Gamma Cephei

Gamma Cephei (γ Cephei, abbreviated Gamma Cep, γ Cep) is a binary star system approximately 45 light-years away in the constellation of Cepheus. The primary (designated Gamma Cephei A, officially named Errai /ɛˈreɪ.iː/, the traditional name of the system)[9][10] is a stellar class K1 orange giant or subgiant star; it has a red dwarf companion (Gamma Cephei B). An exoplanet (designated Gamma Cephei Ab, later named Tadmor) has been confirmed to be orbiting the primary.

Gamma Cephei

Location of γ Cephei (top center)
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Cepheus
Right ascension 23h 39m 20.852s[1]
Declination +77° 37′ 56.19″[1]
Apparent magnitude (V) 3.21[2]
Characteristics
Gamma Cephei A
Spectral type K1III-IV CN1[3]
U−B color index +0.94[2]
B−V color index +1.03[2]
Variable type Suspected[4]
Gamma Cephei B
Spectral type M4V[5]
Astrometry
Radial velocity (Rv)−42.82±0.30[6] km/s
Proper motion (μ) RA: –47.96 ± 0.45[1] mas/yr
Dec.: 126.59 ± 0.40[1] mas/yr
Parallax (π)72.69 ± 0.41[5] mas
Distance44.9 ± 0.3 ly
(13.76 ± 0.08 pc)
Absolute magnitude (MV)2.62[7]
Orbit[5]
PrimaryA
CompanionB
Period (P)67.5 ± 1.4 yr
Semi-major axis (a)1.467 ± 0.046"
(20.18 ± 0.66 AU)
Eccentricity (e)0.4112 ± 0.0063
Inclination (i)119.3 ± 1.0°
Longitude of the node (Ω)18.04 ± 0.98°
Periastron epoch (T)1991.605 ± 0.031
Argument of periastron (ω)
(secondary)		161.01 ± 0.40°
Details
Gamma Cephei A
Mass1.41±0.08[8] M☉
Radius4.93±0.04[8] R☉
Luminosity11.6±0.6[8] L☉
Surface gravity (log g)+3.18[6] cgs
Temperature4792±62[8] K
Metallicity [Fe/H]−0.05[6] dex
Rotational velocity (v sin i)1.63[6] km/s
Age3.25±0.63[8] Gyr
Gamma Cephei B
Mass0.409 ± 0.018[5] M☉
Other designations
Errai, 35 Cep, Gl 903, HR 8974, BD +76°928, HD 222404, GCTP 5725.00, SAO 10818, FK5 893, HIP 116727.
Database references
SIMBADdata
Exoplanet Archivedata
Extrasolar Planets
Encyclopaedia		data

Gamma Cephei is the naked-eye star that will succeed Polaris as the Earth's northern pole star, due to the precession of the equinoxes. It will be closer to the northern celestial pole than Polaris around 3000 CE and will make its closest approach around 4000 CE. The 'title' will pass to Iota Cephei some time around 5200 CE.

Description

Gamma Cephei has an apparent magnitude of 3.21, nearly all of which is accounted for by Gamma Cephei A. The primary is about 3.25 billion years old and has evolved off the main sequence, having burned all of the hydrogen in its core.[8]

The spectrum of this star has served as one of the stable anchor points by which other stars are classified. It was listed as a standard star for the spectral class K1 IV in 1943, 1953, and 1973.[11] However, in 1989, it was given as a spectral standard for K1 III-IV. Its spectrum is notable for the strength of the cyano radical (CN) bands.[3] Analysis of the spectrum in 2018 gave a best match for a spectral type of K1 III.[8]

Gamma Cephei B has a mass approximately 0.409 times that of the Sun.[5] It is probably a red dwarf of class M4, 6.2 magnitudes fainter than the primary.[5] It is assumed to be of similar age to its primary.

γ Cephei is catalogued as a suspected variable star with a brightness range between magnitudes 3.18 and 3.24,[4] based on its inclusion in an 1884 list of suspected variable stars.[12]

Nomenclature

γ Cephei (Latinised to Gamma Cephei) is the system's Bayer designation. Under the rules for naming objects in multiple star systems the two components are designated A and B.[13] Following its discovery the planet was designated Gamma Cephei Ab.

The system bore a traditional name variously spelled as Errai, Er Rai or Alrai, deriving from the Arabic الراعي (ar-rā‘ī), meaning 'the shepherd'. (The star Beta Ophiuchi is sometimes also called Alrai, but it is more commonly known as Cebalrai or Kelb Alrai, meaning 'shepherd's dog'.) In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[14] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[15] included a table of the first two batches of names approved by the WGSN; which included Errai for Gamma Cephei A.

In July 2014 the International Astronomical Union launched a process for giving proper names to certain exoplanets.[16] The process involved public nomination and voting for the new names.[17] In December 2015, the IAU announced the winning name was Tadmor for this planet.[18] It was submitted by the Syrian Astronomical Association and is the ancient Semitic name and modern Arabic name for the city of Palmyra, a (UNESCO) World Heritage Site.[19]

In Chinese, the star is named 少衛增八 (Shàowèi Zēng Bā, literally, the 8th added star of the Xingguan Shaowei, Shaowei: the Minor Guard[20]) belonging to the Left Wall of the Purple Forbidden enclosure (紫微左垣, Zǐwēi Zuǒyuán), which refers to an asterism consisting of Gamma Cephei, Iota Draconis, Theta Draconis, Eta Draconis, Zeta Draconis, Upsilon Draconis, 73 Draconis and 23 Cassiopeiae.[21]

Planetary system

A planet orbiting Gamma Cephei A with a 2.7 year period was discovered in 1988.[22] Its existence was also announced in 1989.[23] This was the first confirmed extrasolar planet and its ostensible discovery was based on the same radial velocity technique later used successfully by others. However, the claim was challenged in 1992 by a paper[24] which favoured K-giant variability with a period equal to the stellar rotation, but in 2002, the existence of a planet with an orbital period of about 2.5 years was confirmed.[25]

The secondary star B orbits A at only 9.8 times the semimajor axis of A's planet. Despite how compact the system is, the planet's orbit is stable if it is coplanar with that of the binary companion.[26]

The Gamma Cephei A planetary system[27]
Companion
(in order from star)		 Mass Semimajor axis
(AU)		 Orbital period
(days)		 Eccentricity Inclination Radius
b (Tadmor) ≥1.85±0.16 MJ 2.05±0.06 903.3±1.5 0.049±0.034 — —

References
  1. van 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. S2CID 18759600.Vizier catalog entry
  2. Hoffleit, D.; Warren, W. H. (1995). "VizieR Online Data Catalog: Bright Star Catalogue, 5th Revised Ed. (Hoffleit+, 1991)". VizieR On-line Data Catalog: V/50. Originally Published in: 1964BS....C......0H. 5050: V/50. Bibcode:1995yCat.5050....0H.
  3. Keenan, Philip C; McNeil, Raymond C (1989). "The Perkins catalog of revised MK types for the cooler stars". The Astrophysical Journal Supplement Series. 71: 245. Bibcode:1989ApJS...71..245K. doi:10.1086/191373.
  4. Samus, N. N.; Durlevich, O. V.; 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.
  5. Neuhäuser, R.; et al. (2007). "Direct detection of exoplanet host star companion γ Cep B and revised masses for both stars and the sub-stellar object". Astronomy and Astrophysics. 462 (2): 777–780. arXiv:astro-ph/0611427. Bibcode:2007A&A...462..777N. doi:10.1051/0004-6361:20066581. S2CID 17530531.
  6. Jofré, E; Petrucci, R; Saffe, C; Saker, L; Artur de la Villarmois, E; Chavero, C; Gómez, M; Mauas, P. J. D (2015). "Stellar parameters and chemical abundances of 223 evolved stars with and without planets". Astronomy & Astrophysics. 574: A50. arXiv:1410.6422. Bibcode:2015A&A...574A..50J. doi:10.1051/0004-6361/201424474. S2CID 53666931.
  7. Park, Sunkyung; et al. (2013), "Wilson-Bappu Effect: Extended to Surface Gravity", The Astronomical Journal, 146 (4): 73, arXiv:1307.0592, Bibcode:2013AJ....146...73P, doi:10.1088/0004-6256/146/4/73, S2CID 119187733.
  8. Baines, Ellyn K.; et al. (2018). "Fundamental Parameters of 87 Stars from the Navy Precision Optical Interferometer". The Astronomical Journal. 155 (1). 30. arXiv:1712.08109. Bibcode:2018AJ....155...30B. doi:10.3847/1538-3881/aa9d8b. S2CID 119427037.
  9. Rumrill, H. B. (June 1936). "Star Name Pronunciation". Publications of the Astronomical Society of the Pacific. San Francisco, California. 48 (283): 139. Bibcode:1936PASP...48..139R. doi:10.1086/124681.
  10. "IAU Catalog of Star Names". Retrieved 28 July 2016.
  11. Garrison, R. F. (December 1993), "Anchor Points for the MK System of Spectral Classification", Bulletin of the American Astronomical Society, 25: 1319, Bibcode:1993AAS...183.1710G, retrieved 2012-02-04
  12. Gore, J. E (1884). "A Catalogue of Suspected Variable Stars. With Notes and Observations". Proceedings of the Royal Irish Academy. Science. 4: 267–410. Bibcode:1885PRIA....4..411G. JSTOR 20635921.
  13. Hartkopf, William I.; Mason, Brian D. "Addressing confusion in double star nomenclature: The Washington Multiplicity Catalog". U.S. Naval Observatory. Archived from the original on 2011-05-17. Retrieved 2016-01-19.
  14. "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
  15. "Bulletin of the IAU Working Group on Star Names, No. 1" (PDF). Retrieved 28 July 2016.
  16. NameExoWorlds: An IAU Worldwide Contest to Name Exoplanets and their Host Stars. IAU.org. 9 July 2014
  17. NameExoWorlds The Process
  18. Final Results of NameExoWorlds Public Vote Released, International Astronomical Union, 15 December 2015.
  19. NameExoWorlds The Approved Names
  20. Richard Hinckley Allen: Star Names — Their Lore and Meaning: Cepheus
  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
  22. Campbell, Bruce; et al. (1988). "A search for substellar companions to solar-type stars". The Astrophysical Journal. 331: 902–921. Bibcode:1988ApJ...331..902C. doi:10.1086/166608.
  23. Lawton, A. T.; Wright, P. (1989). "A planetary system for Gamma Cephei?". British Interplanetary Society. 42: 335–336. Bibcode:1989JBIS...42..335L.
  24. Walker, Gordon A.H.; Bohlender, David A.; Walker, Andrew R. (1992). "Gamma Cephei: Rotation or Planetary Companion?". Astrophysical Journal Letters. 396: L91–L94. Bibcode:1992ApJ...396L..91W. doi:10.1086/186524.
  25. Hatzes, Artie P.; et al. (2003). "A Planetary Companion to Gamma Cephei A". The Astrophysical Journal. 599 (2): 1383–1394. arXiv:astro-ph/0305110. Bibcode:2003ApJ...599.1383H. doi:10.1086/379281. S2CID 11506537.
  26. Torres, Guillermo (2007). "The Planet Host Star γ Cephei: Physical Properties, the Binary Orbit, and the Mass of the Substellar Companion". The Astrophysical Journal. 654 (2): 1095–1109. arXiv:astro-ph/0609638. Bibcode:2007ApJ...654.1095T. doi:10.1086/509715. S2CID 13255217.
  27. Endl, Michael; et al. (2011). News from the γ Cephei Planetary System. PLANETARY SYSTEMS BEYOND THE MAIN SEQUENCE: Proceedings of the International Conference. AIP Conference Proceedings. 1331. pp. 88–94. arXiv:1101.2588. Bibcode:2011AIPC.1331...88E. doi:10.1063/1.3556187.
Preceded by
Polaris		 Pole Star
3000–5200		 Succeeded by
Iota Cephei

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