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

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.

References in corpus (16)

• Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 parsecs: The Southern Sample
• KEPLER's First Rocky Planet: Kepler-10b
• Tertiary companions to close spectroscopic binaries
• WASP-1b and WASP-2b: Two new transiting exoplanets detected with SuperWASP and SOPHIE
• HAT-P-1b: A Large-Radius, Low-Density Exoplanet Transiting one Member of a Stellar Binary
• Precise Radial Velocities of Giant Stars VII. Occurrence Rate of Giant Extrasolar Planets as a Function of Mass and Metallicity
• The Exoplanet Orbit Database II: Updates to exoplanets.org
• The sdB+M Eclipsing System HW Virginis and its Circumbinary Planets
• Two Suns in The Sky: Stellar Multiplicity in Exoplanet Systems
• Stellar parameters and chemical abundances of 223 evolved stars with and without planets
• High resolution spectroscopy for Cepheids distance determination. I. Line asymmetry
• Planetesimal and gas dynamics in binaries
• Precise radial velocities of giant stars: I. Stable stars
• Planet formation in stellar binaries II: overcoming the fragmentation barrier in alpha Centauri and gamma Cephei-like systems
• No evidence of a hot Jupiter around HD 188753 A
• SPOTS: The Search for Planets Orbiting Two Stars. I. Survey description and first observations

Cited by in corpus (8)

• The Radial Velocity Variability of the K-giant Gamma Draconis: Stellar Variability Masquerading as a Planet
• Peculiar architectures for the WASP-53 and WASP-81 planet-hosting systems
• 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
• The conjectured S-type retrograde planet in nu Octantis: more evidence including four years of iodine-cell radial velocities
• Dynamical analysis of the circumprimary planet in the eccentric binary system HD59686
• Multiplicity of stars with planets in the solar neighbourhood
• A photospheric and chromospheric activity analysis of the quiescent retrograde-planet host $\nu$ Octantis A
• Constraining the orbit of the retrograde planet in the $\nu$ Octantis system