for the smallest visually observed angular semi-major axis, ata = 0.663 mas. Two further systems (V447 Sct,V1362 Cyg) are not resolved interferometrically, but other evidence puts them at the same confidence level as LB-1.V2174 Cygis a candidate with very high confidence, but was not observed interferometrically. The remaining systems are either candidates with varying levels of confidence –mainly due to the lack of available spectroscopic or interferometric observations for comparison with the others and orbit determination– or could be rejected as candidates with the followup observations. Conclusions.Of a mostly magnitude-complete sample of 328 Be stars, 0.5–1% are found to have recently completed the mass overflow that led to their formation. Another 5% are systems with a compact subdwarf companion –that is, they are further evolved after a previous overflow– and a further 2% possibly harbor white dwarfs. All these systems are early B subtypes, but if the original sample is restricted to early subtypes (136 objects), these percentages increase by a factor of about 2.5, while dropping to zero for the mid and late subtypes (together 204 objects). This strongly suggests that early-type versus mid- and late-type Be stars follow differently weighted channels to acquire their rapid rotation, namely binary interaction versus evolutionary spin up. 

Abstract This article presents the latest results of our Atacama Large Millimeter/submillimeter Array (ALMA) program to study circumstellar disk characteristics as a function of orbital and stellar properties in a sample of young binary star systems known to host at least one disk. Optical and infrared observations of the eccentric, ∼48 yr period binary DF Tau indicated the presence of only one disk around the brighter component. However, our 1.3 mm ALMA thermal continuum maps show two nearly equal-brightness components in this system. We present these observations within the context of updated stellar and orbital properties, which indicate that the inner disk of the secondary is absent. Because the two stars likely formed together, with the same composition, in the same environment, and at the same time, we expect their disks to be co-eval. However the absence of an inner disk around the secondary suggests uneven dissipation. We consider several processes that have the potential to accelerate inner disk evolution. Rapid inner disk dissipation has important implications for planet formation, particularly in the terrestrial-planet-forming region. 

Aims.We aim to accurately measure the dynamical mass and distance of Cepheids by combining radial velocity measurements with interferometric observations. Cepheid mass measurements are particularly necessary for solving the Cepheid mass discrepancy, while independent distance determinations provide a crucial test of the period–luminosity relation andGaiaparallaxes. Methods.We used the multi-telescope interferometric combiner, the Michigan InfraRed Combiner (MIRC) of the Center for High Angular Resolution Astronomy (CHARA) Array, to detect and measure the astrometric positions of the high-contrast companion orbiting the Galactic Cepheid SU Cygni. We also present new radial velocity measurements from ultraviolet spectra taken with theHubbleSpace Telescope. The combination of interferometric astrometry with optical and ultraviolet spectroscopy provided the full orbital elements of the system, in addition to component masses and the distance to the Cepheid system. Results.We measured the mass of the Cepheid,M_A = 4.859 ± 0.058 M_⊙, and its two companions,M_Ba = 3.595 ± 0.033 M_⊙andM_Bb = 1.546 ± 0.009 M_⊙. This is the most accurate existing measurement of the mass of a Galactic Cepheid (1.2%). Comparing with stellar evolution models, we show that the mass predicted by the tracks is higher than the measured mass of the Cepheid, which is similar to the conclusions of our previous work. We also measured the distance to the system to be 926.3 ± 5.0 pc, obtaining an unprecedented parallax precision of 6 μas (0.5%), which is the most precise and accurate distance for a Cepheid. This precision is similar to what is expected byGaiafor its last data release (DR5 in ∼2030) for single stars fainter thanG = 13, but is not guaranteed for stars as bright as SU Cyg. Conclusions.We demonstrate that evolutionary models remain incapable of accurately reproducing the measured mass of Cepheids, often predicting higher masses for the expected metallicity, even when factors such as rotation or convective core overshooting are taken into account. Our precise distance measurement allowed us to compare predictions from some period–luminosity relations. We find a disagreement of 0.2–0.5 mag with relations calibrated from photometry, while relations calibrated from a direct distance measurement are in better agreement. 

Abstract Close binary systems present challenges to planet formation. As binary separations decrease, so do the occurrence rates of protoplanetary disks in young systems and planets in mature systems. For systems that do retain disks, their disk masses and sizes are altered by the presence of the binary companion. Through the study of protoplanetary disks in binary systems with known orbital parameters, we seek to determine the properties that promote disk retention and therefore planet formation. In this work, we characterize the young binary−disk system FO Tau. We determine the first full orbital solution for the system, finding masses of ${0.35}_{-0.05}^{+0.06}{M}_{\odot }$and 0.34 ± 0.05M_⊙for the stellar components, a semimajor axis of $22{(}_{-1}^{+2})$au, and an eccentricity of $0.21{(}_{-0.03}^{+0.04})$. With long-baseline Atacama Large Millimeter/submillimeter Array interferometry, we detect 1.3 mm continuum and¹²CO (J= 2–1) line emission toward each of the binary components; no circumbinary emission is detected. The protoplanetary disks are compact, consistent with being truncated by the binary orbit. The dust disks are unresolved in the image plane, and the more extended gas disks are only marginally resolved. Fitting the continuum and CO visibilities, we determine the inclination of each disk, finding evidence for alignment of the disk and binary orbital planes. This study is the first of its kind linking the properties of circumstellar protoplanetary disks to a precisely known binary orbit. In the case of FO Tau, we find a dynamically placid environment (coplanar, low eccentricity), which may foster its potential for planet formation. 

Abstract The stellar companion to the weak-line T Tauri star DI Tau A was first discovered by the lunar occultation technique in 1989 and was subsequently confirmed by a speckle imaging observation in 1991. It has not been detected since, despite being targeted by five different studies that used a variety of methods and spanned more than 20 yr. Here, we report the serendipitous rediscovery of DI Tau B during our Young Exoplanets Spectroscopic Survey (YESS). Using radial velocity data from YESS spanning 17 yr, new adaptive optics observations from Keck II, and a variety of other data from the literature, we derive a preliminary orbital solution for the system that effectively explains the detection and (almost all of the) non-detection history of DI Tau B. We estimate the dynamical masses of both components, finding that the large mass difference ( q ∼ 0.17) and long orbital period (≳35 yr) make the DI Tau system a noteworthy and valuable addition to studies of stellar evolution and pre-main-sequence models. With a long orbital period and a small flux ratio (f2/f1) between DI Tau A and B, additional measurements are needed for a better comparison between these observational results and pre-main-sequence models. Finally, we report an average surface magnetic field strength ( B ¯ ) for DI Tau A, of ∼0.55 kG, which is unusually low in the context of young active stars. 

Context . The study of the multiplicity of massive stars gives hints on their formation processes and their evolutionary paths, which are still not fully understood. Large separation binaries (>50 milliseconds of arc, mas) can be probed by adaptive-optics-assisted direct imaging and sparse aperture masking, while close binaries can be resolved by photometry and spectroscopy. However, optical long baseline interferometry is mandatory to establish the multiplicity of Galactic massive stars at the separation gap between 1 and 50 mas. Aims . In this paper, we aim to demonstrate the capability of the new interferometric instrument MIRC-X, located at the CHARA Array, to study the multiplicity of O-type stars and therefore probe the full range of separation for more than 120 massive stars ( H < 7 . 5 mag). Methods . We initiated a pilot survey of bright O-type stars ( H < 6.5 mag) observable with MIRC-X. We observed 29 O-type stars, including two systems in average atmospheric conditions around a magnitude of H = 7.5 mag. We systematically reduced the obtained data with the public reduction pipeline of the instrument. We analyzed the reduced data using the dedicated python software CANDID to detect companions. Results . Out of these 29 systems, we resolved 19 companions in 17 different systems with angular separations between ~0.5 and 50 mas. This results in a multiplicity fraction ƒ m = 17/29 = 0.59 ± 0.09, and an average number of companions ƒ c = 19/29 = 0.66 ± 0.13. Those results are in agreement with the results of the SMASH+ survey in the Southern Hemisphere. Thirteen of these companions have been resolved for the first time, including the companion responsible for the nonthermal emission in Cyg OB2-5 A and the confirmation of the candidate companion of HD 47129 suggested by SMASH+. Conclusions . A large survey on more than 120 northern O-type stars ( H < 7.5) is possible with MIRC-X and will be fruitful.