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Abstract The outer regions of the protoplanetary disc surrounding the T Tauri star HD 143006 show rings, dust asymmetries and shadows. Whilst rings and dust asymmetries can arise from companions and other mechanisms, shadows and misaligned discs in particular are typically attributed to the presence of misaligned planets or stellar-mass companions. To understand the mechanisms that drive these traits, the innermost regions of discs need to be studied. Using CHARA/MIRCX and VLTI/PIONIER, we observed the sub-au region of HD 143006 . We constrain the orientation of the inner disc of HD 143006 and probe whether a misalignment between the inner and outer disc could be the cause of the shadows. Modelling the visibilities using a geometric model, the inclination and position angle are found to be i = 22○ ± 3○ and PA = 158○ ± 8○ respectively, with an inner dust sublimation radius of ~0.04 au. The inner disc is misaligned by 39○ ± 4○ with respect to the outer disc, with the far side of the inner disc to the east and the far side of the outer disc to the west. We constrain h/R (scattering surface/radius of scattered light) of the outer disc at 18 au to be about 13 % by calculating the offset between the shadow position and the central star. No companion was detected, with a magnitude contrast of 4.4 in the H-band and placing an upper mass limit of 0.17M⊙ at separations of 0 − 8 au. Therefore, we cannot confirm or rule out that a low-mass star or giant planet is responsible for the misalignment and dust sub-structures. 

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. 

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. 

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. 

Context . We implement an electron avalanche photodiode (e-APD) in the MIRC-X instrument, which is an upgrade of the six-telescope near-infrared imager MIRC, at the CHARA array. This technology should improve the sensitivity of near-infrared interferometry. Aims . We aim to characterize a near-infrared C-RED ONE camera from First Light Imaging (FLI) using an e-APD from Leonardo (previously SELEX). Methods . We first used the classical mean-variance analysis to measure the system gain and the amplification gain. We then developed a physical model of the statistical distribution of the camera output signal. This model is based on multiple convolutions of the Poisson statistic, the intrinsic avalanche gain distribution, and the observed distribution of the background signal. At low flux level, this model independently constrains the incident illumination level, the total gain, and the excess noise factor of the amplification. Results . We measure a total transmission of 48 ± 3% including the cold filter and the Quantum Efficiency. We measure a system gain of 0.49 ADU/e, a readout noise of 10 ADU, and amplification gains as high as 200. These results are consistent between the two methods and therefore validate our modeling approach. The measured excess noise factor based on the modeling is 1.47 ± 0.03, with no obvious dependency with flux level or amplification gain. Conclusions . The presented model allows the characteristics of the e-APD array to be measured at low flux level independently of a preexisting calibration. With < 0.3 electron equivalent readout noise at kilohertz frame rates, we confirm the revolutionary performances of the camera with respect to the PICNIC or HAWAII technologies. However, the measured excess noise factor is significantly higher than what is claimed in the literature (< 1.25), and explains why counting multiple photons remains challenging with this camera. 

Context. Asymptotic giant branch (AGB) stars are cool luminous evolved stars that are well observable across the Galaxy and populating Gaia data. They have complex stellar surface dynamics, which amplifies the uncertainties on stellar parameters and distances. Aims. On the AGB star CL Lac, it has been shown that the convection-related variability accounts for a substantial part of the Gaia DR2 parallax error. We observed this star with the MIRC-X beam combiner installed at the CHARA interferometer to detect the presence of stellar surface inhomogeneities. Methods. We performed the reconstruction of aperture synthesis images from the interferometric observations at different wavelengths. Then, we used 3D radiative hydrodynamics (RHD) simulations of stellar convection with CO5BOLD and the post-processing radiative transfer code O PTIM 3D to compute intensity maps in the spectral channels of MIRC-X observations. Then, we determined the stellar radius using the average 3D intensity profile and, finally, compared the 3D synthetic maps to the reconstructed ones focusing on matching the intensity contrast, the morphology of stellar surface structures, and the photocentre position at two different spectral channels, 1.52 and 1.70 μ m, simultaneously. Results. We measured the apparent diameter of CL Lac at two wavelengths (3.299 ± 0.005 mas and 3.053 ± 0.006 mas at 1.52 and 1.70 μ m, respectively) and recovered the radius ( R = 307 ± 41 and R = 284 ± 38 R ⊙ ) using a Gaia parallax. In addition to this, the reconstructed images are characterised by the presence of a brighter area that largely affects the position of the photocentre. The comparison with 3D simulation shows good agreement with the observations both in terms of contrast and surface structure morphology, meaning that our model is adequate for explaining the observed inhomogenities. Conclusions. This work confirms the presence of convection-related surface structures on an AGB star of Gaia DR2. Our result will help us to take a step forward in exploiting Gaia measurement uncertainties to extract the fundamental properties of AGB stars using appropriate RHD simulations. 

Context . The Gl 486 system consists of a very nearby, relatively bright, weakly active M3.5 V star at just 8 pc with a warm transiting rocky planet of about 1.3 R ⊕ and 3.0 M ⊕ . It is ideal for both transmission and emission spectroscopy and for testing interior models of telluric planets. Aims . To prepare for future studies, we aim to thoroughly characterise the planetary system with new accurate and precise data collected with state-of-the-art photometers from space and spectrometers and interferometers from the ground. Methods . We collected light curves of seven new transits observed with the CHEOPS space mission and new radial velocities obtained with MAROON-X at the 8.1 m Gemini North telescope and CARMENES at the 3.5 m Calar Alto telescope, together with previously published spectroscopic and photometric data from the two spectrographs and TESS. We also performed near-infrared interferometric observations with the CHARA Array and new photometric monitoring with a suite of smaller telescopes (AstroLAB, LCOGT, OSN, TJO). This extraordinary and rich data set was the input for our comprehensive analysis. Results . From interferometry, we measure a limb-darkened disc angular size of the star Gl 486 at θ LDD = 0.390 ± 0.018 mas. Together with a corrected Gaia EDR3 parallax, we obtain a stellar radius R * = 0.339 ± 0.015 R ⊕ . We also measure a stellar rotation period at P rot = 49.9 ± 5.5 days, an upper limit to its XUV (5-920 A) flux informed by new Hubble /STIS data, and, for the first time, a variety of element abundances (Fe, Mg, Si, V, Sr, Zr, Rb) and C/O ratio. Moreover, we imposed restrictive constraints on the presence of additional components, either stellar or sub-stellar, in the system. With the input stellar parameters and the radial-velocity and transit data, we determine the radius and mass of the planet Gl 486 b at R p = 1.343 −0.062 +0.063 R ⊕ and M p = 3.00 −0.12 +0.13 M ⊕ , with relative uncertainties of the planet radius and mass of 4.7% and 4.2%, respectively. From the planet parameters and the stellar element abundances, we infer the most probable models of planet internal structure and composition, which are consistent with a relatively small metallic core with respect to the Earth, a deep silicate mantle, and a thin volatile upper layer. With all these ingredients, we outline prospects for Gl 486 b atmospheric studies, especially with forthcoming James Webb Space Telescope ( Webb ) observations.