Detection and Astrometry of the Ba-Bb Subsystem in $\alpha$ Piscium: First Dual-Field Interferometry at the CHARA Array

Narsireddy Anugu; Robert Klement; John D. Monnier; Douglas R. Gies; Gail H. Schaefer; Stefan Kraus; Sebastián Carrazco-Gaxiola; Akshat S. Chaturvedi; Mayra Gutierrez; Becky Flores; Jeremy Jones; Colin Kane; Rainer Köhler; Karolina Kubiak; Olli W. Majoinen; Nicholas J. Scott; Kayvon Sharifi

arXiv · 2603.09489

Detection and Astrometry of the Ba-Bb Subsystem in $\alpha$ Piscium: First Dual-Field Interferometry at the CHARA Array

Narsireddy Anugu·Robert Klement·John D. Monnier·Douglas R. Gies·Gail H. Schaefer·Stefan Kraus·Sebastián Carrazco-Gaxiola·Akshat S. Chaturvedi·Mayra Gutierrez·Becky Flores·Jeremy Jones·Colin Kane·Rainer Köhler·Karolina Kubiak·Olli W. Majoinen·Nicholas J. Scott·Kayvon Sharifi

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Abstract

We present the first on-sky demonstration of dual-field interferometry at the CHARA Array and the first direct resolution of the inner Ba--Bb subsystem in the bright hierarchical triple $\alpha$ Piscium. Using $H$-band fringe tracking on component A with MIRC-X to stabilize $K$-band science fringes on component B with MYSTIC, we detected a companion at a projected separation of 7 mas, confirming a long-suspected but previously unresolved short-period subsystem within the B component. The nearly equal $H/K$-band flux ratio indicates that Ba and Bb are near-twin F-type stars, consistent with the two narrow-lined components seen in optical spectra of B. By combining CHARA interferometry with archival VLTI/GRAVITY astrometry and radial velocities from archival and new spectroscopy (NARVAL and ARCES), we derive a well-constrained orbit with a period of $P = 25$ d, eccentricity $e \simeq 0.6$, and inclination $i \simeq 65^\circ$, yielding precise dynamical masses of $1.668\pm0.033\,M_\odot$ and $1.646\pm0.029\,M_\odot$. No additional companion is detected down to $\Delta H \approx 5$ at separations of 0.2--2 AU. We also obtained dual-field differential astrometry of the wide A--B pair with a precision of ~0.234 mas at a separation of $1.85''$, with an error budget dominated by internal delay-line actuators, fringe-tracking performance and chromatic dispersion. While the long-period outer orbit is not refined by these measurements, their agreement with the published astrometric orbit provides an on-sky validation of the CHARA dual-field mode. These results establish $\alpha$ Psc as a well-characterized hierarchical system suitable for future benchmark studies and demonstrate CHARA's new capability for off-axis interferometry and sub-mas astrometry on arcsecond-scale binaries.

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Narsireddy Anugu, Robert Klement, John D. Monnier, Douglas R. Gies, Gail H. Schaefer, Stefan Kraus, Sebastián Carrazco-Gaxiola, Akshat S. Chaturvedi, Mayra Gutierrez, Becky Flores, Jeremy Jones, Colin Kane, Rainer Köhler, Karolina Kubiak, Olli W. Majoinen, Nicholas J. Scott, Kayvon Sharifi. 2026-03-10. Detection and Astrometry of the Ba-Bb Subsystem in $\alpha$ Piscium: First Dual-Field Interferometry at the CHARA Array. https://arxiv.org/abs/2603.09489

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