Astronomy:40 Eridani

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Short description: Triple star system in the constellation Eridanus
For other stars with this Bayer designation, see ο Eridani.
40 Eridani/Keid
Eridanus constellation map.svg
Red circle.svg
Location of 40 Eridani (circled)
Observation data
{{#ifeq:J2000.0|J2000.0 (ICRS)|Epoch J2000.0      Equinox J2000.0 (ICRS)| [[History:Epoch|Epoch J2000.0]]      [[Astronomy:Equinox (celestial coordinates)|Equinox J2000.0}}
Constellation Eridanus
40 Eridani A
Right ascension  04h 15m 16.31962s[1]
Declination −07° 39′ 10.3308″[1]
Apparent magnitude (V) 4.43[2]
40 Eridani B
Right ascension  04h 15m 21.79572s[3]
Declination −07° 39′ 29.2040″[3]
Apparent magnitude (V) 9.52[4]
40 Eridani C
Right ascension  04h 15m 21.53600s[5]
Declination −07° 39′ 20.6946″[5]
Apparent magnitude (V) 11.17[4]
Characteristics
40 Eridani A
Spectral type K0.5V[6]
U−B color index +0.45[4]
B−V color index +0.82[2]
40 Eridani B
Spectral type DA4[4]
U−B color index +0.45[4]
B−V color index +0.03[4]
40 Eridani C
Spectral type M4.5eV[7]
U−B color index +0.83[4]
B−V color 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: −2,240.085[1] mas/yr
Dec.: −3,421.809[1] mas/yr
Parallax (π)199.6080 ± 0.1208[1] mas
Distance16.340 ± 0.010 ly
(5.010 ± 0.003 pc)
Absolute magnitude (MV)5.93[9]
40 Eridani B
Radial velocity (Rv)−21[10] km/s
Proper motion (μ) RA: −2,236.169[3] mas/yr
Dec.: −3,338.955[3] mas/yr
Parallax (π)199.6911 ± 0.0512[3] mas
Distance16.333 ± 0.004 ly
(5.008 ± 0.001 pc)
40 Eridani C
Radial velocity (Rv)−44.06±0.20[5] km/s
Proper motion (μ) RA: −2,247.183 mas/yr
Dec.: −3,409.824 mas/yr
Parallax (π)199.4516 ± 0.0692[5] mas
Distance16.353 ± 0.006 ly
(5.014 ± 0.002 pc)
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±0.68 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
Mass0.78±0.08[14] M☉
Radius0.812±0.017[14] R☉
Luminosity0.457±0.002[15] L☉
Surface gravity (log g)4.45±0.19[14] cgs
Temperature5072±53[14] K
Metallicity [Fe/H]−0.42±0.04[14] dex
Rotation~37–43[16] days
Rotational velocity (v sin i)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
Mass0.2036±0.0064[13] M☉
Radius0.31[20] R☉
Luminosity0.008[note 1] L☉
Temperature3,100[21] K
Age5.6[22] Gyr
1.8[23] Gyr
Other designations
A: {{{names1}}}
B: {{{names2}}}
C: {{{names3}}}
Database references
SIMBADA
B
C

40 Eridani is a triple star system in the constellation of Eridanus, abbreviated 40 Eri. It has the Bayer designation Omicron2 Eridani, which is Latinized from ο2 Eridani and abbreviated Omicron2 Eri or ο2 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,[24] 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.[25]:p73 It was again observed by Friedrich Struve in 1825 and by Otto Struve in 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 a white 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 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.[18]

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).[28] 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").[29] In 2016, the IAU organized a Working 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 entire multiple systems.[31] 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.[24]

Properties

Amateur photo of 40 Eridani

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.[23] B and C orbit each other approximately 400 AU from the primary star, A.[12] Their orbit has a semimajor axis of 35 AU and is rather elliptical with an orbital eccentricity of 0.410).[11]

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

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 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.[12]

Search for planets

40 Eridani A shows periodic radial velocity variations, which may be caused by a planetary companion. The 42-day period is close to the stellar rotation period, making the 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]

A study in 2023 concluded that the radial velocity signal very likely does originate from stellar activity, and not from a planet.[34](pp23-24,44)

The candidate planet would have 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.[14]

The 40 Eridani A planetary system[14]
Companion
(in order from star) 		Mass Semimajor axis
(AU) 		Orbital period
(days) 		Eccentricity Inclination Radius
b (disputed[34]) ≥8.47±0.47 M⊕ 0.22446±0.00004 42.378±0.01 0.04+0.05
−0.03 		— —

See also

  • 40 Eridani in fiction

Notes

  1. ↑ From L=4πR2σTeff4, where L is the luminosity, R is the radius, Teff is the effective surface temperature and σ is the Stefan–Boltzmann constant.
  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 [math]\displaystyle{ \begin{smallmatrix}m\ =\ M_v\ +\ 5\cdot((\log_{10}\ 5.04)\ -\ 1)\ =\ 3.36\end{smallmatrix} }[/math].
  3. ↑ From [math]\displaystyle{ \ h = \frac{\ a\ }{d} \,, }[/math] where h is the apparent height, d is the distance of the object, and a is the actual size of the object.

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 Vallenari, A. et al. (2022). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy & Astrophysics. doi:10.1051/0004-6361/202243940  Gaia DR3 record for this source at VizieR.
  2. ↑ 2.0 2.1 van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics 474 (2): 653–664. doi:10.1051/0004-6361:20078357. Bibcode: 2007A&A...474..653V. 
  3. ↑ 3.0 3.1 3.2 3.3 Vallenari, A. et al. (2022). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy & Astrophysics. doi:10.1051/0004-6361/202243940  Gaia DR3 record for this source at VizieR.
  4. ↑ 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 Gliese Catalogue of Nearby Stars, preliminary 3rd ed., 1991. CDS ID V/70A.
  5. ↑ 5.0 5.1 5.2 5.3 Vallenari, A. et al. (2022). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy & Astrophysics. doi:10.1051/0004-6361/202243940  Gaia DR3 record for this source at VizieR.
  6. ↑ Gray, R. O.; Corbally, C. J.; Garrison, R. F.; McFadden, M. T.; Bubar, E. J.; McGahee, C. E.; O'Donoghue, A. A.; Knox, E. R. (2006). "Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 pc-The Southern Sample". The Astronomical Journal 132 (1): 161–170. doi:10.1086/504637. Bibcode: 2006AJ....132..161G. 
  7. ↑ General Catalogue of Trigonometric Parallaxes, 4th ed., 1995. CDS ID I/238A.
  8. ↑ Samus, N. N. et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)". VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S 1: B/gcvs. Bibcode: 2009yCat....102025S. 
  9. ↑ Holmberg, J. et al. (July 2009), "The Geneva-Copenhagen survey of the solar neighbourhood. III. Improved distances, ages, and kinematics", Astronomy and Astrophysics 501 (3): 941–947, doi:10.1051/0004-6361/200811191, Bibcode: 2009A&A...501..941H. 
  10. ↑ Evans, D. S (1967). "The Revision of the General Catalogue of Radial Velocities". Determination of Radial Velocities and Their Applications 30: 57. Bibcode: 1967IAUS...30...57E. 
  11. ↑ 11.0 11.1 11.2 Heintz, W. D. (1974). "Astrometric study of four visual binaries". Astronomical Journal 79: 819. doi:10.1086/111614. Bibcode: 1974AJ.....79..819H. 
  12. ↑ 12.0 12.1 12.2 "40 Eridani 3 (Omicron² Eridani)". http://www.solstation.com/stars/40erida3.htm. 
  13. ↑ 13.0 13.1 13.2 Mason, Brian D. et al. (November 2017), "Binary Star Orbits. V. The Nearby White Dwarf/Red Dwarf Pair 40 Eri BC", The Astronomical Journal 154 (5): 9, doi:10.3847/1538-3881/aa803e, 200, Bibcode: 2017AJ....154..200M. 
  14. ↑ 14.0 14.1 14.2 14.3 14.4 14.5 14.6 14.7 14.8 Ma, Bo et al. (2018). "The first super-Earth Detection from the High Cadence and High Radial Velocity Precision Dharma Planet Survey". Monthly Notices of the Royal Astronomical Society 480 (2): 2411. doi:10.1093/mnras/sty1933. Bibcode: 2018MNRAS.480.2411M. 
  15. ↑ Brown, A. G. A. (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics 616: A1. doi:10.1051/0004-6361/201833051. Bibcode: 2018A&A...616A...1G.  Gaia DR2 record for this source at VizieR.
  16. ↑ 16.0 16.1 16.2 Díaz, Matías R.; Jenkins, James S.; Tuomi, Mikko; Butler, R. Paul; Soto, Maritza G.; Teske, Johanna K.; Feng, Fabo; Shectman, Stephen A. et al. (2018). "The test case of HD26965: Difficulties disentangling weak Doppler signals from stellar activity". The Astronomical Journal 155 (3): 126. doi:10.3847/1538-3881/aaa896. Bibcode: 2018AJ....155..126D. https://www.researchgate.net/publication/322498205. 
  17. ↑ Provencal, J. L.; Shipman, H. L.; Høg, Erik; Thejll, P. (1998). "Testing the White Dwarf Mass-Radius Relation with HIPPARCOS". The Astrophysical Journal 494 (2): 759. doi:10.1086/305238. Bibcode: 1998ApJ...494..759P. 
  18. ↑ 18.0 18.1 Keid , Jim Kaler, STARS web page, accessed 15/5/2007, 10/12/2011.
  19. ↑ Finley, David S.; Koester, Detlev; Basri, Gibor (1997). "The Temperature Scale and Mass Distribution of Hot DA White Dwarfs". The Astrophysical Journal 488 (1): 375–396. doi:10.1086/304668. Bibcode: 1997ApJ...488..375F. 
  20. ↑ Catalogue of nearest stars until 10pc, V. A. Zakhozhaj. Revised 1996. CDS ID V/101.
  21. ↑ Johnson, H. M.; Wright, C. D. (1983). "Predicted infrared brightness of stars within 25 parsecs of the sun". Astrophysical Journal Supplement Series 53: 643. doi:10.1086/190905. Bibcode: 1983ApJS...53..643J. 
  22. ↑ Mamajek, Eric E.; Hillenbrand, Lynne A. (November 2008). "Improved Age Estimation for Solar-Type Dwarfs Using Activity-Rotation Diagnostics". The Astrophysical Journal 687 (2): 1264–1293. doi:10.1086/591785. Bibcode: 2008ApJ...687.1264M. 
  23. ↑ 23.0 23.1 Bond, Howard E.; Bergeron, P.; Bédard, A. (October 2017). "Astrophysical Implications of a New Dynamical Mass for the Nearby White Dwarf 40 Eridani B". The Astrophysical Journal 848 (1): 16. doi:10.3847/1538-4357/aa8a63. 16. Bibcode: 2017ApJ...848...16B. 
  24. ↑ 24.0 24.1 "Naming Stars". IAU.org. https://www.iau.org/public/themes/naming_stars/. 
  25. ↑ Herschel, William (1785). "Catalogue of Double Stars. By William Herschel, Esq. F. R. S". Philosophical Transactions of the Royal Society of London 75: 40–126. doi:10.1098/rstl.1785.0006. Bibcode: 1785RSPT...75...40H. 
  26. ↑ Van Den Bos, W. H. (1926). "The orbit and the masses of 40 Eridani BC". Bulletin of the Astronomical Institutes of the Netherlands 3: 128. Bibcode: 1926BAN.....3..128V. 
  27. ↑ White Dwarfs, E. Schatzman, Amsterdam: North-Holland, 1958. , p. 1
  28. ↑ Hessman, F.V.; Dhillon, V.S.; Winget, D.E.; Schreiber, M.R.; Horne, K.; Marsh, T.R.; et al. (2010). "On the naming convention used for multiple star systems and extrasolar planets". arXiv:1012.0707 [astro-ph.SR].
  29. ↑ "Beid". http://stars.astro.illinois.edu/sow/beid.html. 
  30. ↑ "Working Group on Star Names (WGSN)". International Astronomical Union. https://www.iau.org/science/scientific_bodies/working_groups/280/. 
  31. ↑ Working Group on Star Names Triennial Report (2015-2018) (Report). International Astronomical Union. 2018. p. 5. https://www.iau.org/static/science/scientific_bodies/working_groups/280/wg-starnames-triennial-report-2015-2018.pdf. Retrieved 2018-07-14. 
  32. ↑ Rosenthal, Lee J.; Fulton, Benjamin J.; Hirsch, Lea A.; Isaacson, Howard T.; Howard, Andrew W.; Dedrick, Cayla M.; Sherstyuk, Ilya A.; Blunt, Sarah C. et al. (2021). "The California Legacy Survey. I. A Catalog of 178 Planets from Precision Radial Velocity Monitoring of 719 Nearby Stars over Three Decades". The Astrophysical Journal Supplement Series 255 (1): 8. doi:10.3847/1538-4365/abe23c. Bibcode: 2021ApJS..255....8R. 
  33. ↑ Zhao, Lily L. et al. (2022). "The EXPRES Stellar Signals Project II. State of the Field in Disentangling Photospheric Velocities". The Astronomical Journal 163 (4): 171. doi:10.3847/1538-3881/ac5176. Bibcode: 2022AJ....163..171Z. 
  34. ↑ 34.0 34.1 Laliotis, Katherine et al. (February 2023). "Doppler Constraints on Planetary Companions to Nearby Sun-like Stars: An Archival Radial Velocity Survey of Southern Targets for Proposed NASA Direct Imaging Missions". The Astronomical Journal 165 (4): 176. doi:10.3847/1538-3881/acc067. Bibcode: 2023AJ....165..176L. 

External links

Coordinates: Sky map 04h 15m 16.32s, −07° 39′ 10.34″



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