Nu Octantis

ν Octantis, Latinised as Nu Octantis, is a star in the constellation of Octans. Unusually for having such a late greek letter in its name, it is the brightest star in this faint constellation at apparent magnitude +3.7.^[2] It is a spectroscopic binary^[9] star with a period around 2.9 years. Parallax measurements place it at 22.1 parsecs (72 ly) from Earth.^[7]

ν Octantis

Location of ν Octantis in Octans (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Octans
Right ascension	21h 41m 28.64977s[1]
Declination	−77° 23′ 24.1563″[1]
Apparent magnitude (V)	3.73[2]
Characteristics
A
Evolutionary stage	Giant star
Spectral type	K1III[3]
U−B color index	+0.89[4]
B−V color index	+1.00[4]
B
Evolutionary stage	Either a main sequence star or a white dwarf[5]
Spectral type	K7–M0V or WD[5]
Astrometry
Radial velocity (Rv)	+34.40[6] km/s
Proper motion (μ)	RA: +66.41 mas/yr[1] Dec.: −239.10 mas/yr[1]
Parallax (π)	45.25±0.25 mas[7]
Distance	72.1 ± 0.4 ly (22.1 ± 0.1 pc)
Absolute magnitude (MV)	2.3±0.16[5]
Orbit[8]
Period (P)	1050.69+0.05 −0.07 d
Semi-major axis (a)	2.62959+0.00009 −0.00011 AU
Eccentricity (e)	0.23680±0.00007
Inclination (i)	70.8±0.9°
Longitude of the node (Ω)	87±1.2°
Argument of periastron (ω) (secondary)	74.970±0.016°
Semi-amplitude (K1) (primary)	7.032±0.003 km/s
Details
Nu Octantis A
Mass	1.61[8] M☉
Radius	5.81±0.12[8] R☉
Luminosity	17.53[2] L☉
Surface gravity (log g)	3.12±0.10[8] cgs
Temperature	4,860±40[7] K
Metallicity [Fe/H]	+0.18±0.04[8] dex
Rotational velocity (v sin i)	2.0[8] km/s
Age	~2.5-3[8] Gyr
Nu Octantis B[5]
Mass	0.585[8] M☉
Other designations
nu Oct, CD−77 1079, CCDM J21415-7723, FK5 810, GC 30289, GJ 835.1, GJ 9744, HIP 107089, HR 8254, HD 205478, SAO 257948, WDS J21415-7723
Database references
SIMBAD	data

The primary has a spectral type of K1III,^[3] 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.^[8] Its photosphere has cooler to an effective temperature of 4,860 K^[7] and now is radiating 18 times as much luminosity as the Sun.^[2] It possibly hosts an extrasolar planet, a jovian planet on a retograde orbit.^[5]

The secondary star is likely either a red dwarf or a white dwarf, from its relatively low mass.^[5] 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.^[8]

Planetary system

In 2009, the system was hypothesised to contain a superjovian exoplanet based on perturbations in the orbital period.^[7] A prograde solution was quickly ruled out^[10] but a retrograde solution remains a possibility, although the variations may instead be due to the secondary star being itself a close binary,^[11] since the formation of a planet in such a system would be difficult due to dynamic perturbations.^[12] Further evidence ruling out a stellar variability and favouring the existence of the planet was gathered by 2021.^[5]

The Nu Octantis A planetary system^[8]

Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (days)	Eccentricity	Inclination	Radius
b	2.1059 MJ	1.276	414.8	0.086	112.5°	—

See also

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