Nu Octantis Explained

ν Octantis, Latinised as Nu Octantis, is a star in the constellation of Octans. Unusually, it is the brightest star in this faint constellation at apparent magnitude +3.7. It is a spectroscopic binary^[1] star with a period around 2.9 years. Parallax measurements place it at 19.4pc from Earth.

The primary has a spectral type of K1III, with the luminosity class III indicating that it is a giant star that has burned up the hydrogen at its core and has expanded. Nu Octantis A has 1.6 times the mass of the Sun, but has expanded to 5.8 times the radius of the Sun. Its photosphere has cooler to an effective temperature of 4,860 K and now is radiating 18 times as much luminosity as the Sun. It possibly hosts an extrasolar planet, a jovian planet on a retograde orbit.

The secondary star is likely either a red dwarf or a white dwarf, from its relatively low mass. This star is estimated to have around 60% the mass of the Sun. It shares a center of mass with the primary, completing an orbit around it every 2 years and 11 months. The orbit has an eccentricity of 0.24 and a semi-major axis of 2.63 au.

Planetary system

In 2009, the system was hypothesised to contain a superjovian exoplanet based on perturbations in the orbital period.^[2] A prograde solution was quickly ruled out^[3] but a retrograde solution remains a possibility, although the variations may instead be due to the secondary star being itself a close binary,^[4] since the formation of a planet in such a system would be difficult due to dynamic perturbations.^[5] Further evidence ruling out a stellar variability and favouring the existence of the planet was gathered by 2021.^[6]

See also

• Gamma Cephei and Nu² Canis Majoris, another similar-sized giant stars hosting a jovian planet
• Beta Hydri

Notes and References

1. Eggleton . P. P. . Tokovinin . A. A. . A catalogue of multiplicity among bright stellar systems . . 389 . 2 . 869–879 . September 2008 . 10.1111/j.1365-2966.2008.13596.x . free . 2008MNRAS.389..869E . 0806.2878. 14878976 .
2. Ramm . D. J. . Pourbaix . D. . Hearnshaw . J. B. . Komonjinda . S. . Spectroscopic orbits for K giants β Reticuli and ν Octantis: what is causing a low-amplitude radial velocity resonant perturbation in ν Oct? . Monthly Notices of the Royal Astronomical Society . 394 . 3 . 1695–1710 . April 2009 . 10.1111/j.1365-2966.2009.14459.x . 2009MNRAS.394.1695R. free .
3. Eberle. J.. Cuntz. M.. On the reality of the suggested planet in the ν Octantis system. The Astrophysical Journal. 721. 2. October 2010. L168–L171. 2010ApJ...721L.168E. 10.1088/2041-8205/721/2/L168. free.
4. Morais. M. H. M.. Correia. A. C. M.. Precession due to a close binary system: an alternative explanation for ν-Octantis?. Monthly Notices of the Royal Astronomical Society. 419. 4. February 2012. 3447–3456. 10.1111/j.1365-2966.2011.19986.x. free . 2012MNRAS.419.3447M. 1110.3176 . 119152109.
5. Gozdziewski. K.. Slonina. M.. Migaszewski. C.. Rozenkiewicz. A.. Testing a hypothesis of the ν Octantis planetary system. Monthly Notices of the Royal Astronomical Society. 430. 1. March 2013. 533–545. 10.1093/mnras/sts652. free . 2013MNRAS.430..533G. 1205.1341 .
6. Ramm. D J. Robertson. P. Reffert. S. Gunn. F. Trifonov. T. Pollard. K. Cantalloube. F. 2. A photospheric and chromospheric activity analysis of the quiescent retrograde-planet host ν Octantis A. Monthly Notices of the Royal Astronomical Society. 2021. 502. 2. 2793–2806. 2101.06844. 10.1093/mnras/stab078. free .