# Astrometric Solution for HD 196885 Ab

## Astrometric Solution for HD 196885 Ab

Planetary systems in close binary stars: the case of HD196885
http://arxiv.org/abs/1009.5851

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

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Sirius_Alpha

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## Re: Astrometric Solution for HD 196885 Ab

From a brief scan of the paper, it looks like they haven't detected the planet astrometrically, but only the binary star orbit.

Lazarus
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## Re: Astrometric Solution for HD 196885 Ab

Against all odds? Forming the planet of the HD196885 binary
http://arxiv.org/abs/1103.3900

HD196885Ab is the most "extreme" planet-in-a-binary discovered to date, whose orbit places it at the limit for orbital stability. The presence of a planet in such a highly perturbed region poses a clear challenge to planet-formation scenarios. We investigate this issue by focusing on the planet-formation stage that is arguably the most sensitive to binary perturbations: the mutual accretion of kilometre-sized planetesimals. To this effect we numerically estimate the impact velocities $dv$ amongst a population of circumprimary planetesimals. We find that most of the circumprimary disc is strongly hostile to planetesimal accretion, especially the region around 2.6AU (the planet's location) where binary perturbations induce planetesimal-shattering $dv$ of more than 1km/s. Possible solutions to the paradox of having a planet in such accretion-hostile regions are 1) that initial planetesimals were very big, at least 250km, 2) that the binary had an initial orbit at least twice the present one, and was later compacted due to early stellar encounters, 3) that planetesimals did not grow by mutual impacts but by sweeping of dust (the "snowball" growth mode identified by Xie et al., 2010b), or 4) that HD196885Ab was formed not by core-accretion but by the concurent disc instability mechanism. All of these 4 scenarios remain however highly conjectural.

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## Re: Astrometric Solution for HD 196885 Ab

Hmmm maybe if we start searching close binaries for lower mass planets, i.e. ones that are more likely to have formed via core accretion rather than disc instability, things might become a little clearer.

Wonder what the formation conditions for Gliese 86 would be like assuming the planet formed before the secondary became a white dwarf.

Lazarus
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## Re: Astrometric Solution for HD 196885 Ab

If I recall, Gliese 86 is a candidate system for having a "second generation planet" and would thus have formed after the mass reshuffling of the system.

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## Re: Astrometric Solution for HD 196885 Ab

Maybe it is, I quite like Gliese 86 as a possible second generation planet candidate. Would still be exceptional if it was first generation though, would like to see that scenario getting modelled.

Wonder how the various Alpha Centauri campaigns are doing...

Lazarus
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## Re: Astrometric Solution for HD 196885 Ab

Dynamical analysis and constraints for the HD 196885 system
http://arxiv.org/abs/1203.5249

The HD\,196885 system is composed of a binary star and a planet orbiting the primary. The orbit of the binary is fully constrained by astrometry, but for the planet the inclination with respect to the plane of the sky and the longitude of the node are unknown. Here we perform a full analysis of the HD\,196885 system by exploring the two free parameters of the planet and choosing different sets of angular variables. We find that the most likely configurations for the planet is either nearly coplanar orbits (prograde and retrograde), or highly inclined orbits near the Lidov-Kozai equilibrium points, i = 44^{\circ} or i = 137^{\circ} . Among coplanar orbits, the retrograde ones appear to be less chaotic, while for the orbits near the Lidov-Kozai equilibria, those around \omega= 270^{\circ} are more reliable, where \omega_k is the argument of pericenter of the planet's orbit with respect to the binary's orbit.
From the observer's point of view (plane of the sky) stable areas are restricted to (I1, \Omega_1) \sim (65^{\circ}, 80^{\circ}), (65^{\circ},260^{\circ}), (115^{\circ},80^{\circ}), and (115^{\circ},260^{\circ}), where I1 is the inclination of the planet and \Omega_1 is the longitude of ascending node.

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## Re: Astrometric Solution for HD 196885 Ab

Chaotic dynamics of the planet in HD 196885 AB

The advent in the detection technology has led to the confirmation of hundreds of exoplanets among which many are found to orbit binary stars where depending on the planetary orbit around the host star(s), a planet could orbit either one or both stars as S-type or P-type, respectively. We have analysed the orbital stability of the S-type planetary system in HD 196885 AB with an emphasis on higher orbital inclination of planet with the binary plane. The mean exponential growth factor of nearby orbits (MEGNO) maps are used as an indicator to determine regions of chaos for the various choices of planet's semi-major axis, eccentricity and inclination with respect to the previously determined observational uncertainties. Based on our analysis we have quantitatively mapped chaotic and quasi-periodic regions of the system's phase space which indicate regions of likely stability. In addition, we inspect the resonant angle to determine whether alternation between libration and circulation occurs as a consequence of Kozai oscillations, a probable mechanism that leads the system towards highly inclined planetary orbit. Also, we demonstrate the possible higher mass limit of the planet and improve upon the current ephemeris with a more consistent dynamical model based on our stability analysis.

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