40 Eridani

40 Eridani (abbreviated 40 Eri), also designated Omicron² Eridani (ο² Eridani, abbreviated Omicron² Eri, ο² Eri), is a triple star system in the constellation of Eridanus. Based on parallax measurements taken during the Hipparcos mission, it is less than 17 light-years from the Sun.

For other stars with this Bayer designation, see ο Eridani.
40 Eridani

A star chart of the Eridanus constellation showing the position of 40 Eridani (circled)
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Eridanus
40 Eridani A
Right ascension  04h 15m 16.31963s[1]
Declination −07° 39′ 10.3404″[1]
Apparent magnitude (V) 4.43[1]
40 Eridani B
Right ascension  04h 15m 21.786s[2]
Declination −07° 39′ 29.22″[2]
Apparent magnitude (V) 9.52[3]
40 Eridani C
Right ascension  04h 15m 21.50s[4]
Declination −07° 39′ 22.3″[4]
Apparent magnitude (V) 11.17[3]
Characteristics
40 Eridani A
Spectral type K0.5V[5]
U−B color index +0.45[3]
B−V color index +0.82[1]
40 Eridani B
Spectral type DA4[3]
U−B color index +0.45[3]
B−V color index +0.03[3]
40 Eridani C
Spectral type M4.5eV[6]
U−B color index +0.83[3]
B−V color index +1.67[3]
Variable type Flare star[7]
Astrometry
40 Eridani A
Radial velocity (Rv)−43.0[8] km/s
Proper motion (μ) RA: −2,240.12[1] mas/yr
Dec.: −3,420.27[1] mas/yr
Parallax (π)200.62 ± 0.23[1] mas
Distance16.26 ± 0.02 ly
(4.985 ± 0.006 pc)
Absolute magnitude (MV)5.93[9]
40 Eridani B
Radial velocity (Rv)−21[10] km/s
Proper motion (μ) RA: −2,228.3[2] mas/yr
Dec.: −3,377.1[2] mas/yr
40 Eridani C
Radial velocity (Rv)−46[3] km/s
Proper motion (μ) RA: −2,250[11] mas/yr
Dec.: −3,408[11] mas/yr
Parallax (π)199.4552 ± 0.3204[11] mas
Distance16.35 ± 0.03 ly
(5.014 ± 0.008 pc)
Orbit
Primary40 Eridani A
Companion40 Eridani BC
Period (P)~8,000[12] yr
Semi-major axis (a)~400[13] AU
Orbit[14]
Primary40 Eridani B
Companion40 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
Mass0.84 [8] M☉
Radius0.81[15] R☉
Luminosity0.457±0.002[16] L☉
Temperature5,300[8] K
Metallicity [Fe/H]−0.19[17] dex
Rotation~37–43[18] days
Rotational velocity (v sin i)1.23 ± 0.28[18] km/s
Age5.6[19] Gyr
40 Eridani B
Mass0.573±0.018[14] M☉
Radius0.014[20] R☉
Luminosity0.013[21] L☉
Temperature16,500[22] K
Age∼1.8 (cooling age ∼0.122)[23] Gyr
40 Eridani C
Mass0.2036±0.0064[14] M☉
Radius0.31[15] R☉
Luminosity0.008[note 1] L☉
Temperature3,100[24] K
Other designations
ο² Eri, 40 Eri, ADS 3093, CCDM J04153-0739, GCTP 945, Gliese 166
A: Keid,[25] HD 26965, HIP 19849, HR 1325, LHS 23, LTT 1907, SAO 131063, BD-07° 780
B: BD-07° 781, G 160-060, HD 26976, LHS 24, LTT 1908, SAO 131065
C: DY Eri, BD-07°781 C, LHS 25, LTT 1909
Database references
SIMBADThe system
A
B
C

The primary star of the system, designated 40 Eridani A, formally named Keid /ˈkaɪd/ from the traditional name for the system,[26][25] 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.[27]:p73 It was again observed by Friedrich Struve in 1825 and by Otto Struve in 1851.[12][28]

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.[29] 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.[21]

Nomenclature

40 Eridani is the system's Flamsteed designation and ο² Eridani (Latinised to Omicron² 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).[30] 40 Eridani C also bears the variable star designation DY Eridani.

The system bore the traditional name Keid derived from the Arabic word قيض qayḍ meaning "(egg)shells" (cf. neighboring Beid "eggs"). In 2016, the IAU organized a Working Group on Star Names (WGSN)[31] to catalogue and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems.[32] 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.[25]

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.[13] Their orbit has a semimajor axis of 35 AU (which is the approximate average distance between B and C) and is rather elliptical (eccentricity 0.410).[12]

As seen from the 40 Eridani system, the Sun is a 3.4-magnitude star in Hercules, near the border with Serpens Caput.[note 2]

Potential for life
Comparison of the habitable zones of the Sun and 40 Eridani A (here labeled Vulcan, after the fictional planet from Star Trek)

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%[note 3] 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 (magnitudes -8 and -6) white and reddish-orange stars in the night sky.

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.[13]

Planetary system

In 2018, a planet was discovered orbiting 40 Eridani A with a minimum mass of 8.47±0.47 Earth masses.[33] The planet has an orbit of 42 days, and lies considerably interior to the habitable zone, receiving 9 times more stellar flux than Earth, which is an even greater stellar flux amount than Mercury, the innermost planet in our solar system, on average receives from our Sun. It is one of the closest Super-Earths known, the closest discovered to date (as of 2 October 2018) within a multiple star system.

The 40 Eridani A planetary system[33]
Companion
(in order from star)		 Mass Semimajor axis
(AU)		 Orbital period
(days)		 Eccentricity Inclination Radius
b 8.47 ± 0.47 M⊕ 0.22446±0.00004 42.378 ± 0.01 days 0.04+0.05
−0.03		 — —
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gollark: RFC 1149 best RFC.
gollark: Or r-room.
gollark: Rename it to rec-room again.
gollark: <@583380661130493990> <@175456582098878464> You too.

See also
  • 40 Eridani in fiction

Notes
  1. From L=R2(Teff/5772)4, where L is the luminosity in units of L☉, R is the radius in units of R☉, Teff is the effective surface temperature in Kelvin, and 5772 is the nominal effective temperature of the sun
  2. From 40 Eridani the Sun would appear on the diametrically opposite side of the sky at the coordinates RA= 16h 15m 16.32s, 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 .
  3. From , 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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  2. Second U.S. Naval Observatory CCD Astrograph Catalog (UCAC-2); CDS ID I/289.
  3. Gliese Catalogue of Nearby Stars, preliminary 3rd ed., 1991. CDS ID V/70A.
  4. Salim, Samir; Gould, Andrew (2003). "Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog". The Astrophysical Journal. 582 (2): 1011–1031. arXiv:astro-ph/0206318. Bibcode:2003ApJ...582.1011S. doi:10.1086/344822.; CDS ID J/ApJ/582/1011.
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