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40 Eridani

Coordinates:Sky map04h15m16.32s,−07° 39′ 10.34″
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For other stars with thisBayer designation,seeο Eridani.
40 Eridani / Keid
Location of 40 Eridani (circled)
Observation data
EpochJ2000.0EquinoxJ2000.0
Constellation Eridanus
40 Eridani A
Right ascension 04h15m16.31962s[1]
Declination −07° 39′ 10.3308″[1]
Apparent magnitude(V) 4.43[2]
40 Eridani B
Right ascension 04h15m21.79572s[3]
Declination −07° 39′ 29.2040″[3]
Apparent magnitude(V) 9.52[4]
40 Eridani C
Right ascension 04h15m21.53600s[5]
Declination −07° 39′ 20.6946″[5]
Apparent magnitude(V) 11.17[4]
Characteristics
40 Eridani A
Spectral type K0.5V[6]
U−Bcolor index +0.45[4]
B−Vcolor index +0.82[2]
40 Eridani B
Spectral type DA4[4]
U−Bcolor index +0.45[4]
B−Vcolor index +0.03[4]
40 Eridani C
Spectral type M4.5eV[7]
U−Bcolor index +0.83[4]
B−Vcolor index +1.67[4]
Variable type Flare star[8]
Astrometry
40 Eridani A
Radial velocity(Rv)−42.47±0.12[1]km/s
Proper motion(μ)RA:−2,240.085mas/yr[1]
Dec.:−3,421.809mas/yr[1]
Parallax(π)199.6080 ± 0.1208mas[1]
Distance16.340 ± 0.010ly
(5.010 ± 0.003pc)
Absolute magnitude(MV)5.93[9]
40 Eridani B
Radial velocity(Rv)−21[10]km/s
Proper motion(μ)RA:−2,236.169mas/yr[3]
Dec.:−3,338.955mas/yr[3]
Parallax(π)199.6911 ± 0.0512mas[3]
Distance16.333 ± 0.004ly
(5.008 ± 0.001pc)
40 Eridani C
Radial velocity(Rv)−44.06±0.20[5]km/s
Proper motion(μ)RA:−2,247.183mas/yr[5]
Dec.:−3,409.824mas/yr[5]
Parallax(π)199.4516 ± 0.0692mas[5]
Distance16.353 ± 0.006ly
(5.014 ± 0.002pc)
Orbit
Primary40 Eridani A
Companion40 Eridani BC
Period(P)~8,000[11]yr
Semi-major axis(a)~400[12]AU
Orbit[13]
Primary40 Eridani B
Companion40 Eridani C
Period(P)230.30±0.68yr
Semi-major axis(a)6.930±0.050"
(~35AU)
Eccentricity(e)0.4294±0.0027
Inclination(i)107.56±0.29°
Longitude of the node(Ω)151.44±0.12°
Periastronepoch(T)1847.7±1.1
Argument of periastron(ω)
(secondary)		318.4±1.1°
Details
40 Eridani A
Mass0.78±0.08[14]M☉
Radius0.804±0.006[15]R☉
Luminosity0.4±0.01[15]L☉
Surface gravity(logg)4.35±0.1[15]cgs
Temperature5126±30[15]K
Metallicity[Fe/H]−0.36±0.02[15]dex
Rotation~37–43[16]days
Rotational velocity(vsini)1.23 ± 0.28[16]km/s
Age6.9±4.7[14]Gyr
40 Eridani B
Mass0.573±0.018[13]M☉
Radius0.014[17]R☉
Luminosity0.013[18]L☉
Temperature16,500[19]K
40 Eridani C
Mass0.271±0.001[20]M☉
Radius0.298±0.009[20]R☉
Luminosity0.00651±0.00013[20]L☉
Surface gravity(logg)~5.5[20]cgs
Temperature3,100[21]K
Age5.6[22]Gyr
1.8[23]Gyr
Other designations
ο2Eri,40 Eri,GJ166,ADS3093,CCDMJ04153-0739
A:Keid[24],BD−07° 780,HD26965,HIP19849,HR1325,SAO131063,LHS23,LTT1907
B:BD−07° 781,HD26976,SAO131065,G160-060,LHS24,LTT1908
C:DY Eri,BD−07°781 C,LHS25,LTT1909
Database references
SIMBADA
B
C

40 Eridaniis atriple star systemin theconstellationofEridanus,abbreviated40 Eri.It has theBayer designationOmicron2Eridani,which isLatinizedfromο2Eridaniand abbreviated Omicron2Eri orο2Eri.Based onparallaxmeasurements taken by theGaiamission, it is about 16.3light-yearsfrom theSun.

The primary star of the system, designated 40 Eridani A and namedKeid,[24]is easily visible to the naked eye. It is orbited by abinary pairwhose two components are designated 40 Eridani B and C, and which were discovered on January 31, 1783, byWilliam Herschel.[25]: p73 It was again observed byFriedrich Struvein 1825 and byOtto Struvein 1851.[11][26]

In 1910, it was discovered that although component B was a faint star, it was white in color. This meant that it had to be a small star; in fact it was awhite dwarf,the first discovered.[27]Although it is neither the closest white dwarf, nor the brightest in the night sky, it is by far the easiest to observe; it is nearly three magnitudes brighter thanVan Maanen's Star,the nearest solitary white dwarf, and unlike the companions ofProcyonandSiriusit is not swamped in the glare of a much brighter primary.[18]

Nomenclature

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40 Eridani is the system'sFlamsteed designationand ο² Eridani (Latinisedto Omicron2Eridani) itsBayer designation.The designations of the sub-components –40 Eridani A,B and C – derive from the convention used by theWashington Multiplicity Catalog(WMC) formultiple star systems,and adopted by theInternational Astronomical Union(IAU).[28]40 Eridani Calso bears thevariable star designationDY Eridani.

The system bore the traditional nameKeidderived from theArabicword قيض (alqayḍ) meaning "theeggshells,"alluding to its neighbourBeid(Arabic "egg" ).[29]In 2016, the IAU organized aWorking Group on Star Names(WGSN)[30] to catalogue and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entiremultiple systems.[31] It approved the nameKeidfor the component40 Eridani Aon 12 September 2016 and it is now so included in the List of IAU-approved Star Names.[24]

Properties

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Amateur photo of 40 Eridani

40 Eridani A is amain-sequencedwarfofspectral typeK1, 40 Eridani B is a 9th magnitudewhite dwarfof spectral type DA4, and 40 Eridani C is an 11th magnitudered dwarfflare starof spectral type M4.5e. When component B was a main-sequence star, it is thought to have been the most massive member of the system, but ejected most of its mass before it became a white dwarf.[23]B and Corbiteach other approximately 400AUfrom the primary star, A.[12]Their orbit has a semimajor axis of 35AUand is rather elliptical with anorbital eccentricityof 0.410).[11]

As seen from the 40 Eridani system, the Sun is a 3.4-magnitude star inHercules,near the border withSerpens Caput.[note 1]

Potential for life

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Thehabitable zoneof40 Eridani A,where a planet could exist with liquid water, is near 0.68AUfrom A. At this distance a planet would complete a revolution in 223 Earth days (according to the third ofKepler's laws) and40 Eridani Awould appear nearly 20%[note 2]wider than the Sun does on Earth. An observer on a planet in the40 Eridani Asystem would see the B-C pair as unusually bright white and reddish-orange stars in the night sky –magnitudes−8 and −6, slightly brighter than the appearance ofVenusseen from Earth as theevening star.

It is unlikely that habitable planets exist around40 Eridani Bbecause they would have been sterilized by its evolution into a white dwarf. As for40 Eridani C,it is prone to flares, which cause large momentary increases in the emission ofX-raysas well as visible light. This would be lethal toEarth-type life on planets near the flare star.[12]

Search for planets

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40 Eridani A shows periodicradial velocity variations,which were suggested to be caused by a planetary companion. The 42-day period is close to the stellar rotation period, which made the possible planetary nature of the signal difficult to confirm.[16]A 2018 study found that most evidence supports a planetary origin for the signal,[14]but this has remained controversial, with a 2021 study characterizing the signal as a false positive.[32]As of 2022, the cause of the radial velocity variations remained inconclusive.[33]

Further studies in 2023[34]: 23–24, 44 and 2024 concluded that the radial velocity signal very likely does originate from stellar activity, and not from a planet.[35]

The candidate planet would have had aminimum massof8.47±0.47M🜨,and lie considerably interior to the habitable zone, receiving nine times more stellar flux than Earth, which is an even greater amount thanMercury,the innermost planet in the Solar System, on average receives from the Sun.[14]

In fiction

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In theStar Trekfranchise, the planetVulcanorbits 40 Eridani A.[36]Vulcan has been referenced in relation to the real-life search for exoplanets in this system.[37][35]The hypothetical planet 40 Eridani A b is also mentioned in the bookProject Hail Maryas the home of the eponymous Eridian species.[38]

Notes

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  1. ^From 40 Eridani the Sun would appear on the diametrically opposite side of the sky at the coordinates RA=16h15m16.32s,Dec=07° 39′ 10.34″, which is located near the border ofHercules (constellation)andSerpens Caput,the closest bright star beingAlpha Serpentis.The absolute magnitude of the Sun is 4.85, so, at a distance of 5.04 parsecs, the Sun would have an apparent magnitude.
  2. ^Fromwherehis the apparent height,dis the distance of the object, andais the actual size of the object.

References

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  2. ^abvan Leeuwen, F. (2007). "Validation of the new Hipparcos reduction".Astronomy and Astrophysics.474(2): 653–664.arXiv:0708.1752.Bibcode:2007A&A...474..653V.doi:10.1051/0004-6361:20078357.S2CID18759600.
  3. ^abcdVallenari, A.; et al. (Gaia collaboration) (2023)."GaiaData Release 3. Summary of the content and survey properties ".Astronomy and Astrophysics.674:A1.arXiv:2208.00211.Bibcode:2023A&A...674A...1G.doi:10.1051/0004-6361/202243940.S2CID244398875. Gaia DR3 record for this sourceatVizieR.
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