{{Short description|Brightest star in the constellation Octans}} {{Starbox begin | name = ν Octantis }} {{Starbox image |image= {{Location mark |image=Octans constellation map.svg|alt=|float=center|width=280 |label=|position=right |mark=Red circle.svg|mark_width=10|mark_link=Nu Octantis |x=536|y=334 }} |caption=Location of ν Octantis in Octans (circled) }} {{Starbox observe | epoch = J2000 | constell = Octans | ra = {{RA|21|41|28.64977}}<ref name=vanLeeuwen2007>{{cite journal | arxiv=0708.1752 | bibcode=2007A&A...474..653V | doi=10.1051/0004-6361:20078357 | title=Validation of the new Hipparcos reduction | journal=Astronomy and Astrophysics | volume=474 | issue=2 | pages=653–664 | year=2007 | last1=Van Leeuwen | first1=F. | s2cid=18759600 }} [http://vizier.u-strasbg.fr/viz-bin/VizieR-S?HIP%20107089 Vizier catalog entry]</ref> | dec = {{DEC|-77|23|24.1563}}<ref name=vanLeeuwen2007/> | appmag_v = 3.73<ref name=XHIP>{{cite journal | bibcode=2012AstL...38..331A | doi=10.1134/S1063773712050015 | arxiv=1108.4971 | title=XHIP: An extended hipparcos compilation | journal=Astronomy Letters | volume=38 | issue=5 | pages=331 | year=2012 | last1=Anderson | first1=E. | last2=Francis | first2=Ch. | s2cid=119257644 }} [http://vizier.u-strasbg.fr/viz-bin/VizieR-4?-source=V/137D/XHIP&HIP=107089 Vizier catalog entry]</ref> }} {{Starbox character | component1= A | type= Subgiant<ref name=Cheng2025/> | class= K1IV<ref name=Cheng2025/> | b-v = +1.00<ref name=Mallama2014>{{cite journal | bibcode=2014JAVSO..42..443M | title=Sloan Magnitudes for the Brightest Stars | journal=The Journal of the American Association of Variable Star Observers | volume=42 | pages=443 | last1=Mallama | first1=A. | year=2014 | issue=2 }}[http://vizier.u-strasbg.fr/viz-bin/VizieR-4?-source=J/other/JAVSO/42.443/ugriz&HD=205478 Vizier catalog entry]</ref> | u-b = +0.89<ref name=Mallama2014/> | component2=B | type2 = white dwarf<ref name=Cheng2025/> }} {{Starbox astrometry | radial_v = +34.40<ref name=Wilson1953>{{cite journal | last=Wilson | first=R. E. | year=1953 | title=General Catalogue of Stellar Radial Velocities | journal=Carnegie Institute Washington D.C. Publication | publisher=Carnegie Institution for Science | bibcode=1953GCRV..C......0W | lccn=54001336 }}</ref> | prop_mo_ra = +66.41 | prop_mo_dec = −239.10 | pm_footnote = <ref name=vanLeeuwen2007/> | parallax = 45.25 | p_error = 0.25 | parallax_footnote = <ref name="Ramm"/> | dist_pc = {{val|22.54|0.21}} | dist_ly = {{val|{{convert|22.54|pc|ly|disp=number}}|{{convert|0.21|pc|ly|disp=number}}}} | dist_footnote = <ref name=Cheng2025/> | absmag_v = {{val|2.3|0.16}}<ref name=Ramm3/> }} {{Starbox orbit |reference=<ref name=Cheng2025/> | period_unitless ={{val|1050.74|+0.20|-0.10}} days | eccentricity = {{val|0.2366|0.0003}} | axis_unitless = {{val|2.61|0.03}} AU | inclination = {{val|71.8|0.7|0.6}} | node = {{val|86.5|0.3}} | k2 = {{val|7.0648|0.0050|0.0009}} | periarg = {{val|74.88|0.06|0.14}} <!-- note: mean anomaly is 339.286 +0.023, -0.019 and epoch is not given --> }} {{Starbox detail | source = <ref name=Cheng2025/> | component1 = Nu Octantis A | mass = {{Val|1.57|0.06}} | radius = {{val|5.04|0.10}} | temperature = {{val|4811.0|4.2|fmt=commas}}<ref name=Ramm2015/> | luminosity = {{val|13.2|0.3}} | rotational_velocity = 2.0<ref name=Ramm2>{{cite journal|title=The conjectured S-type retrograde planet in ν Octantis: more evidence including four years of iodine-cell radial velocities|author1=Ramm, D. J.|display-authors=etal|journal=Monthly Notices of the Royal Astronomical Society|volume=460|issue=4|pages=3706–3719|date=2016|bibcode=2016MNRAS.460.3706R|doi=10.1093/mnras/stw1106|doi-access=free |arxiv=1605.06720}}</ref> | gravity = {{val|3.23|0.02|0.03}} | metal_fe={{val|+0.18|0.04}}<ref name=Ramm2/> | age_gyr = {{Val|2.70|0.35}} | component2 = Nu Octantis B | mass2 = {{val|0.57|0.01}} }} {{Starbox catalog | names = {{odlist | B=ν Oct | CCDM=J21415-7723 | CD=-77 1079 | FK5=810 | GC=30289 | GJ=835.1 | GJ2=9744 | HD=205478 | HIP=107089 | HR=8254 | SAO=257948 | WDS=J21415-7723}}<ref name=SIMBAD>{{cite simbad |title=* nu. Oct}}</ref> }} {{Starbox reference | Simbad = HD+205478 | NSTED = nu Oct }} {{Starbox end}}
'''ν Octantis''', Latinised as '''Nu Octantis''', is a binary 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. It is located at {{convert|22.54|pc|ly|abbr=off}} from Earth, and is moving away at a radial velocity of +34.4 km/s. The primary star has an exoplanet whose orbit lies halfway between both stars.
==Characteristics== This is a spectroscopic binary system,<ref name="Eggleton2008">{{cite journal | last1=Eggleton | first1=P. P. | last2=Tokovinin | first2=A. A. | title=A catalogue of multiplicity among bright stellar systems | journal=Monthly Notices of the Royal Astronomical Society | volume=389 | issue=2 | pages=869–879 | date=September 2008 | doi=10.1111/j.1365-2966.2008.13596.x | doi-access=free | bibcode=2008MNRAS.389..869E | arxiv=0806.2878 | s2cid=14878976 }}</ref> meaning the binarity was inferred from periodic Doppler shifts in the spectral lines, which correspond to the motion of the stars.<ref name=Struve1958>{{Cite book |last1=Struve |first1=Otto |title=Spectroscopic Binaries |date=1958 |work=Astrophysik I: Sternoberflächen-Doppelsterne / Astrophysics I: Stellar-Surfaces-Binaries |pages=243–273 |editor-last=Flügge |editor-first=S. |url=https://doi.org/10.1007/978-3-642-45906-1_8 |access-date=2025-05-26 |place=Berlin, Heidelberg |publisher=Springer |language=en |doi=10.1007/978-3-642-45906-1_8 |isbn=978-3-642-45906-1 |last2=Huang |first2=Su Shu}}</ref> Both stars take {{convert|1050|day|year|abbr=off}} to complete an orbit around each other, being separated by a semi-major axis of 2.61 astronomical units at a somewhat elliptical orbit.<ref name=Cheng2025/>
The primary has a spectral type of K1IV, with the luminosity class IV indicating that it is a subgiant star that has fused up all the hydrogen at its core and has expanded. Nu Octantis A has 1.57 times the mass of the Sun, but has expanded to 5.04 times the radius of the Sun.<ref name=Ramm2/> Its photosphere has cooled to an effective temperature of 4,811 K<ref name=Ramm2015>{{Cite journal |last=Ramm |first=D. J. |date=2015-06-01 |title=Line-depth-ratio temperatures for the close binary ν Octantis: new evidence supporting the conjectured circumstellar retrograde planet |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=449 |issue=4 |pages=4428–4442 |doi=10.1093/mnras/stv533 |doi-access=free |arxiv=1605.05453 |bibcode=2015MNRAS.449.4428R |issn=0035-8711}}</ref> and now is radiating 13.2 times as much luminosity as the Sun.<ref name=Cheng2025/>
The secondary star is a white dwarf with 0.57 times the mass of the Sun. When it was on the main sequence, it had a mass of {{val|2.36|0.13|0.15|u=solar mass}} and was closer to its primary, at {{val|1.31|0.07|u=AU}}. When it evolved to a red giant, and then to a white dwarf, it lost most of its mass, thus increasing the orbital separation. The primary star accreted about {{solar mass|0.2}} from the secondary during this period.<ref name=Cheng2025/>
Nu Octantis is unusual on that its Bayer designation would suggest it is one of the faintest stars in the constellation, but is actually the brightest, over one magnitude brighter than Alpha Octantis. It seems that Lacaille (who lettered the Bayer stars in Octans) believed that Nu Octantis was a double star (like Mu Octantis) of small angular separation, rather than a single bright star.<ref>{{Cite journal |last=Lynn |first=W. T. |date=April 1911 |title=The constellation Octans |url=https://ui.adsabs.harvard.edu/abs/1911Obs....34..160L/abstract |journal=The Observatory |language=en |volume=34 |pages=160 |bibcode=1911Obs....34..160L |issn=0029-7704}}</ref>
==Planetary system== thumb|left|Orbital configuration of the Nu Octantis system, centered on the subgiant primary. In 2009, the system was hypothesised to contain a superjovian exoplanet based on variations in the radial velocity.<ref name="Ramm">{{cite journal |last1=Ramm |first1=D. J. |last2=Pourbaix |first2=D. |last3=Hearnshaw |first3=J. B. |last4=Komonjinda |first4=S. |title=Spectroscopic orbits for K giants β Reticuli and ν Octantis: what is causing a low-amplitude radial velocity resonant perturbation in ν Oct? |journal=Monthly Notices of the Royal Astronomical Society |volume=394 |issue=3 |pages=1695–1710 |date=April 2009 |doi=10.1111/j.1365-2966.2009.14459.x |bibcode=2009MNRAS.394.1695R|doi-access=free }}</ref> A prograde solution was quickly ruled out<ref name="Eberle">{{cite journal|last1=Eberle|first1=J.|last2=Cuntz|first2=M.|title=On the reality of the suggested planet in the ν Octantis system|journal=The Astrophysical Journal|volume=721|issue=2|date=October 2010|pages=L168–L171|bibcode=2010ApJ...721L.168E|doi=10.1088/2041-8205/721/2/L168|doi-access=free}}</ref> but a retrograde solution remains a possibility, although a study posited that it may instead be due to the secondary star being itself a close binary,<ref name="Morais">{{cite journal|last1=Morais|first1=M. H. M.|last2=Correia|first2=A. C. M.|title=Precession due to a close binary system: an alternative explanation for ν-Octantis?|journal=Monthly Notices of the Royal Astronomical Society|volume=419|issue=4|date=February 2012|pages=3447–3456|doi=10.1111/j.1365-2966.2011.19986.x|doi-access=free |bibcode=2012MNRAS.419.3447M|arxiv = 1110.3176 |s2cid=119152109}}</ref> since the formation of a planet in such a system would be difficult due to gravitational perturbations.<ref name="Gozdziewski">{{cite journal|last1=Gozdziewski|first1=K.|last2=Slonina|first2=M.|last3=Migaszewski|first3=C.|last4=Rozenkiewicz|first4=A.|title=Testing a hypothesis of the ν Octantis planetary system|journal=Monthly Notices of the Royal Astronomical Society|volume=430|issue=1|date=March 2013|pages=533–545|doi=10.1093/mnras/sts652|doi-access=free |bibcode=2013MNRAS.430..533G|arxiv = 1205.1341 }}</ref> Further evidence ruling out a stellar variability and favouring the existence of the planet was gathered by 2021.<ref name="Ramm3">{{cite journal|last1=Ramm|first1=D J|last2=Robertson|first2=P|last3=Reffert|first3=S|last4=Gunn|first4=F|last5=Trifonov|first5=T|last6=Pollard|first6=K|last7=Cantalloube|first7=F|display-authors=2|title=A photospheric and chromospheric activity analysis of the quiescent retrograde-planet host ν Octantis A|journal=Monthly Notices of the Royal Astronomical Society|year=2021|volume=502|issue=2|pages=2793–2806|arxiv=2101.06844|doi=10.1093/mnras/stab078|doi-access=free }}</ref> With new radial velocity measurements, a study in 2025 confirmed the planet's existence.<ref name=Cheng2025>{{Cite journal |last1=Cheng |first1=Ho Wan |last2=Trifonov |first2=Trifon |last3=Lee |first3=Man Hoi |last4=Cantalloube |first4=Faustine |last5=Reffert |first5=Sabine |last6=Ramm |first6=David |last7=Quirrenbach |first7=Andreas |date=May 2025 |title=A retrograde planet in a tight binary star system with a white dwarf |url=https://rdcu.be/enkbR |journal=Nature |language=en |volume=641 |issue=8064 |pages=866–870 |doi=10.1038/s41586-025-09006-x |pmid=40399630 |bibcode=2025Natur.641..866C |issn=1476-4687}}</ref>
The binary components in the Nu Octantis system were initially separated by 1.3 AU, which overlap with the current orbital separation of the planet. Therefore, the planet did not form in its current orbit, but either migrated from a longer, circumbinary orbit, or originated from a protoplanetary disc that formed after the death of the white dwarf's progenitor.<ref name=Cheng2025/>
{{OrbitboxPlanet begin | name = Nu Octantis A | table_ref=<ref name=Cheng2025/> }} {{OrbitboxPlanet | exoplanet = b | mass = {{val|2.19|0.11}} | semimajor = {{Val|1.24|0.02}} | period = {{Val|402.4|7.7|6.0}} | eccentricity = {{Val|0.195|0.050|0.037}} | inclination = 108.2 }} {{Orbitbox end}}
==See also== * Gamma Cephei and Nu<sup>2</sup> Canis Majoris, another similar-sized giant stars hosting a jovian planet * Beta Hydri
==References== {{reflist}}
{{Stars of Octans}}
{{DEFAULTSORT:Nu Octantis}} Category:Octans Category:K-type giants Category:Spectroscopic binaries Octantis, Nu CD-77 1079 107089 8254 205478 9744 Category:Planetary systems with one confirmed planet Category:Population I stars Category:Lucidae