HD 59686 - A giant planet 1 AU from a 13-AU separation binary

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HD 59686 - A giant planet 1 AU from a 13-AU separation binary

Post by Sirius_Alpha on 2nd August 2016, 8:22 pm

Precise radial velocities of giant stars IX. HD 59686 Ab: a massive circumstellar planet orbiting a giant star in a ~13.6 au eccentric binary system
http://arxiv.org/abs/1608.00963

Context: For over 12 years, we have carried out a precise radial velocity survey of a sample of 373 G and K giant stars using the Hamilton \'Echelle Spectrograph at Lick Observatory. There are, among others, a number of multiple planetary systems in our sample as well as several planetary candidates in stellar binaries. Aims: We aim at detecting and characterizing substellar+stellar companions to the giant star HD 59686 A (HR 2877, HIP 36616). Methods: We obtained high precision radial velocity (RV) measurements of the star HD 59686 A. By fitting a Keplerian model to the periodic changes in the RVs, we can assess the nature of companions in the system. In order to discriminate between RV variations due to non-radial pulsation or stellar spots we used infrared RVs taken with the CRIRES spectrograph at the Very Large Telescope. Additionally, to further characterize the system, we obtain high-resolution images with LMIRCam at the Large Binocular Telescope. Results: We report the likely discovery of a giant planet with a mass of mp sini=6.92+0.18−0.24 MJup orbiting at ap=1.0860+0.0006−0.0007 au from the giant star HD 59686 A. Besides the planetary signal, we discover an eccentric (eB=0.729+0.004−0.003) binary companion with a mass of mB sini=0.5296+0.0011−0.0008 MSun orbiting at a semi-major axis of just aB=13.56+0.18−0.14 au. Conclusions: The existence of the planet HD 59686 Ab in a tight eccentric binary system severely challenges standard giant planet formation theories and requires substantial improvements to such theories in tight binaries. Otherwise, alternative planet formation scenarios such as second generation planets or dynamical interactions in an early phase of the system's lifetime should be seriously considered in order to better understand the origin of this enigmatic planet.

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Re: HD 59686 - A giant planet 1 AU from a 13-AU separation binary

Post by Lazarus on 3rd August 2016, 2:46 pm

What a weird system, particularly the near-circular orbit for HD 59686 Ab. Would be very interesting to know whether the companion is a white dwarf or a main sequence star.
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Re: HD 59686 - A giant planet 1 AU from a 13-AU separation binary

Post by Shellface on 3rd August 2016, 8:06 pm

This planet was announced way back in 2003. 12 and a half years is probably the longest interval between announcement and publishing for a planet so far. I vaguely recall an arxiv preprint from a few years ago showing a plot of the RVs, but I could not locate it. More recently it was mentioned in this abstract, with the system apparently understood.

This is indeed a very unusual system - the delay in publishing was probably warranted. It does seem that a second-generation (SG) planet is more tenable than a first-generation one, but that hinges on the identification of the companion as a WD.

In principle, this system would satisfy several expected observational characteristics from SG planets. As stated by Perets (2010), SG planets are expected to be almost exclusively found in evolved binary systems with compact objects, like WD or neutron stars. They are also likely to be more massive than normal first generation planets; with a mass roughly constrained between ~7-9 MJup, HD 59686 Ab is among the most massive exoplanets detected so far. SG planets could reside in regions of orbital phase space forbidden to pre-existing planets by dynamical arguments. HD 59686 Ab is marked as unstable or on the border of stability by some dynamical criteria (Holman & Wiegert 1999; Mardling & Aarseth 2001), although detailed N-body integrations allow stability for a certain parameter space including both prograde and retrograde orbital configurations (Trifonov et al. 2016, in prep.). For the prograde case, the bootstrap dynami cal test yielded a small sample of long-term stable fits consistent with the bootstrap distributions at the 1 sigma confidence level. These prograde fits are locked in secular resonance with aligned orbital periapsis. The best dynamical fits assuming a retrograde orbit have slightly better quality (smaller χ2) and are long-term stable.
Stronger conclusions will be possible when the dynamical study is published.

It would also be much more feasible to identify a WD companion in the ultraviolet, as the contrast will be much better, even in unresolved photometry.

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Re: HD 59686 - A giant planet 1 AU from a 13-AU separation binary

Post by Sirius_Alpha on 3rd August 2016, 8:30 pm

Shellface wrote:I vaguely recall an arxiv preprint from a few years ago showing a plot of the RVs, but I could not locate it.
I think you're thinking of this paper from 2011, in which it goes under the identifier HIP 33616.

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Re: HD 59686 - A giant planet 1 AU from a 13-AU separation binary

Post by Shellface on 3rd August 2016, 9:36 pm

Yes, that's it. Darn papers with ridiculously modest titles.

I wonder of there's going to be a proper Lick study of ν Oph soon - I believe it's the last of the "ancient" Lick discoveries that hasn't been formally published (though it has been studied by the Okayama group).

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Re: HD 59686 - A giant planet 1 AU from a 13-AU separation binary

Post by Lazarus on 4th August 2016, 6:17 pm

Either a WD or an MS companion star would be interesting. The latter would presumably lend credence to the possibility that the system experienced a scattering event, but the circularity of the planetary orbit is definitely curious under such a scenario (unless we happen to be observing it at just the right point in its eccentricity cycle, but presumably it can't get too high or it would become unstable during the periastron passages of the binary companion). This system is definitely one for which full 3D information about the orbits would be very valuable.
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