40 Eridani - Misplaced Pages

(Redirected from Keid) Triple star system in the constellation Eridanus For other stars with this Bayer designation, see ο Eridani.

40 Eridani / Keid
Observation data Epoch J2000.0 Equinox J2000.0
Location of 40 Eridani (circled)
Constellation	Eridanus
40 Eridani A
Right ascension	04 15 16.31962
Declination	−07° 39′ 10.3308″
Apparent magnitude (V)	4.43
40 Eridani B
Right ascension	04 15 21.79572
Declination	−07° 39′ 29.2040″
Apparent magnitude (V)	9.52
40 Eridani C
Right ascension	04 15 21.53600
Declination	−07° 39′ 20.6946″
Apparent magnitude (V)	11.17
Characteristics
40 Eridani A
Spectral type	K0.5V
U−B color index	+0.45
B−V color index	+0.82

40 Eridani B
Spectral type	DA4
U−B color index	+0.45
B−V color index	+0.03
40 Eridani C
Spectral type	M4.5eV
U−B color index	+0.83
B−V color index	+1.67
Variable type	Flare star
Astrometry
40 Eridani A
Radial velocity (R_v)	−42.47±0.12 km/s
Proper motion (μ)	RA: −2,240.085 mas/yr Dec.: −3,421.809 mas/yr
Parallax (π)	199.6080 ± 0.1208 mas
Distance	16.340 ± 0.010 ly (5.010 ± 0.003 pc)
Absolute magnitude (M_V)	5.93


40 Eridani B
Radial velocity (R_v)	−21 km/s
Proper motion (μ)	RA: −2,236.169 mas/yr Dec.: −3,338.955 mas/yr
Parallax (π)	199.6911 ± 0.0512 mas
Distance	16.333 ± 0.004 ly (5.008 ± 0.001 pc)
40 Eridani C
Radial velocity (R_v)	−44.06±0.20 km/s
Proper motion (μ)	RA: −2,247.183 mas/yr Dec.: −3,409.824 mas/yr
Parallax (π)	199.4516 ± 0.0692 mas
Distance	16.353 ± 0.006 ly (5.014 ± 0.002 pc)
Orbit
Primary	40 Eridani A
Companion	40 Eridani BC
Period (P)	~8,000 yr
Semi-major axis (a)	~400 AU
Orbit
Primary	40 Eridani B
Companion	40 Eridani C
Period (P)	230.30±0.68 yr
Semi-major axis (a)	6.930±0.050" (~35 AU)
Eccentricity (e)	0.4294±0.0027
Inclination (i)	107.56±0.29°
Longitude of the node (Ω)	151.44±0.12°
Periastron epoch (T)	1847.7±1.1
Argument of periastron (ω) (secondary)	318.4±1.1°
Details
40 Eridani A
Mass	0.78±0.08 `M`_☉
Radius	0.804±0.006 `R`_☉
Luminosity	0.4±0.01 `L`_☉
Surface gravity (log g)	4.35±0.1 cgs
Temperature	5126±30 K
Metallicity	−0.36±0.02 dex
Rotation	~37–43 days
Rotational velocity (v sin i)	1.23 ± 0.28 km/s
Age	6.9±4.7 Gyr


40 Eridani B
Mass	0.573±0.018 `M`_☉
Radius	0.01308±0.00020 `R`_☉
Luminosity	0.01349±0.00054 `L`_☉
Surface gravity (log g)	7.957±0.020 cgs
Temperature	17,200±110 K
40 Eridani C
Mass	0.222±0.22 `M`_☉
Radius	0.274±0.011 `R`_☉
Luminosity	0.00651±0.00013 `L`_☉
Surface gravity (log g)	~5.5 cgs
Temperature	3,167±60 K
Age	1.8 Gyr
Other designations
ο Eri, 40 Eri, GJ 166, ADS 3093, CCDM J04153-0739
A: Keid, BD−07° 780, HD 26965, HIP 19849, HR 1325, SAO 131063, LHS 23, LTT 1907
B: BD−07° 781, HD 26976, SAO 131065, G 160-060, LHS 24, LTT 1908
C: DY Eri, BD−07°781 C, LHS 25, LTT 1909
Database references
SIMBAD	A
	B
	C

40 Eridani is a triple star system in the constellation of Eridanus, abbreviated 40 Eri. It has the Bayer designation Omicron Eridani, which is Latinized from ο Eridani and abbreviated Omicron Eri or ο Eri. Based on parallax measurements taken by the Gaia mission, it is about 16.3 light-years from the Sun.

The primary star of the system, designated 40 Eridani A and named Keid, is easily visible to the naked eye. It is orbited by a binary pair whose two components are designated 40 Eridani B and C, and which were discovered on January 31, 1783, by William Herschel. It was again observed by Friedrich Struve in 1825 and by Otto Struve in 1851.

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 a white dwarf, the first discovered. 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 than Van Maanen's Star, the nearest solitary white dwarf, and unlike the companions of Procyon and Sirius it is not swamped in the glare of a much brighter primary.

Nomenclature

40 Eridani is the system's Flamsteed designation and ο² Eridani (Latinised to Omicron Eridani) its Bayer designation. The designations of the sub-components – 40 Eridani A, B and C – derive from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems, and adopted by the International Astronomical Union (IAU). 40 Eridani C also bears the variable star designation DY Eridani.

The system bore the traditional name Keid derived from the Arabic word قيض (alqayḍ) meaning "the eggshells," alluding to its neighbour Beid (Arabic "egg"). In 2016, the IAU organized a Working Group on Star Names (WGSN) to catalogue and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems. It approved the name Keid for the component 40 Eridani A on 12 September 2016 and it is now so included in the List of IAU-approved Star Names.

Properties

40 Eridani A is a main-sequence dwarf of spectral type K1, 40 Eridani B is a 9th magnitude white dwarf of spectral type DA4, and 40 Eridani C is an 11th magnitude red dwarf flare star of 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. B and C orbit each other approximately 400 AU from the primary star, A. Their orbit has a semimajor axis of 35 AU and is rather elliptical with an orbital eccentricity of 0.410).

As seen from the 40 Eridani system, the Sun is a 3.4-magnitude star in Hercules, near the border with Serpens Caput.

Potential for life

The habitable zone of 40 Eridani A, where a planet could exist with liquid water, is near 0.68 AU from A. At this distance a planet would complete a revolution in 223 Earth days (according to the third of Kepler's laws) and 40 Eridani A would appear nearly 20% wider than the Sun does on Earth. An observer on a planet in the 40 Eridani A system 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 of Venus seen from Earth as the evening star.

It is unlikely that habitable planets exist around 40 Eridani B because they would have been sterilized by its evolution into a white dwarf. As for 40 Eridani C, it is prone to flares, which cause large momentary increases in the emission of X-rays as well as visible light. This would be lethal to Earth-type life on planets near the flare star.

Search for planets

40 Eridani A shows periodic radial 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. A 2018 study found that most evidence supports a planetary origin for the signal, but this has remained controversial, with a 2021 study characterizing the signal as a false positive. As of 2022, the cause of the radial velocity variations remained inconclusive.

Further studies in 2023 and 2024 concluded that the radial velocity signal very likely does originate from stellar activity, and not from a planet.

The candidate planet would have had a minimum mass of 8.47±0.47 M_🜨, and lie considerably interior to the habitable zone, receiving nine times more stellar flux than Earth, which is an even greater amount than Mercury, the innermost planet in the Solar System, on average receives from the Sun.

In fiction

In the Star Trek franchise, the planet Vulcan orbits 40 Eridani A. Vulcan has been referenced in relation to the real-life search for exoplanets in this system. The hypothetical planet 40 Eridani A b is also mentioned in the book Project Hail Mary as the home of the eponymous Eridian species.

Notes

From 40 Eridani the Sun would appear on the diametrically opposite side of the sky at the coordinates RA=16 15 16.32, Dec=07° 39′ 10.34″, which is located near the border of Hercules (constellation) and Serpens Caput, the closest bright star being Alpha 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 ${\begin{smallmatrix}m\ =\ M_{v}\ +\ 5\cdot ((\log _{10}\ 5.04)\ -\ 1)\ =\ 3.36\end{smallmatrix}}$ .
From $\ h={\frac {\ a\ }{d}}\,,$ where h is the apparent height, d is the distance of the object, and a is the actual size of the object.

References

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External links

Keid at Jim Kaler's STARS.
Omicron(2) Eridani entry at the Internet Stellar Database.

← Celestial objects within 15–20 light-years

Primary member type

Celestial objects by systems.

Subgiant stars

G-type	Delta Pavonis (19.893±0.015 ly)

Main-sequence
stars

A-type	Altair (Alpha Aquilae) (16.730±0.049 ly)
G-type	Alsafi (Sigma Draconis) (18.7993±0.0081 ly) Achird (Eta Cassiopeiae) (19.3314±0.0025 ly) K-type main-sequence star B e (82 G.) Eridani (19.7045±0.0093 ly) 3 (6?) planets: b, c?, d (c), e?, f?, g?, d
K-type	Groombridge 1618 (15.8857±0.0017 ly) Omicron (40) Eridani (16.3330±0.0042 ly) white dwarf B red dwarf C 70 Ophiuchi (16.7074±0.0087 ly) K-type main-sequence star B Gliese 570 (19.1987±0.0074 ly) 2 red dwarfs: B, C T-type brown dwarf D 36 Ophiuchi (19.4185±0.0036 ly) 2 K-type main-sequence stars: B, C HR 7703 (19.609±0.013 ly) red dwarf B
M-type (red dwarfs)	GJ 1245 (15.2001±0.0034 ly) 2 red dwarfs: B, C Gliese 876 (15.2382±0.0025 ly) 4 planets: d, c, b, e LHS 288 (15.7586±0.0034 ly) GJ 1002 (15.8060±0.0036 ly) 2 planets: b, c Gliese 412 (15.9969±0.0026 ly) red dwarf B AD Leonis (16.1939±0.0024 ly) Gliese 832 (16.2005±0.0019 ly) planet b Gliese 682 (16.3328±0.0026 ly) 2? planets: b?, c? EV Lacertae (16.4761±0.0018 ly) G 9-38 (16.800±0.011 ly) red dwarf B GJ 3379 (16.9861±0.0027 ly) Gliese 445 (17.1368±0.0017 ly) 2M1540 (17.3738±0.0046 ly) GJ 3323 (17.5309±0.0026 ly) 2 planets: b, c Gliese 526 (17.7263±0.0024 ly) Stein 2051 (17.9925±0.0020 ly) white dwarf B Gliese 251 (18.2146±0.0028 ly) planet b LP 816-60 (18.3305±0.0038 ly) LSR J1835+3259 (18.5534±0.0049 ly) Gliese 205 (18.6042±0.0022 ly) Gliese 229 (18.7906±0.0018 ly) T-type brown dwarf B 2 planets: Ab, Ac Ross 47 (18.8883±0.0031 ly) Gliese 693 (19.2078±0.0053 ly) Gliese 754 (19.2724±0.0067 ly) Gliese 908 (19.2745±0.0032 ly) Gliese 752 (19.2922±0.0027 ly) red dwarf B (vB 10) planet Ab Gliese 588 (19.2996±0.0031 ly) 2? planets: b?, c? YZ Canis Minoris (19.5330±0.0040 ly) GJ 1005 (19.577±0.035 ly) red dwarf B Gliese 268 (19.7414±0.0076 ly) red dwarf B

Degenerate
stars

White dwarfs	Gliese 440 (15.1226±0.0013 ly)

Brown dwarfs

L-type	DENIS 0255−4700 (15.877±0.014 ly)
T-type	WISE 1741+2553 (15.22±0.20 ly) WISE 1506+7027 (16.856±0.052 ly) DENIS 0817-6155 (17.002±0.037 ly) 2MASS 0939-2448 (17.41±0.44 ly) T-type brown dwarf B 2MASS 1114-2618 (18.20±0.14 ly) 2MASS 0415-0935 (18.62±0.18 ly) SIMP0136 (19.955±0.057 ly) 2MASS 0937+2931 (19.96 _−0.21 ly)
Y-type	WISE 1639−6847 (15.450±0.041 ly) WISE 0350−5658 (18.49±0.24 ly)

Sub-brown dwarfs
and rogue planets

Y-type	WISE 1541−2250 (19.54±0.24 ly)

Constellation of Eridanus

Stars

Bayer	α (Achernar) β (Cursa) γ (Zaurak) δ (Rana) ε (Ran) ζ (Zibal) η (Azha) θ (Acamar) θ ι κ λ μ ν ξ ο (Beid) ο (40, Keid) π ρ ρ ρ τ τ (Angetenar) τ τ τ τ τ τ τ υ υ (Theemin) υ (Beemim) υ φ χ ψ ω
Flamsteed	14 15 17 (v) 39 (A) 45 51 (c) 53 (Sceptrum) 54 58 64 66
Variable	DO DZ EF EK EM (d) EP KT
HR	486 (p) 487 (p) 506 (q) 789 (s) 859 1008 (e) 1106 (y) 1143 (h) 1189 (f) 1190 (f) 1195 (g) 1483 1536 1614
HD	18742 (Ayeyarwady) 19467 28185 30562 30856 (Mouhoun) 30963
Other	2MASS J04151954−0935066 DENIS 0255−4700 Gliese 86 Gliese 180 GJ 1062 GJ 3293 GJ 3323 HIP 12961 (Koeia) LTT 1445 LP 890-9 Mothra WASP-22 (Tojil) WASP-35 WASP-50 (Chaophraya) WASP-78 WASP-79 (Montuno) WISE J0336−0143

Exoplanets	51 Eridani b ε Eridani b (AEgir) Gliese 86 b HD 10647 b HD 18742 b (Bagan) HD 28185 b HD 30562 b HD 30856 Ab (Nakanbé) WASP-79b (Pollera)

Nebulae

NGC	1535

Galaxies

NGC	782 852 939 1084 1132 1140 1154 1155 1157 1158 1172 1187 1189 1190 1191 1192 1199 1222 1232 1234 1241 1253 1262 1266 1269 1291 1300 1309 1325 1332 1337 1345 1347 1353 1357 1369 1386 1395 1400 1403 1404 1407 1426 1427A 1436 1437 1439 1452 1460 1487 1531 1532 1592 1600 1614 1637 1721 1723 1725 1728 1741
Other	Eridanus II ESO 198-13 ESO 0313-192 IC 1953 IC 2006 LEDA 74886 MRC 0406-244 PKS 0208-512 PKS 0405–385 PKS 0420-014 UGC 2812

Galaxy clusters	Abell 383 Eridanus Cluster Eridanus Group MACS J0416.1-2403

Astronomical events	GRB 080913 HE0435-1223