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

Narsireddy Anugu (The CHARA Array of Georgia State University, Mount Wilson Observatory, Mount Wilson, CA 91023, USA), Robert Klement (European Organisation for Astronomical Research in the Southern Hemisphere, Université Côte d'Azur, Observatoire de la Côte d'Azur, CNRS, Boulevard de l'Observatoire, CS 34229, 06304 Nice Cedex 4, France, The CHARA Array of Georgia State University, Mount Wilson Observatory, Mount Wilson, CA 91023, USA), John D. Monnier (Department of Astronomy, University of Michigan, Ann Arbor, MI 48109, USA), Douglas R. Gies (Center for High Angular Resolution Astronomy and Department of Physics and Astronomy, Georgia State University, P.O. Box 5060, Atlanta, GA 30302-5060, USA), Gail H. Schaefer (The CHARA Array of Georgia State University, Mount Wilson Observatory, Mount Wilson, CA 91023, USA), Stefan Kraus (Astrophysics Group, Department of Physics and Astronomy, University of Exeter, Stocker Road, Exeter, EX4 4QL, UK), Sebastián Carrazco-Gaxiola (Center for High Angular Resolution Astronomy and Department of Physics and Astronomy, Georgia State University, P.O. Box 5060, Atlanta, GA 30302-5060, USA), Akshat S. Chaturvedi (Center for High Angular Resolution Astronomy and Department of Physics and Astronomy, Georgia State University, P.O. Box 5060, Atlanta, GA 30302-5060, USA), Mayra Gutierrez (Department of Astronomy, University of Michigan, Ann Arbor, MI 48109, USA), Becky Flores (Center for High Angular Resolution Astronomy and Department of Physics and Astronomy, Georgia State University, P.O. Box 5060, Atlanta, GA 30302-5060, USA), Jeremy Jones (Center for High Angular Resolution Astronomy and Department of Physics and Astronomy, Georgia State University, P.O. Box 5060, Atlanta, GA 30302-5060, USA), Colin Kane (Center for High Angular Resolution Astronomy and Department of Physics and Astronomy, Georgia State University, P.O. Box 5060, Atlanta, GA 30302-5060, USA), Rainer Köhler (The CHARA Array of Georgia State University, Mount Wilson Observatory, Mount Wilson, CA 91023, USA), Karolina Kubiak (The CHARA Array of Georgia State University, Mount Wilson Observatory, Mount Wilson, CA 91023, USA), Olli W. Majoinen (The CHARA Array of Georgia State University, Mount Wilson Observatory, Mount Wilson, CA 91023, USA), Nicholas J. Scott (The CHARA Array of Georgia State University, Mount Wilson Observatory, Mount Wilson, CA 91023, USA), Kayvon Sharifi (Center for High Angular Resolution Astronomy and Department of Physics and Astronomy, Georgia State University, P.O. Box 5060, Atlanta, GA 30302-5060, USA)

Published Wed, 11 Ma

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Here is an explanation of the paper, translated from "astronomer-speak" into everyday language with some creative analogies.

The Big Picture: A New Telescope "Superpower"

Imagine the CHARA Array as a giant, high-tech camera made of six separate telescopes spread out across a mountain in California. Usually, this camera is like a very sharp pair of binoculars: it can see tiny details, but it has to look at one thing at a time. If you try to look at two stars that are close together, the camera gets confused, like trying to focus on a friend's face while standing next to a bright streetlamp.

This paper announces that the team has successfully installed a new "superpower" for this camera called Dual-Field Interferometry.

The Analogy: Think of it like a security guard at a busy airport.

Old Way: The guard could only watch one person (the bright star) to keep them steady, but if they tried to watch a second person standing nearby (the faint star), the camera would shake and blur.
New Way: The guard now has two pairs of eyes. One pair locks onto the bright, easy-to-see person to keep the camera steady (fringe tracking). The other pair uses that stability to take a crystal-clear photo of the second, fainter person standing right next to them.

The Target: A Cosmic Family Reunion

The team tested this new superpower on a star system called Alpha Piscium (or $\alpha$ Psc), which is like a famous, well-known family in the neighborhood.

The Parents (Star A & Star B): For centuries, astronomers knew this system had two main stars, A and B, orbiting each other far apart. Star A is a bit of a show-off (a "chemically peculiar" star with a strong magnetic field), and Star B was thought to be a single, quiet star.
The Mystery: For a long time, astronomers suspected Star B wasn't actually a single person. They thought it might be a "twin" pair of stars (let's call them Ba and Bb) hugging each other so tightly that no one could see them separately. It was like seeing a single silhouette in the fog and guessing there were two people standing back-to-back.

The Discovery: Unmasking the Twins

Using their new "Dual-Field" mode, the CHARA team did two amazing things:

1. They caught the twins in the act.
By using the bright Star A to stabilize the camera, they looked at Star B and finally saw the split. They discovered that Star B is actually two nearly identical stars (Ba and Bb) dancing around each other.

The Analogy: It's like looking at a distant streetlight through a foggy window and realizing it's actually two identical bulbs right next to each other.
The Details: These twins are so similar they are almost perfect clones (both are F-type stars). They orbit each other incredibly fast, completing a lap in just 25 days. They are also very close, separated by only about 7 milliarcseconds. To put that in perspective: if you were looking at a human hair from 10 kilometers (6 miles) away, that's roughly the size of the gap between these two stars.

2. They measured the family's dance.
Because they could see the twins so clearly, they could measure exactly how they move. By combining their new telescope data with old radio telescope data and computer models, they calculated:

Mass: Both twins weigh about the same as our Sun (roughly 1.6 times heavier).
Shape: Their orbit is very stretched out (like a flattened circle), not a perfect circle.
Distance: They confirmed the whole family is about 48 light-years away.

Why This Matters (The "So What?")

1. Solving a 40-Year Mystery
For decades, astronomers tried to figure out the orbit of these twins using spectroscopy (splitting light into rainbows), but it was like trying to hear two people whispering the same words at the same time in a noisy room. The new telescope allowed them to "see" the separation directly, finally solving the puzzle.

2. Proving the New Tech Works
The most important part of this paper isn't just the discovery; it's proving the new camera mode works.

They also measured the distance between the "parents" (Star A and Star B) with incredible precision.
The Analogy: Imagine you are trying to measure the distance between two houses on a hill. Usually, your ruler is a bit wobbly. With this new method, they measured the distance between the houses to within the width of a human hair, even though the houses are miles apart. This proves the new "Dual-Field" mode is ready for serious science.

3. Future Potential
Now that they know this trick works, they can use it to find faint planets or dim stars hiding next to bright ones all over the northern sky. It opens the door to finding "dark secrets" in the universe that were previously too close to their bright neighbors to be seen.

Summary in a Nutshell

The CHARA telescope team taught their camera a new trick: stabilize on a bright star to see a faint one next to it. They tested this on a famous star system, discovered that one of the stars was actually a pair of twins orbiting each other every 25 days, and proved that this new technology is precise enough to measure cosmic distances with the accuracy of a laser ruler. It's a major step forward in our ability to map the hidden architecture of the universe.

Here is a detailed technical summary of the paper "Detection and Astrometry of the Ba–Bb Subsystem in α Piscium: First Dual-Field Interferometry at the CHARA Array."

1. Problem Statement

The paper addresses two primary challenges in high-angular-resolution astronomy:

Unresolved Subsystems in Hierarchical Stars: The bright visual binary $\alpha$ Piscium ( $\alpha$ Psc) consists of an Ap star (Component A) and an Am star (Component B). While Component B was long suspected to be a spectroscopic binary (SB2) with nearly equal components, its inner structure remained unresolved due to the small angular separation and the limitations of single-field interferometry.
Limitations of Single-Field Interferometry: Traditional interferometers like CHARA operate in single-field mode, where fringe tracking and science observation occur on the same target. This limits sensitivity to faint companions near bright stars and prevents precise differential astrometry on wide binaries (separations $>1''$ ) because the fainter component cannot be tracked independently without losing coherence.

2. Methodology

The authors conducted a commissioning run to demonstrate dual-field interferometry at the CHARA Array, utilizing a novel configuration to observe $\alpha$ Psc.

Instrumentation:
- Telescopes: All six 1-meter telescopes of the CHARA Array (baselines 34–331 m).
- Beam Combiners:
  - MIRC-X: Operating in the H-band (1.5–1.75 $\mu$ m).
  - MYSTIC: Operating in the K-band (2.0–2.4 $\mu$ m).
- Dual-Field Strategy: The team implemented a cross-band dual-field mode.
  - Fringe Tracking: The brighter Component A was injected into one combiner (e.g., MIRC-X) to provide real-time group-delay fringe tracking.
  - Science Observation: The fainter Component B was injected into the second combiner (e.g., MYSTIC) to record science fringes.
  - Differential Delay: Internal Differential Delay Lines (DDLs) were used to compensate for the geometric optical path difference (OPD) between the two stars, allowing coherent integration on the fainter component while the bright component stabilized the fringes.
- Configuration Swaps: To validate the system and mitigate chromatic dispersion, the team performed four configurations: (A in MIRC-X/B in MYSTIC), (B in MIRC-X/A in MYSTIC), and their reverses.
Data Synthesis:
- Interferometry: Combined new CHARA data with archival VLTI/GRAVITY dual-field calibration data (7 snapshots).
- Spectroscopy: Analyzed 56 archival spectra from NARVAL (TBL telescope), 3 new spectra from ARCES (Apache Point Observatory), and historical RVs from Abt et al. (1980, 1985).
- Photometry: Analyzed TESS light curves for Component A to characterize rotation and search for companions.
- Orbital Fitting: Used orbfit-lib for the inner Ba–Bb orbit and orbitize! for the outer A–B orbit, combining astrometric and spectroscopic constraints.

3. Key Contributions

First On-Sky Dual-Field Demonstration at CHARA: This is the first successful implementation of dual-field interferometry at the CHARA Array, proving the feasibility of off-axis fringe tracking and sub-mas astrometry on arcsecond-scale binaries.
Direct Resolution of the Ba–Bb Subsystem: The study provides the first direct spatial resolution of the inner binary within Component B, which had previously only been inferred spectroscopically.
Comprehensive System Characterization: The paper delivers a complete dynamical solution for the hierarchical triple system, including precise masses, orbital elements, and distances for both the inner (Ba–Bb) and outer (A–B) orbits.

4. Key Results

A. The Inner Ba–Bb Subsystem

Detection: Directly resolved a companion at a projected separation of $\sim7$ mas.
Nature: The components are near-twin F-type stars with an H/K-band flux ratio of $\approx 1.0$ (equal brightness), explaining the difficulty in previous spectroscopic orbit determination.
Orbital Parameters:
- Period ( $P$ ): $24.9994 \pm 0.00003$ days.
- Eccentricity ( $e$ ): $0.6004 \pm 0.0018$.
- Inclination ( $i$ ): $65.20 \pm 0.40^\circ$.
- Semimajor Axis: $0.2493 \pm 0.0020 $AU ($ 5.175 \pm 0.018$ mas).
Dynamical Masses:
- $M_{Ba} = 1.668 \pm 0.033 M_\odot$
- $M_{Bb} = 1.646 \pm 0.029 M_\odot$
- Total inner mass: $3.315 \pm 0.060 M_\odot$.
Distance: The dynamical parallax yields a distance of $48.17 \pm 0.34$ pc, consistent with Gaia EDR3 data for the wide tertiary companion.

B. Component A (Primary)

Resolution: Partially resolved with a uniform-disk diameter of $\theta_{UD} \approx 0.45 \pm 0.05$ mas ( $R \approx 2.33 R_\odot$ ).
Companions: No close companions detected down to $\Delta H \approx 5$ at separations of 0.2–2 AU.
Rotation: TESS data confirms a rotation period of $P_{rot} = 1.490975$ days, consistent with magnetic modulation observed in radio and photometry.

C. Wide A–B Astrometry

Measurement: Dual-field differential astrometry measured the separation of the wide pair ( $\rho = 1850.063 \pm 0.234$ mas) with a position angle of $256.567 \pm 0.009^\circ$.
Precision: Achieved a precision of $\sim0.234$ mas (234 $\mu$ as).
Validation: The measurements align perfectly with the long-period (3.3 kyr) orbital trajectory predicted by the ORB6 catalog, serving as an on-sky validation of the dual-field mode rather than a revision of the outer orbit.

5. Significance and Future Outlook

Technological Milestone: This work establishes CHARA as a facility capable of off-axis interferometry and sub-mas astrometry on wide binaries, a capability previously unique to VLTI/GRAVITY but now extended to the Northern Hemisphere and wider separations ( $\sim5''$ ).
Astrophysical Implications:
- The $\alpha$ Psc system is now a fully characterized hierarchical triple, providing a benchmark for studying chemical peculiarities (Ap/Am stars) in intermediate-mass systems.
- The mutual orientation of the inner and outer orbits is nearly perpendicular ( $\Phi \approx 80^\circ$ or $119^\circ$), suggesting the system may undergo von Zeipel–Lidov–Kozai oscillations, which could influence the eccentricity and evolution of the inner binary.
Error Budget & Future Upgrades: The current astrometric floor ( $\sim227$ $\mu$ as) is dominated by DDL repeatability and fringe-tracking residuals. The authors outline that future upgrades (SPICA-FT for phase tracking, Longitudinal Dispersion Correctors) could push precision toward the 100 $\mu$ as regime, enabling the detection of fainter companions and more complex hierarchical systems.

In summary, this paper successfully commissions a new observing mode at CHARA, solves a long-standing mystery regarding the structure of $\alpha$ Psc, and opens a new window for high-precision astrometry on wide binary systems in the Northern sky.

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