The conjectured S-type retrograde planet in ν Octantis: more evidence including four years of iodine-cell radial velocities

Abstract

Released ABSTRACTWe report 1212 radial-velocity (RV) measurements obtained in the years 2009-2013 using an iodine cell for the spectroscopic binary ν Octantis (K1III/IV). This system (a bin ∼ 2.6 au, P ∼ 1050 days) is conjectured to have a Jovian planet with a semi-major axis half that of the binary host. The extreme geometry only permits long-term stability if the planet is in a retrograde orbit. Whilst the reality of the planet (P ∼ 415 days) remains uncertain, other scenarios (stellar variability or apsidal motion caused by a yet unobserved third star) continue to appear substantially less credible based on CCF bisectors, line-depth ratios and many other independent details. If this evidence is validated but the planet is disproved, the claims of other planets using RVs will be seriously challenged.We also describe a significant revision to the previously published RVs and the full set of 1437 RVs now encompasses nearly 13 years. The sensitive orbital dynamics allow us to constrain the 3D architecture with a broad prior probability distribution on the mutual inclination, which with posterior samples obtained from an N -body Markov chain Monte Carlo is found to be 152 • .5± 0.7 0.6 . None of these samples are dynamically stable beyond 10 6 years. However, a grid search around the best-fitting solution finds a region that has many models stable for 10 7 years, and includes one model within 1-sigma that is stable for at least 10 8 years. The planet's exceptional nature demands robust independent verification and makes the theoretical understanding of its formation a worthy challenge.

Eccentric Companions to Kepler-448b and Kepler-693b: Clues to the Formation of Warm Jupiters

“…Planets in S-type orbits around tight binaries with periastron distances less than 10 au have also been reported around KOI-1257 (Santerne et al 2014), Kepler-444 (Dupuy et al 2016), HD 59686 (Ortiz et al 2016), and possibly ν Octantis (Ramm et al 2016). If confirmed to be a low-mass star, Kepler-693c has the smallest peristron among such stellar companions.…”

The HD 181433 Planetary System: Dynamics and a New Orbital Solution

“…This result is not unexpected -such retrograde solutions are almost always highly stable unless they feature mutually crossing orbits (e.g. Eberle & Cuntz 2010;Horner et al 2011Horner et al , 2012bWittenmyer et al 2013a,b;Ramm et al 2016).…”

Precise radial velocities of giant stars XV. Mysterious nearly periodic radial velocity variations in the eccentric binary $ε$ Cygni

“…In Morais & Correia (2012) the authors develop a secular theory for such systems and derive an expression for the precession rate of the orbit of the companion pair around the main component. Furthermore they suggest this effect as an alternative explanation to the planet hypothesis for the ν Octantis system (Ramm et al 2009(Ramm et al , 2016, which is known to be a close single-line spectroscopic binary where the two stellar components orbit each other with a period of 1050 d. In addition to the long-period RV signal caused by the stellar companion, Ramm et al (2009) observed short-period RV variations with a period of 417 d, which they interpreted as a possible planet with a minimum mass of 2.5 M jup . Stable orbital configurations have been published by Ramm et al (2016), but previously the stability of the proposed planet seemed questionable.…”