Epsilon Tauri

Epsilon Tauri / Ain
	Location of ε Tauri (circled)
Observation data; Epoch J2000 Equinox J2000
Constellation	Taurus
Right ascension	04h 28m 37.0003s
Declination	+19° 10′ 49.563″
Apparent magnitude (V)	+3.53
Characteristics
Evolutionary stage	red clump
Spectral type	K0III
B−V color index	1.014
Astrometry
Radial velocity (Rv)	38.420±0.0004 km/s
Proper motion (μ)	RA: 107.526±0.193 mas/yr ; Dec.: −36.200±0.126 mas/yr
Parallax (π)	22.3654±0.1721 mas
Distance	146 ± 1 ly ; (44.7 ± 0.3 pc)
Absolute magnitude (MV)	0.145
Details
Mass	2.458±0.073 M☉
Radius	12.46±0.26 R☉
Luminosity	79.4±3.4 L☉
Habitable zone inner limit	8.06±0.24 AU
Habitable zone outer limit	15.82±0.47 AU
Surface gravity (log g)	2.66+0.03; −0.05 cgs
Temperature	4,880±67 K
Metallicity [Fe/H]	+0.15±0.02 dex
Rotation	141.1 days
Rotational velocity (v sin i)	3.0 km/s
Age	600+150; −50 Myr
Other designations
	Ain, Oculus Boreus, Epsilon Tau, ε Tau, 74 Tau, BD+18°640, FK5 164, GC 5430, HD 28305, HIP 20889, HR 1409, SAO 93954, CCDM J04286+1911, WDS J04286+1911A
Database references
SIMBAD	data
Exoplanet Archive	data

Epsilon Tauri or ε Tauri, formally named Ain (/ˈeɪn/),^[10] is an orange giant star located approximately 146 light-years (45 parsecs) from the Sun in the constellation of Taurus. An exoplanet (designated Epsilon Tauri b, later named Amateru) is believed to be orbiting the star.

It is a member of the Hyades open cluster. As such its age is well constrained at 625 million years.^[2] It is claimed to be the heaviest among planet-harboring stars with reliable initial masses.^[2] Given its large mass, this star, though presently of spectral type K0 III, was formerly of spectral type A that has now evolved off the main sequence into the giant phase. It is regarded as a red clump giant; that is, a core-helium burning star.^[2]

Since Epsilon Tauri lies near the plane of the ecliptic, it is sometimes occulted by the Moon and (very rarely) by planets.

It has an 11th magnitude companion 182 arcseconds away, although this is an unrelated background star.^[11]

Nomenclature

ε Tauri (Latinised to Epsilon Tauri) is the star's Bayer designation; it also bears the Flamsteed designation of 74 Tauri. On discovery, the planet was designated Epsilon Tauri b (or Ain b).

The star bore the traditional name Ain (Arabic عين for "eye") and was given the name Oculus Boreus (Latin for "Northern eye") by John Flamsteed.^[12]^[9] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)^[13] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016^[14] included a table of the first two batches of names approved by the WGSN; which included Ain for this star.

In July 2014, the International Astronomical Union launched NameExoWorlds, a process for giving proper names to certain exoplanets.^[15] The process involved public nomination and voting for the new names.^[16] In December 2015, the IAU announced the winning name was Amateru for this planet.^[17]

The winning name was based on that submitted by the Kamagari Astronomical Observatory of Kure, Hiroshima Prefecture, Japan: namely 'Amaterasu', the Shinto goddess of the Sun, born from the left eye of the god Izanagi. The IAU substituted 'Amateru' – which is a common Japanese appellation for shrines when they enshrine Amaterasu – because 'Amaterasu' is already used for an asteroid (10385 Amaterasu).^[18]

In Chinese, 畢宿 (Bì Xiù), meaning Net, refers to an asterism consisting ε Tauri, δ³ Tauri, δ¹ Tauri, γ Tauri, Aldebaran, θ² Tauri, 71 Tauri and λ Tauri.^[19] Consequently, the Chinese name for ε Tauri itself is 畢宿一 (Bì Xiù yī), "the First Star of Net".^[20]

Planetary system

In 2007, a massive exoplanet was reported orbiting the star with a period of 1.6 years in a somewhat eccentric orbit. It was the first planet ever discovered in an open cluster.^[2] A 2023 study updated this planet's parameters, and detected additional radial velocity variations that are likely caused by stellar activity.^[8]

The Epsilon Tauri planetary system^[8]
Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (days)	Eccentricity	Inclination	Radius
b (Amateru)	≥7.190±0.056 $M$ _J	1.878±0.001	585.82+0.26 −0.33	0.076+0.009 −0.008	—	—

References

^ ^a ^b ^c ^d Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data 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. S2CID 244398875. Gaia DR3 record for this source at VizieR.
^ ^a ^b ^c ^d ^e ^f ^g Sato, Bun'ei; et al. (2007). "A Planetary Companion to the Hyades Giant ε Tauri". The Astrophysical Journal. 661 (1): 527–531. Bibcode:2007ApJ...661..527S. doi:10.1086/513503.
^ "Entry for star HIP 20889". Vizier Catalogue Service. Retrieved 2015-09-13.
^ Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.
^ ^a ^b Böhm-Vitense, Erika; et al. (December 2000), "Ultraviolet Emission Lines in BA and Non-BA Giants", The Astrophysical Journal, 545 (2): 992–999, Bibcode:2000ApJ...545..992B, doi:10.1086/317850.
^ ^a ^b ^c Arentoft, T.; Grundahl, F.; et al. (February 2019). "Asteroseismology of the Hyades red giant and planet host ɛ Tauri". Astronomy & Astrophysics. 622: A190. arXiv:1901.06187. Bibcode:2019A&A...622A.190A. doi:10.1051/0004-6361/201834690. S2CID 119066215.
^ ^a ^b ^c ^d ^e Baines, Ellyn K.; Jones, Jeremy; Clark, James H.; Schmitt, Henrique R.; Stone, Jordan M. (January 2025). "Eighteen Exoplanet Host Stars from the NPOI Data Archive". The Astronomical Journal. 169 (2): 83. arXiv:2506.02934. Bibcode:2025AJ....169...83B. doi:10.3847/1538-3881/ad9bb1. ISSN 1538-3881.
^ ^a ^b ^c ^d Teng, Huan-Yu; Sato, Bun'ei; et al. (August 2023). "Revisiting planetary systems in the Okayama Planet Search Program: A new long-period planet, RV astrometry joint analysis, and a multiplicity-metallicity trend around evolved stars". Publications of the Astronomical Society of Japan. 75 (6): 1030–1071. arXiv:2308.05343. Bibcode:2023PASJ...75.1030T. doi:10.1093/pasj/psad056.
^ ^a ^b Allen, Richard Hickley (1963). Star Names – Their Lore and Meaning. Dover Books. p. 391.
^ "IAU Catalog of Star Names". Retrieved 28 July 2016.
^ "UCAC2 38562433". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2 December 2023.
^ Flamsteed, John (1725). Historia Coelestis Britannica. H. Meere. p. 47.
^ "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
^ "Bulletin of the IAU Working Group on Star Names, No. 1" (PDF). Retrieved 28 July 2016.
^ NameExoWorlds: An IAU Worldwide Contest to Name Exoplanets and their Host Stars. IAU.org. 9 July 2014
^ "NameExoWorlds The Process". Archived from the original on 2015-08-15. Retrieved 2015-09-05.
^ Final Results of NameExoWorlds Public Vote Released, International Astronomical Union, 15 December 2015.
^ "NameExoWorlds The Approved Names". Archived from the original on 2018-02-01. Retrieved 2015-12-21.
^ (in Chinese) 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, ISBN 978-986-7332-25-7.
^ (in Chinese) 香港太空館 – 研究資源 – 亮星中英對照表 Archived 2008-10-25 at the Wayback Machine, Hong Kong Space Museum. Accessed on line November 23, 2010.

External links

"Star Names". Frosty Drew Observatory. Retrieved 2008-06-24.
"eps Tau". SIMBAD. Centre de données astronomiques de Strasbourg.

[GaiaDR3-1] Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data 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. S2CID 244398875. Gaia DR3 record for this source at VizieR.

[Sato2007-2] ^ ^a ^b ^c ^d ^e ^f ^g Sato, Bun'ei; et al. (2007). "A Planetary Companion to the Hyades Giant ε Tauri". The Astrophysical Journal. 661 (1): 527–531. Bibcode:2007ApJ...661..527S. doi:10.1086/513503.

[VizieR-3] "Entry for star HIP 20889". Vizier Catalogue Service. Retrieved 2015-09-13.

[GaiaDR2-4] Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.

[BohmVitense2000-5] Böhm-Vitense, Erika; et al. (December 2000), "Ultraviolet Emission Lines in BA and Non-BA Giants", The Astrophysical Journal, 545 (2): 992–999, Bibcode:2000ApJ...545..992B, doi:10.1086/317850.

[Arentoft2019-6] Arentoft, T.; Grundahl, F.; et al. (February 2019). "Asteroseismology of the Hyades red giant and planet host ɛ Tauri". Astronomy & Astrophysics. 622: A190. arXiv:1901.06187. Bibcode:2019A&A...622A.190A. doi:10.1051/0004-6361/201834690. S2CID 119066215.

[Baines2025-7] Baines, Ellyn K.; Jones, Jeremy; Clark, James H.; Schmitt, Henrique R.; Stone, Jordan M. (January 2025). "Eighteen Exoplanet Host Stars from the NPOI Data Archive". The Astronomical Journal. 169 (2): 83. arXiv:2506.02934. Bibcode:2025AJ....169...83B. doi:10.3847/1538-3881/ad9bb1. ISSN 1538-3881.

[Teng2023-8] Teng, Huan-Yu; Sato, Bun'ei; et al. (August 2023). "Revisiting planetary systems in the Okayama Planet Search Program: A new long-period planet, RV astrometry joint analysis, and a multiplicity-metallicity trend around evolved stars". Publications of the Astronomical Society of Japan. 75 (6): 1030–1071. arXiv:2308.05343. Bibcode:2023PASJ...75.1030T. doi:10.1093/pasj/psad056.

[allen-9] Allen, Richard Hickley (1963). Star Names – Their Lore and Meaning. Dover Books. p. 391.

[IAU-CSN-10] "IAU Catalog of Star Names". Retrieved 28 July 2016.

[SIMBAD2-11] "UCAC2 38562433". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2 December 2023.

[12] Flamsteed, John (1725). Historia Coelestis Britannica. H. Meere. p. 47.

[WGSN-13] "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.

[WGSN1-14] "Bulletin of the IAU Working Group on Star Names, No. 1" (PDF). Retrieved 28 July 2016.

[15] NameExoWorlds: An IAU Worldwide Contest to Name Exoplanets and their Host Stars. IAU.org. 9 July 2014

[16] "NameExoWorlds The Process". Archived from the original on 2015-08-15. Retrieved 2015-09-05.

[17] Final Results of NameExoWorlds Public Vote Released, International Astronomical Union, 15 December 2015.

[18] "NameExoWorlds The Approved Names". Archived from the original on 2018-02-01. Retrieved 2015-12-21.

[19] (in Chinese) 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, ISBN 978-986-7332-25-7.

[20] (in Chinese) 香港太空館 – 研究資源 – 亮星中英對照表 Archived 2008-10-25 at the Wayback Machine, Hong Kong Space Museum. Accessed on line November 23, 2010.

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Observation data Epoch J2000 Equinox J2000
Location of ε Tauri (circled)
Constellation	Taurus
Right ascension	04^h 28^m 37.0003^s^[1]
Declination	+19° 10′ 49.563″^[1]
Apparent magnitude (V)	+3.53^[2]
Characteristics
Evolutionary stage	red clump^[2]
Spectral type	K0III^[2]
B−V color index	1.014^[3]
Astrometry

Radial velocity (R_v)	38.420±0.0004^[4] km/s
Proper motion (μ)	RA: 107.526±0.193 mas/yr^[1] Dec.: −36.200±0.126 mas/yr^[1]
Parallax (π)	22.3654±0.1721 mas^[1]
Distance	146 ± 1 ly (44.7 ± 0.3 pc)
Absolute magnitude (M_V)	0.145^[5]

Details

Mass	2.458±0.073^[6] `M`_☉
Radius	12.46±0.26^[7] `R`_☉
Luminosity	79.4±3.4^[7] `L`_☉
Habitable zone inner limit	8.06±0.24^[7] AU
Habitable zone outer limit	15.82±0.47^[7] AU
Surface gravity (log g)	2.66+0.03 −0.05^[8] cgs
Temperature	4,880±67^[7] K
Metallicity [Fe/H]	+0.15±0.02^[6] dex
Rotation	141.1^[8] days
Rotational velocity (v sin i)	3.0^[5] km/s
Age	600+150 −50^[6] Myr

Other designations
Ain, Oculus Boreus^[9], Epsilon Tau, ε Tau, 74 Tau, BD+18°640, FK5 164, GC 5430, HD 28305, HIP 20889, HR 1409, SAO 93954, CCDM J04286+1911, WDS J04286+1911A
Database references
SIMBAD	data
Exoplanet Archive	data

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