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

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 $α$ 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 $ΔH \approx 5$ 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 $α$ 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.

Figures (13)

• Figure 1: Light from the six 1-m CHARA telescopes is relayed to the beam-combiner laboratory and imaged on the STST (Six Telescope Star Tracker) camera with a field of view of $\sim 5^{\prime\prime}$Anugu2025SPIE. Using the STST camera coordinates of the $\alpha$ Psc A--B pair, flux injection into the single-mode fibers is optimized: the brighter A component into the MIRC-X H-band fibers (blue), and the fainter B component into the MYSTIC K-band fibers (red). Each fiber pair (two-telescope fringes shown for clarity) produces a corresponding white-light fringe packet, whose relative offset is set by the sky separation of the two stars. Here we define the differential optical path difference (OPD) between the two stars (and thus between the two channels) as $\delta\mathrm{OPD}$. The measured fringe-packet separation $\delta\mathrm{OPD}$ therefore yields the instantaneous on-sky astrometric offset between the two stars with sub-mas precision. This dual-field scheme enables simultaneous fringe tracking on the bright component and long-exposure science measurements on the faint component. See detailed optical layout in Figure \ref{['fig:dual_star_optical_layout']} and implementation in Appendix \ref{['app:dualfield']}.
• Figure 2: Astrometric and spectroscopic orbit of the inner Ba--Bb subsystem in $\alpha$ Psc. (a): Relative astrometry of Ba--Bb, with measurements denoted by 10$\sigma$ error ellipses; CHARA dual-field measurements obtained in this work are shown in red, while the archival VLTI/GRAVITY measurements are shown in gray. The best-fit orbit of the Bb component is represented by the dashed curve, relative to the Ba component ("+" symbol). The positions on the best-fit orbit for the epochs of our observations are shown as "x" symbols. The magenta and cyan "+" symbols indicate the position of the periastron and the ascending node, respectively. The line of nodes is shown as a gray line. (b): RV curves for Ba and Bb components (solid and dashed curves, respectively), and the corresponding RV measurements ("x" symbols and open circles). The historical RVs taken from the literature are shown in blue, and our RV measurements from the archival NARVAL and newly obtained ARCES spectra are shown in black and magenta, respectively. The horizontal line corresponds to the systemic velocity. The error bars are too small to be seen for most RVs. The lower panel shows the residuals in units of the measurement errors $\sigma$, with the dotted lines showing $\pm3\sigma$. (c): Detail of the astrometric orbit showing the measurements as 3$\sigma$ error ellipses.
• Figure 3: Orbit of the $\alpha$ Piscium A--B wide binary. Gray curves show 200 posterior samples from our MCMC fit to the complete Washington Double Star astrometric record (1779--2024). Red points are the historical measurements compiled in ORB6, and the two black open circles mark the new dual-field astrometric detections obtained with CHARA in 2025. North is up and East is to the left. The CHARA measurements lie exactly along the predicted orbital path at $\rho \simeq 1.85"$ and ${\rm PA} \simeq 256.6^{\circ}$, demonstrating that the dual-field mode delivers sub-mas relative astrometry at arcsecond-level separations. Including the CHARA points does not materially change the orbital solution; all elements remain consistent with the published ORB6 orbit at the $\lesssim 1\%$ level.
• Figure 4: Schematic optical layout of the CHARA dual-field mode. For clarity we show only two telescopes and suppress the second-telescope AO/CHARA delay-line path. The $\sim5^{\prime\prime}$ field is acquired at the telescopes; visible light feeds the AO/WFS via the telescope dichroic (TelDic), while the IR beam is relayed to the beam-combiner lab and main CHARA delay lines. The full field is imaged by STST, then a small pickoff (STST-BS) sends light to MIRC-X ($H$ band) and MYSTIC ($K$ band). MIRC-X fringe-tracks on component A and drives the main delay lines; MYSTIC records science fringes on component B (see Figure \ref{['fig:opd_trend']}). The instrument DDLs apply the additional differential delay $\boldsymbol{B}\cdot\boldsymbol{\theta}$ between A and B, shifting the science beam relative to the fringe-tracking path. The fiber injection is set from the STST astrometric positions. Unlike VLTI/GRAVITY, CHARA does not have an internal metrology system; differential delays are inferred from DDL telemetry and A-B/B-A swap tests. This schematic illustrates the functional architecture rather than the full optical complexity of the six-telescope CHARA beam train.
• Figure 5: Waterfall plots of MYSTIC science-channel OPD residuals during dual-field observations of $\alpha$ Psc B (${\rm MJD} = 60918$). The waterfall plot shows the amplitude and position of the Fourier transform of a scan through the interferogram over time. Stable fringe packets are visible on all 15 baselines despite fringe-tracking residuals, demonstrating successful phase referencing from component A and enabling direct detection of the Ba--Bb subsystem. Each subpanel corresponds to one of the 15 baselines, labeled by the beam pair (e.g., 12 = Telescope 1--2) for the beam order E1--W2--W1--S2--S1--E2. Time increases horizontally and optical path difference (OPD) is plotted vertically; brighter lines in each waterfall window indicate higher fringe contrast and successful fringe detection. The brighter double lines in some of the large baseline waterfall windows are an indication of BaBb binary is resolved. The coherent fringe integration time for this diagnostic is 10 ms.