Planet around Nu Octantis?

In press, but sounds good.

Spectroscopic orbits for K giants β Reticuli and ν Octantis: what is causing a low-amplitude radial velocity resonant perturbation in ν Oct?

Quote :

New astrometric–spectroscopic orbital solutions for the single-line K-giant binaries β Reticuli (P≈ 5.2 yr, e= 0.3346 ± 0.0004) and ν Octantis (P≈ 2.9 yr, e= 0.2358 ± 0.0003) have been derived based on high-precision spectroscopic radial velocities (RVs) and the Hipparcos astrometry. For the case of ν Oct, the simultaneous solution is particularly robust and an inclination of i= 70.8 ± 0.9° has been derived. This is one of the most precise inclinations yet calculated based on a spectroscopic solution and the Hipparcos astrometry.

We have also discovered low-amplitude periodic behaviour in the residuals of the orbital solution for ν Oct. This RV perturbation has a semi-amplitude of 50 m s⁻¹ and a 418-d period which is coherent over several years. The RV curve of the perturbation is apparently in resonance with that of the binary: every second maximum of the binary coincides with every fifth minimum of the perturbation, hence the periods have the simple ratio 5:2.

The possible causes of such a perturbation are rotational modulation of surface phenomenon, pulsations or an orbiting body. We have assessed these alternatives in terms of the suspected photometric stability (H_p= 3.8981 ± 0.0004) , a lack of evidence of other RV periodicities, no correlation of cross-correlation function bisectors with the residual velocities, no compelling evidence of wavelength dependency for the amplitude or relative phase of the perturbation, our bounds on the rotational period of the primary star and the need for long-lived relatively fixed surface features. The results of these analyses lack consistency with both rotational modulation and pulsations and so imply that a planetary mass is a realistic cause. The planet hypothesis, however, is strongly constrained and challenged by our precise binary orbit. The hypothetical planet 2 Mj would have an orbit (e≈ 0.1, a₃≈ 1.2 au) about mid-way between the stars whose periastron distance is only 1.9 au. This orbit, supposedly in resonance with the binary system, appears to be highly unlikely based on current planet formation and orbit-stability expectations.

Without knowing the cause of the perturbation, we cannot be certain if the suspected RV and hence period resonance are merely coincidental or not. Establishing the true cause of the perturbation requires renewed observation of the system, re-assessment of the possible resonance if this is redetected and the acquisition of similar and additional precise diagnostic parameters with respect to each of the possible causative mechanisms.

Emphasis mine.
Moreover such a planetary object would seems a trojan planet witihin a binary system, am I wrong?

A planet in a 5:2 resonance with a star? That would be very odd. Not a Trojan planet though, that would be a 1:1 resonance.

If I'm working things out correctly from the numbers in the abstract, the mass of Nu Octantis is being taken as 1.5 solar masses, which would give the secondary star roughly 0.6 solar masses. (Using a mass function of 0.0418 solar masses from here)

The Trojan points of the binary star would thus be unstable. (Need a mass ratio m₁/m₂ exceeding 24.96)

Listed amongst unconfirmed planets at EPE too

http://exoplanet.eu/star.php?st=HR+8254

Still unconfirmed.
http://exoplanet.eu/planet.php?p1=nu+Oct&p2=b

Quote :

14 Sep. 2010: Eberle & Cuntz (2010) find that the orbit of this planet in the nu Oct binary system is stable only if it is counter-rotating with respect to the stellar orbits.

On the Reality of the Suggested Planet in the ν Octantis System
http://iopscience.iop.org/2041-8205/721/2/L168

Quote :

The aim of this study is to explore an enigmatic finding about the ν Octantis binary system that indicates the possible existence of a Jupiter-type planet even though the planet seems to be located outside the zone of orbital stability. We perform a detailed analysis of orbital stability based on previous studies that carefully considers the ν Octantis system parameters including their observationally deduced uncertainties. In our analysis, we confront the probability distribution of the parameter space of the system with the requirements of planetary orbital stability. Our results indicate that the suggested planet, if in a prograde orbit with respect to the motion of the binary components, is virtually impossible. However, the estimated probability of existence for a planet in a retrograde orbit is nearly 60%, an estimate that encapsulates the probability distribution of the mass ratio of the stellar components. This estimate increases if a relatively low stellar mass ratio (within the error bars) is assumed. The principal possibility of a planet in a retrograde orbit is also consistent with long-term orbital stability simulations pursued as part of our study. Thus, the existence of the suggested planet in the ν Octantis system constitutes a realistic possibility.

Would be weird if true. I guess astrometric determinations are necessary to resolve this issue. Have to wonder whether the other known planets in close binaries are also in retrograde orbits: I heard that there have been measurements made to determine the mutual inclination of gamma Cephei's planetary system.

An interesting blog post about ν Oct.
http://cosmicdiary.org/blogs/john_hearnshaw/?p=185

And my attempt at a Celestia rendition.

Leads also to the question of just how this system could have formed the planet: the binary has a separation of about 2.55 AU, which suggests the disc would have been truncated well inside the ice line. Gas giants shouldn't have been able to form there.

That would be nice.

This will increase the % for more planets.

Would a planet like this setup that orbits polar survive?

Precession due to a close binary system: An alternative explanation for ν-Octantis?
http://arxiv.org/abs/1110.3176

Quote :

We model the secular evolution of a star's orbit when it has a nearby binary system. We assume a hierarchical triple system where the inter-binary distance is small in comparison with the distance to the star. We show that the major secular effect is precession of the star's orbit around the binary system's centre of mass. We explain how we can obtain this precession rate from the star's radial velocity data, and thus infer the binary system's parameters. We show that the secular effect of a nearby binary system on the star's radial velocity can sometimes mimic a planet. We analyze the radial velocity data for {\nu}-octantis A which has a nearby companion ({\nu}-octantis B) and we obtain retrograde precession of (-0.86 \pm 0.02)\degree/yr. We show that if {\nu}-octantis B was itself a double star, it could mimic a signal with similarities to that previously identified as a planet of {\nu}-octantis A. Nevertheless, we need more observations in order to decide in favor of the double star hypothesis.

I had this originally in the HU Aqr planets thread after getting horrible confused between Nu Oct and NN Ser.

That does seem somewhat more plausible than the retrograde planet idea, which would cause various problems in terms of explaining the formation of the system. So perhaps this is a preferable idea.

Nevertheless the universe is full of surprises.

The Stability of the Suggested Planet in the nu Octantis System: A Numerical and Statistical Study

Stability analysis taking into account some variation of the initial parameters of the possible planet around nu Octantis A. Again it is found that retrograde orbits seem to offer the best possibility for stability.

Whether the signal is due to a planet is still not clear (see this thread).

New paper in MNRAS (no arXiv as far as I can see)

http://mnras.oxfordjournals.org/content/449/4/4428

From the abstract to suggest the planet as the most plausible option, unfortunately cannot access to see why they regard the additional star hypothesis as implausible.

(We seem to have three threads about this system, wasn't sure which one best to put this in...)
https://solar-flux.forumotion.com/t275-planet-around-nu-octantis
https://solar-flux.forumotion.com/t999-more-fun-with-nu-octantis

Good call. Merged.

http://aas.org/meetings/aas227

Abstract of the AAS 227th meeting, next week :

Increasing evidence for Nu Octantis (A)b...

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

Quote :

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.

Link should be http://arxiv.org/abs/1605.05453

The paper was published in MNRAS in 2015.

A new update to the Nu Octantis system:

Ramm et al. "The conjectured S-type retrograde planet in nu Octantis: more evidence including four years of iodine-cell radial velocities"
http://arxiv.org/abs/1605.06720

Includes some estimates of the 3D structure of the system, mutual inclination seems to be around 152.5°.

Celestia definition for the system here.

Persisting evidence for a planet in Nu Octantis system

A photospheric and chromospheric activity analysis of the quiescent retrograde-planet host ν Octantis A

Quote :

The single-lined spectroscopic binary ν Octantis provided evidence of the first conjectured circumstellar planet demanding an orbit retrograde to the stellar orbits. The planet-like behaviour is now based on 1437 radial velocities (RVs) acquired from 2001-2013. ν Oct's semimajor axis is only 2.6 AU with the candidate planet orbiting ν Oct A about midway between. These details seriously challenge our understanding of planet formation and our decisive modelling of orbit reconfiguration and stability scenarios, whilst having a host that is equally inconsistent with accepted stellar variability scenarios. All non-planetary explanations are less credible based on multiple qualitative and quantitative tests including previous spectroscopic studies of bisectors and line-depth ratios, photometry from Hipparcos and the more recent space missions TESS and GAIA (whose increased parallax classifies ν Oct A closer still to a subgiant ~ K1 IV). We conducted the first large survey of ν Oct A's chromosphere: 198 Ca II H-line and 1160 H α indices using spectra from a previous RV campaign (2009-2013). We also acquired 135 spectra (2018-2020) primarily used for additional line-depth ratios, which are extremely sensitive to the photosphere's temperature. We found no significant RV-correlated variability. Our line-depth ratios indicate temperature variations of only ± 4 K, as achieved previously. Our atypical Ca II analysis models the indices in terms of S/N and includes covariance significantly in their errors. The H α indices have a quasi-periodic variability which we demonstrate is due to telluric lines. Our new evidence provides further multiple arguments realistically only in favor of the planet.