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1. MYSTIC: a high angular resolution K-band imager at CHARA

   https://doi.org/10.1117/12.2629437  

   Setterholm, Benjamin R. ; Monnier, John D. ; Le Bouquin, Jean-Baptiste ; Anugu, Narsireddy ; Ennis, Jacob ; Flores, Becky ; Gardner, Tyler ; Ibrahim, Nour ; Jocou, Laurent ; Kraus, Stefan ; et al ( August 2022 , SPIE Astronomical Instrumentation)

   Mérand, Antoine ; Sallum, Stephanie ; Sanchez-Bermudez, Joel (Ed.)

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

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2. Ultrafast laser inscription of asymmetric integrated waveguide 3  dB couplers for astronomical K-band interferometry at the CHARA array

   https://doi.org/10.1364/JOSAB.423727  

   Benoît, Aurélien ; Pike, Fraser A. ; Sharma, Tarun K. ; MacLachlan, David G. ; Dinkelaker, Aline N. ; Nayak, Abani S. ; Madhav, Kalaga ; Roth, Martin M. ; Labadie, Lucas ; Pedretti, Ettore ; et al ( August 2021 , Journal of the Optical Society of America B)

   We present the fabrication and characterization of 3 dB asymmetric directional couplers for the astronomical K-band at wavelengths between 2.0 and 2.4 µm. The couplers were fabricated in commercial Infrasil silica glass using an ultrafast laser operating at 1030 nm. After optimizing the fabrication parameters, the insertion losses of straight single-mode waveguides were measured to be$\sim <\#comment/>1.2\pm <\#comment/>0.5\mathrm{d}\mathrm{B}$across the full K-band. We investigate the development of asymmetric 3 dB directional couplers by varying the coupler interaction lengths and by varying the width of one of the waveguide cores to detune the propagation constants of the coupled modes. In this manner, we demonstrate that ultrafast laser inscription is capable of fabricating asymmetric 3 dB directional couplers for future applications in K-band stellar interferometry. Finally, we demonstrate that our couplers exhibit an interferometric fringe contrast of$><\#comment/>90\mathrm{\%<\#comment/>}$. This technology paves the path for the development of a two-telescope K-band integrated optic beam combiner for interferometry to replace the existing beam combiner (MONA) in Jouvence of the Fiber Linked Unit for Recombination (JouFLU) at the Center for High Angular Resolution Astronomy (CHARA) telescope array.

    

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3. Long Term Evolution of Surface Features on the Red Supergiant AZ Cyg

   https://doi.org/10.3847/1538-4357/ac0c7e  

   Norris, Ryan P. ; Baron, Fabien R. ; Monnier, John D. ; Paladini, Claudia ; Anderson, Matthew D. ; Martinez, Arturo O. ; Schaefer, Gail H. ; Che, Xiao ; Chiavassa, Andrea ; Connelley, Michael S. ; et al ( October 2021 , The Astrophysical Journal)

   We presentH-band interferometric observations of the red supergiant (RSG) AZ Cyg that were made with the Michigan Infra-Red Combiner (MIRC) at the six-telescope Center for High Angular Resolution Astronomy (CHARA) Array. The observations span 5 yr (2011–2016), which offers insight into the short and long-term evolution of surface features on RSGs. Using a spectrum of AZ Cyg obtained with SpeX on the NASA InfraRed Telescope Facility (IRTF) and synthetic spectra calculated from spherical MARCS, spherical PHOENIX, and SAtlas model atmospheres, we deriveT_effis between 3972 K and 4000 K and$\log g$between −0.50 and 0.00, depending on the stellar model used. Using fits to the squared visibility and GAIA parallaxes, we measure its average radius$R={911}_{-50}^{+57}{R}_{\odot }$. Reconstructions of the stellar surface using our model-independent imaging codes SQUEEZE and OITOOLS.jl show a complex surface with small bright features that appear to vary on a timescale of less than one year and larger features that persist for more than one year. The 1D power spectra of these images suggest a characteristic size of 0.52–0.69R_⋆for the larger, long lived features. This is close to the values of 0.51–0.53R_⋆that are derived from 3D RHD models of stellar surfaces. We conclude that interferometric imaging of this star is in line with predictions of 3D RHD models but that short-term imaging is needed to more stringently test predictions of convection in RSGs.

    

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4. Dusty disk winds at the sublimation rim of the highly inclined, low mass young stellar object SU Aurigae

   https://doi.org/10.1051/0004-6361/201935331  

   Labdon, Aaron ; Kraus, Stefan ; Davies, Claire L. ; Kreplin, Alexander ; Kluska, Jacques ; Harries, Tim J. ; Monnier, John D. ; ten Brummelaar, Theo ; Baron, Fabien ; Millan-Gabet, Rafael ; et al ( July 2019 , Astronomy & Astrophysics)

   Context. T Tauri stars are low-mass young stars whose disks provide the setting for planet formation. Despite this, their structure is poorly understood. We present new infrared interferometric observations of the SU Aurigae circumstellar environment that offer resolution that is three times higher and a better baseline position angle coverage than previous observations. Aims. We aim to investigate the characteristics of the circumstellar material around SU Aur, constrain the disk geometry, composition and inner dust rim structure. Methods. The CHARA array offers unique opportunities for long baseline observations, with baselines up to 331 m. Using the CLIMB three-telescope combiner in the K -band allows us to measure visibilities as well as closure phase. We undertook image reconstruction for model-independent analysis, and fitted geometric models such as Gaussian and ring distributions. Additionally, the fitting of radiative transfer models constrain the physical parameters of the disk. For the first time, a dusty disk wind is introduced to the radiative transfer code TORUS to model protoplanetary disks. Our implementation is motivated by theoretical models of dusty disk winds, where magnetic field lines drive dust above the disk plane close to the sublimation zone. Results. Image reconstruction reveals an inclined disk with slight asymmetry along its minor-axis, likely due to inclination effects obscuring the inner disk rim through absorption of incident star light on the near-side and thermal re-emission and scattering of the far-side. Geometric modelling of a skewed ring finds the inner rim at 0.17 ± 0.02 au with an inclination of 50.9 ± 1.0° and minor axis position angle 60.8 ± 1.2°. Radiative transfer modelling shows a flared disk with an inner radius at 0.18 au which implies a grain size of 0.4 μ m assuming astronomical silicates and a scale height of 15.0 at 100 au. Among the tested radiative transfer models, only the dusty disk wind successfully accounts for the K -band excess by introducing dust above the mid-plane. 

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5. MIRC-X polarinterferometry at CHARA

   https://doi.org/10.1117/12.2562407  

   Setterholm, Benjamin R. ; Monnier, John D. ; Le Bouquin, Jean-Baptiste ; Anugu, Narsireddy ; Labdon, Aaron ; Ennis, Jacob ; Johnson, Keith J. ; Kraus, Stefan ; ten Brummelaar, Theo A. ( December 2020 , Proceedings of the SPIE)

   Mérand, Antoine ; Sallum, Stephanie ; Tuthill, Peter G. (Ed.)

   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. 

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