Line-depth-ratio temperatures for the close binary nu Octantis: new evidence supporting the conjectured retrograde planet

Line-depth-ratio temperatures for the close binary nu Octantis: new evidence supporting the conjectured retrograde planet

Author:

Ramm et al

Abstract:

We explore the possibly that either star-spots or pulsations are the cause of a periodic radial-velocity signal (P~400 days) from the K-giant binary nu Octantis (P~1050 days, e~0.25), alternatively conjectured to have a retrograde planet. Our study is based on temperatures derived from 22 line-depth ratios (LDRs) for nu Oct and twenty calibration stars. Empirical evidence and stability modelling provide unexpected support for the planet since other standard explanations (star-spots, pulsations and additional stellar masses) each have credibility problems. However, the proposed system presents formidable challenges to planet-formation and stability theories: it has by far the smallest stellar separation of any claimed planet-harbouring binary (a_bin~2.6 au) and an equally unbelievable separation ratio (a_pl/a_bin~0.5), hence the necessity that the circumstellar orbit be retrograde.

The LDR analysis of 215 nu Oct spectra acquired between 2001--2007, from which the RV perturbation was first revealed, have no significant periodicity at any frequency. The LDRs recover the original 21 stellar temperatures with an average accuracy of 45+/-25 K. The 215 nu Oct temperatures have a standard deviation of only 4.2 K. Assuming the host primary is not pulsating, the temperatures converted to magnitude differences strikingly mimic the very stable photometric Hipparcos observations 15 years previously, implying the long-term stability of the star and demonstrating a novel use of LDRs as a photometric gauge. Our results provide substantial new evidence that conventional star-spots and pulsations are unlikely causes of the RV perturbation. The controversial system deserves continued attention, including with higher resolving-power spectra for bisector and LDR analyses.

nu Octantis b: a Retrograde S Type Circumbinary Gas Giant...or not?

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

Authors:

Ramm et al

Abstract:

We report 1212 radial-velocity (RV) measurements obtained in the years 2009-2013 using an iodine cell for the spectroscopic binary nu 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 three-dimensional 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 158.4 +/- 1.2 deg. None of these samples are dynamically stable beyond 1 Myr. However, a grid search around the best-fitting solution finds a region that has many models stable for 10 Myr, and includes one model within 1-sigma that is stable for at least 100 Myr. The planet's exceptional nature demands robust independent verification and makes the theoretical understanding of its formation a worthy challenge.