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Ground-based optical long-baseline interferometry has the power to measure the orbits of close binary systems at ~10 micro-arcsecond precision. This precision makes it possible to detect "wobbles" in the binary motion due to the gravitational pull from additional short period companions. We started the ARrangement for Micro-Arcsecond Differential Astrometry (ARMADA) survey with the MIRC-X instrument at the CHARA array for the purpose of detecting giant planets and stellar companions orbiting individual stars in binary systems. We describe our observations for the survey, and introduce the wavelength calibration scheme that delivers precision at the tens of micro-arcseconds level for, 25 pages, 15 figures, accepted for publication in AJ

We present interferometric observations of the Be star Zeta Tau obtained using the MIRC beam combiner at the CHARA Array. We resolved the disk during four epochs in 2007-2009. We fit the data with a geometric model to characterize the circumstellar disk as a skewed elliptical Gaussian and the central Be star as a uniform disk. The visibilities reveal a nearly edge-on disk with a FWHM major axis of ~ 1.8 mas in the H-band. The non-zero closure phases indicate an asymmetry within the disk. Interestingly, when combining our results with previously published interferometric observations of Zeta Tau, we find a correlation between the position angle of the disk and the spectroscopic V/R ratio, suggesting that the tilt of the disk is precessing. This work is part of a multi-year monitoring campaign to investigate the development and outward motion of asymmetric structures in the disks of Be stars., Comment: Accepted for publication in the Astronomical Journal. 27 pages, 7 Figures

We present the modification of the orbits of χ Draconis and HD 184467, and a completely new orbit for HD 198084, including data taken at the Center for High Angular Resolution Astronomy (CHARA) Array. These data were obtained using a modification of the technique of separated fringe packets (SFPs). The accuracy of the SFP data surpasses that of data taken by speckle, but the technique is much more time and labor intensive. Additionally, using SFPs with the CHARA Array, it is possible to obtain separations below the detection range of speckle interferometry (30 mas) above the range in “classic” long-baseline interferometry where fringes from a binary overlap are no longer separated (10 mas). Using spectroscopic binary systems with published speckle orbits, we are able to test our new measurements against their ephemerides to calibrate the method as well as produce entirely new orbits for systems with no current astrometric observations.

We conducted interferometric observations with the CHARA Array of transiting super-Earth host HD 97658 and measured its limb-darkened angular diameter to be $\theta_{\text{LD}}=0.314\pm0.004$ mas. The combination of the angular diameter with the Gaia EDR3 parallax value with zero-point correction ($\pi=46.412\pm0.022$ mas, $d=21.546\pm0.011$ pc) yields a physical radius of $R_\star=0.728\pm0.008$ $R_\odot$. We also measured the bolometric flux of the star to be $F_\text{bol}=2.42\pm 0.05\times 10^{-8}$erg s$^{-1}$ cm$^{-2}$, which, together with angular size, allows a measurement of the effective temperature $T_{\text{eff}}=5212\pm43$ K. Our directly determined physical stellar properties are in good agreement with previous estimates derived from spectroscopy. We used our measurements in combination with stellar evolutionary models and properties of the transit of HD 97658 b to determine the mass and age of HD 97658 as well as constrain the properties of the planet. Our results and our analysis of the TESS lightcurve on the planet (TOI-1821) corroborate previous studies of this system with tighter uncertainties., Comment: 20 pages, 6 figures, and 3 tables. Accepted for publication in AJ

We present interferometric observations of the sigma Orionis triple system using the CHARA Array, NPOI, and VLTI. Using these measurements, we spatially resolve the orbit of the close spectroscopic binary (Aa,Ab) for the first time and present a revised orbit for the wide pair (A,B). Combining the visual orbits with previously published radial velocity measurements and new radial velocities measured at CTIO, we derive dynamical masses for the three massive stars in the system of M_Aa = 16.99 +/- 0.20 Msun, M_Ab = 12.81 +/- 0.18 Msun, and M_B = 11.5 +/- 1.2 Msun. The inner and outer orbits in the triple are not coplanar, with a relative inclination of 120-127 deg. The orbital parallax provides a precise distance of 387.5 +/- 1.3 pc to the system. This is a significant improvement over previous estimates of the distance to the young sigma Orionis cluster., Accepted by the Astronomical Journal. 23 pages, 11 figures, 11 tables