DDA 2015 – Dynamical Analysis of the 6:1 Resonance of the Brown Dwarfs Orbiting the K Giant Star ν Ophiuchi

This is one of a series of notes taken during the 2015 meeting of the AAS Division on Dynamical Astronomy, 3-7 May, at CalTech. An index to this series (all the papers presented at the meeting) is here.

Session: Dynamical Constraints from Exoplanet Observaons II

Man Hoi Lee (University of Hong Kong)

Abstract

The K giant star ν Oph has two brown dwarf companions (with minimum masses of about 22 and 25 times the mass of Jupiter), whose orbital periods are about 530 and 3200 days and close to 6:1 in ratio. We present a dynamical analysis of this system, using 150 precise radial velocities obtained at the Lick Observatory in combination with data already available in the literature. We investigate a large set of orbital fits by applying systematic $\chi^2$ grid-search techniques coupled with self-consistent dynamical fitting. We find that the brown dwarfs are indeed locked in an aligned 6:1 resonant configuration, with all six mean-motion resonance angles librating around 0°, but the inclination of the orbits is poorly constrained. As with resonant planet pairs, the brown dwarfs in this system were most likely captured into resonance through disk-induced convergent migration. Thus the ν Oph system shows that brown dwarfs can form like planets in disks around stars.

Notes

  • Lick G & K giants RV survey
    • 373 bright G & K giant stars
    • 0.6-m Coude
    • ~1999-2012
    • RV precision ~5 m/s
  • $\nu$Oph
    • K0III HB star, 2.73 $M_\odot$
    • brown dwarf companion, P = 530 d
    • 150 Lick RV measurements
    • Fitting codes: Tan et al. 2013
    • Grid search to minimize $\chi^2$
    • SyMBA 10 Myr integrations
  • Best fit:
    • $M_1 = 22 M_J$, $P_1 = 530$ d, $a_1 = 1.79$ AU, $e_1 = 0.124$
    • $M_2 = 25 M_J$, $P_2 = xxx$ d, $a_2 = 6.02$ AU, $e_2 = 0.1xx$
    • 6:1 MMR at $3\sigma$
  • Stability: all fits stable (numerically) to 10 Myr
  • No constraints on inclination
  • Origin
    • Resonant capture via migration
      • Type II (Ward 1997)
      • $\left|\dfrac{\dot{a}}{a}\right| = \dfrac{3\nu}{2a^2}$
  • Conclusions
    • 2 brown dwarf companions
      • minimum mass $22 M_J$ and $25 M_J$
      • 6:1 MMR
    • 6:1MMR couldindicate formation & migration in a disk
      • But resonant capture requires slow migration and nonzero eccentricities

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