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

Abstract

We explore the possibly that either starspots or pulsations are the cause of a periodic radialvelocity signal (P ∼ 400 days) from the K-giant binary ν 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 ν Oct and twenty calibration stars. Empirical evidence and stability modelling provide unexpected support for the planet since other standard explanations (starspots, 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 ν 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 ν 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 starspots 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.

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

“…striking resemblance to the brightness variations recorded by Hipparcos and estimated by Ramm (2015), the three sets of ∆m values now spanning about 23 years (Fig. 8).…”

“…Our investigations of Ca II H-line emission, CCF bisectors and line-depth ratios are also all consistent with the primary star having negligible dynamical surface behaviour. The LDR and bisector results challenge any possibility that surface-activity cycles or pulsations are the cause: both here and in Ramm (2015) the extremely temperature-sensitive LDRs converted to magnitude differences ∆m strikingly mimic the Hipparcos photometric variations. Furthermore, the predicted spot-filling factor fsp ∼ 1% from the ∆m values (see Ramm 2015, fig.…”

“…The LDR and bisector results challenge any possibility that surface-activity cycles or pulsations are the cause: both here and in Ramm (2015) the extremely temperature-sensitive LDRs converted to magnitude differences ∆m strikingly mimic the Hipparcos photometric variations. Furthermore, the predicted spot-filling factor fsp ∼ 1% from the ∆m values (see Ramm 2015, fig. 10) can be used to estimate the corresponding RV amplitude (see Hatzes 2002).…”

“…Gray 1997;Hatzes, Cochran & Bakker 1998). In a recent paper Ramm (2015) likewise investigated the 215 ν Oct spectra reported in Ramm et al (2009), derivng 22 LDRs from ten spectral lines in the wavelength region 6230-6260 Å. Spectra of 20 similarly evolved stars were also used to create a temperature scale. Ramm recovered the original 21 stellar temperatures with an accuracy and standard deviation of 45 ± 25 K and found no significant periodic behaviour in any of the LDRs.…”

Precise radial velocities of giant stars

“…Interestingly, similar to HD 59686 AB, the ν Oct system is composed of a single-lined K-giant binary, with a secondary star mass of ∼ 0.55 M . Moreover, the ν Oct system is slighlty eccentric: e ∼ 0.25 (Ramm 2015). As we discussed below, the existence of giant planets in both systems is very hard to explain by traditional theories.…”