The Orbital Architecture of Qatar-6: A Fully Aligned 3-Body System?

TL;DR

This study measures the orbital alignment of the Qatar-6 system, revealing a nearly aligned 3-body configuration through spectroscopic and astrometric data, advancing understanding of planetary system architectures.

Contribution

It provides the first detailed measurement of spin-orbit and orbit-orbit alignment in the Qatar-6 binary system using combined spectroscopic and Gaia data.

Findings

The system is nearly aligned with a true obliquity of about 22 degrees.

The binary star system is edge-on, indicating line-of-sight orbit-orbit alignment.

Current data are consistent with a fully aligned 3-body system.

Abstract

The evolutionary history of an extrasolar system is, in part, fossilized through its planets' orbital orientations relative to the host star's spin axis. However, spin-orbit constraints for warm Jupiters -- particularly in binary star systems, which are amenable to a wide range of dynamical processes -- are relatively scarce. We report a measurement of the Rossiter-McLaughlin effect, observed with the Keck/HIRES spectrograph, across the transit of Qatar-6 A b: a warm Jupiter orbiting one star within a binary system. From this measurement, we obtain a sky-projected spin-orbit angle $\lambda={0.1\pm2.6}^{\circ}$. Combining this new constraint with the stellar rotational velocity of Qatar-6 A that we measure from TESS photometry, we derive a true obliquity $\psi={21.82^{+8.86}_{-18.36}}^{\circ}$ -- consistent with near-exact alignment. We also leverage astrometric data from Gaia DR3 to show that the Qatar-6 binary star system is edge-on ($i_{B}={90.17^{+1.07}_{-1.06}}^{\circ}$), such that the stellar binary and the transiting exoplanet orbit exhibit line-of-sight orbit-orbit alignment. Ultimately, we demonstrate that all current constraints for the 3-body Qatar-6 system are consistent with both spin-orbit and orbit-orbit alignment. High-precision measurements of the projected stellar spin rate of the host star and the sky-plane geometry of the transit relative to the binary plane are required to conclusively verify the full 3D configuration of the system.