Chasing extreme planetary architectures: I- HD196885Ab, a super-Jupiter dancing with two stars?

TL;DR

This study combines high-resolution imaging, radial velocity data, and astrometry to characterize the complex orbit of the HD 196885 system, confirming the inner planet as a super-Jupiter in a highly inclined orbit within a binary star system.

Contribution

The paper provides the first astrometric confirmation of the inner planet's nature and detailed orbital parameters in a binary system with a super-Jupiter, using multi-technique observations.

Findings

The binary HD 196885 AB has an inclined, retrograde orbit with a semi-major axis of 19.78 au.

The inner planet HD 196885 Ab is a Jupiter-like planet with a mass of 3.39 MJup and an inclination of about 143 degrees.

The system is dynamically stable, with potential eccentricity and inclination variations due to Kozai-Lidov cycles.

Abstract

Planet(s) in binaries are unique architectures for testing predictions of planetary formation and evolution theories in very hostile environments. We used the IRDIS dual-band imager of SPHERE at VLT, and the speckle interferometric camera HRCAM of SOAR, to acquire high-angular resolution images of HD 196885 AB between 2015 and 2020. Radial velocity observations have been extended over almost 40 yr extending the radial velocity measurements HD 196885 A and resolving both the binary companion and the inner giant planet HD 196885 Ab. Finally, we took advantage of the exquisite astrometric precision of the dual-field mode of VLTI/GRAVITY (down to 30 {\mu}as) to monitor the relative position of HD 196885 A and B to search for the 3.6 yr astrometric wobble of the circumprimary planet Ab imprinted on the binary separation. Our observations enable to accurately constrain the orbital properties of the binary HD 196885 AB, seen on an inclined and retrograde orbit (iAB = 120.43 deg) with a semi-major axis of 19.78 au, and an eccentricity of 0.417. The GRAVITY measurements confirm for the first time the nature of the inner planet HD 196885 Ab by rejecting all families of pole-on solutions in the stellar or brown dwarf masses. The most favored island of solutions is associated with a Jupiter-like planet (MAb = 3.39 MJup), with moderate eccentricity (eAaAb = 0.44), and inclination close to 143.04 deg. This results points toward a significant mutual inclination (Phi = 24.36 deg) between the orbital planes (relative to the star) of the binary companion B and the planet Ab. Our dynamical simulations indicate that the system is dynamically stable over time. Eccentricity and mutual inclination variations could be expected for moderate von Zipele Kozai Lidov cycles that may affect the inner planet.