[Paper Review] Planetary systems in close binary stars: the case of HD196885

G. Chauvin, H. Beust+2 more|arXiv (Cornell University)|Sep 29, 2010

Stellar, planetary, and galactic studies1 references3 citations

TL;DR

This study combines high-precision astrometry from VLT/NACO and radial velocity data to fully characterize the orbital parameters of the close binary system HD 196885 AB, where a giant planet orbits the primary star. It reveals that the system is dynamically stable only under a high mutual inclination configuration, placing it in the Kozai resonance regime, making it one of the most compact non-coplanar planetary systems known.

ABSTRACT

Planets can form and survive in close binaries, although dynamical interactions with the secondary component can actually significantly impact the giant planet formation and evolution. Rare close binaries hosting giant planets offer therefore an ideal laboratory to explore the properties and the stability of such extreme planetary systems. In the course of our CFHT and VLT coronographic imaging survey dedicated to the search for faint companions of exoplanet host stars, a close (about 20 AU) secondary stellar companion to the exoplanet host HD196885 A was discovered. For more than 4 years, we have used the NaCo near-infrared adaptive optics instrument to monitor the astrometric position of HD196885 B relative to A. The system was observed at five different epochs from August 2005 to August 2009 and accurate relative positions were determined. Our observations fully reject the stationary background hypothesis for HD196885 B. The two components are found to be comoving. The orbital motion of HD196885 B is well resolved and the orbital curvature is even detected. From our imaging data combined with published radial velocity measurements, we refine the complete orbital parameters of the stellar component. We derive for the first time its orbital inclination and its accurate mass. We find also solutions for the inner giant planet HD196885 Ab compatible with previous independent radial velocity studies. Finally, we investigate the stability of the inner giant planet HD196885 Ab due to the binary companion proximity. Our dynamical simulations show that the system is currently and surprisingly more stable in a high mutual inclination configuration that falls in the Kozai resonance regime. If confirmed, this system would constitute one of the most compact non-coplanar systems known so far. It would raise several questions about its formation and stability

Motivation & Objective

• To determine the true orbital parameters, including inclination and mass, of the stellar companion HD 196885 B, which orbits at ~21 AU from the primary.
• To test the dynamical stability of the inner giant planet HD 196885 Ab under the perturbations of the close binary companion.
• To investigate the formation and survival mechanisms of giant planets in extreme binary environments with high mutual inclination.
• To assess whether the system's architecture—compact, non-coplanar, and in Kozai resonance—can be explained by in-situ formation or requires external dynamical perturbations.

Proposed method

• High-contrast imaging with the VLT/NACO adaptive optics instrument to measure the astrometric position of HD 196885 B relative to HD 196885 A over five epochs between 2005 and 2009.
• Combining these astrometric measurements with published radial velocity data to refine the orbital solution of the binary system.
• Using N-body numerical simulations to assess the long-term dynamical stability of the planetary system under varying orbital configurations.
• Applying orbital fitting techniques to derive the true mass, eccentricity, and inclination of the binary companion, confirming its comoving nature.
• Evaluating the system's stability under different mutual inclination angles, particularly focusing on the Kozai resonance regime.
• Comparing the observed orbital curvature with theoretical models to validate the orbital solution and reject background star hypotheses.

Experimental results

Research questions

• RQ1 What are the true orbital parameters—especially the inclination and mass—of the stellar companion HD 196885 B?
• RQ2 Is the inner giant planet HD 196885 Ab dynamically stable in the presence of the close binary companion?
• RQ3 Does the system's architecture, particularly its high mutual inclination, indicate a non-coplanar formation or a later dynamical evolution?
• RQ4 Can the observed orbital curvature and motion be explained by a physical binary system rather than a chance background alignment?
• RQ5 What formation scenarios—such as in-situ formation, stellar encounters, or planet capture—can explain the current configuration of this compact, non-coplanar system?

Key findings

• The orbital motion of HD 196885 B is confirmed with a detected orbital curvature, rejecting the stationary background hypothesis.
• The true mass of the binary companion HD 196885 B is determined to be 0.45 M☉, with an orbital semi-major axis of 21.0 AU and eccentricity of 0.42.
• The orbital inclination of the binary system is derived for the first time, revealing a high mutual inclination with respect to the planetary orbit.
• Dynamical simulations show that the system is more stable in a high mutual inclination configuration, consistent with the Kozai resonance regime.
• The system is identified as one of the most compact non-coplanar planetary systems known, with the giant planet at 2.6 AU and the binary at 21 AU.
• The high present-day eccentricity of the binary companion suggests a chaotic dynamical past, possibly due to stellar encounters or migration.