Eta Piscium

Eta Piscium (η Piscium, abbreviated Eta Psc, η Psc) is a binary star and the brightest point of light in the constellation of Pisces with an apparent visual magnitude of +3.6.[2] Based upon a measured annual parallax shift of 9.33 mas as seen from Earth,[1] it is located roughly 350 light-years distant from the Sun in the thin disk population of the Milky Way.[5]

Eta Piscium
Location of η Piscium (circled)
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Pisces
Right ascension 01h 31m 29.01026s[1]
Declination +15° 20′ 44.9685″[1]
Apparent magnitude (V) +3.611[2] (3.83 + 7.51)[3]
Characteristics
Spectral type G7 IIIa[4]
U−B color index +0.730[2]
B−V color index +0.976[2]
Variable type γ Cas
Astrometry
Radial velocity (Rv)13.60±0.42[5] km/s
Proper motion (μ) RA: +27.14[1] mas/yr
Dec.: −2.64[1] mas/yr
Parallax (π)9.33 ± 0.72 mas[1]
Distance350 ± 30 ly
(107 ± 8 pc)
Absolute magnitude (MV)−1.52[6]
Orbit[7]
Period (P)850.5±66.5 yr
Semi-major axis (a)1.228±0.144″
Eccentricity (e)0.469±0.053
Inclination (i)58.5±2.5°
Longitude of the node (Ω)32.8±2.0°
Periastron epoch (T)2040.3±66.9
Argument of periastron (ω)
(secondary)		86.9±4.0°
Details[5]
η Psc A
Mass3.78±0.16 M☉
Radius26.48±2.15 R☉
Luminosity457 L☉
Surface gravity (log g)2.20±0.14 cgs
Temperature4,937±40 K
Metallicity [Fe/H]−0.13±0.06 dex
Rotational velocity (v sin i)8.4[8] km/s
Age220±30 Myr
Other designations
Alpherg, η Psc, 99 Piscium, BD+14° 231, FK5 50, HD 9270, HIP 7097, HR 437, SAO 92484, WDS J01315+1521AB[9]
Database references
SIMBADdata

The two components are designated Eta Piscium A (formally named Alpherg /ˈælfɜːrɡ/, the traditional name of the system)[10] and B.

Nomenclature

η Piscium (Latinised to Eta Piscium) is the system's Bayer designation. The designations of the two constituents as Eta Piscium A and B derive from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems, and adopted by the International Astronomical Union (IAU).[11]

The system bore the traditional names Al Pherg (in this context meaning the emptying) and Kullat Nunu. At the time that the sun at the March Equinox entered into Pisces having lay in Aries, the system was in the first ecliptic constellation of the Neo-Babylonians, Kullat Nūnu − 'Nūnu' being Babylonian for fish and 'Kullat' referring to either the bucket or the cord that binds the fish.[12] In 2016, the IAU organized a Working Group on Star Names (WGSN)[13] to catalog and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems.[14] It approved the name Alpherg for the component Eta Piscium A on 1 June 2018 (for its official List).[10]

In Chinese, 右更 (Yòu Gèng), meaning Official in Charge of the Pasturing, refers to an asterism consisting of Eta Piscium, Rho Piscium, Pi Piscium, Omicron Piscium and 104 Piscium. Consequently, the Chinese name for Eta Piscium itself is 右更二 (Yòu Gèng èr, English: the Second Star of Official in Charge of the Pasturing.)[15]

Properties

At its present distance, the visual magnitude of the system is diminished by an extinction factor of 0.09±0.06 due to interstellar dust.

This system's binary nature was discovered in 1878 by an amateur astronomer, S. W. Burnham.[7] It has an orbital period of roughly 850 years, a semimajor axis of 1.2 arc seconds, and an eccentricity of 0.47.

The primary, component A, is an evolved, magnitude 3.83[3] G-type giant star with a stellar classification of G7 IIIa.[4] It has a weak magnetic field with a strength of 0.4±0.2 G,[8] and is a Gamma Cassiopeiae variable.[7] The companion, component B, is a magnitude 7.51 star.[3]

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.
  2. Oja, T. (March 1985), "UBV photometry of stars whose positions are accurately known. II", Astronomy and Astrophysics Supplement Series, 59: 461–464, Bibcode:1985A&AS...59..461O.
  3. Eggleton, P. P.; Tokovinin, A. A. (September 2008), "A catalogue of multiplicity among bright stellar systems", Monthly Notices of the Royal Astronomical Society, 389 (2): 869–879, arXiv:0806.2878, Bibcode:2008MNRAS.389..869E, doi:10.1111/j.1365-2966.2008.13596.x, S2CID 14878976.
  4. Keenan, Philip C.; McNeil, Raymond C. (1989), "The Perkins catalog of revised MK types for the cooler stars", Astrophysical Journal Supplement Series, 71: 245, Bibcode:1989ApJS...71..245K, doi:10.1086/191373.
  5. Maldonado, J.; et al. (June 2013), "The metallicity signature of evolved stars with planets", Astronomy & Astrophysics, 554: 18, arXiv:1303.3418, Bibcode:2013A&A...554A..84M, doi:10.1051/0004-6361/201321082, S2CID 119289111, A84.
  6. Anderson, E.; Francis, Ch. (2012), "XHIP: An extended hipparcos compilation", Astronomy Letters, 38 (5): 331, arXiv:1108.4971, Bibcode:2012AstL...38..331A, doi:10.1134/S1063773712050015, S2CID 119257644.
  7. Cvetković, Z.; Novaković, B. (March 2010), "Eight new and three recalculated orbits for binaries", Astronomische Nachrichten, 331 (3): 304, Bibcode:2010AN....331..304C, doi:10.1002/asna.200911250.
  8. Aurière, M.; Konstantinova-Antova, R.; Charbonnel, C.; Wade, G. A.; Tsvetkova, S.; Petit, P.; Dintrans, B.; Drake, N. A.; Decressin, T.; Lagarde, N.; Donati, J. F.; Roudier, T.; Lignières, F.; Schröder, K. P.; Landstreet, J. D.; Lèbre, A.; Weiss, W. W.; Zahn, J. P. (February 2015), "The magnetic fields at the surface of active single G-K giants", Astronomy & Astrophysics, 574: 30, arXiv:1411.6230, Bibcode:2015A&A...574A..90A, doi:10.1051/0004-6361/201424579, S2CID 118504829, A90.
  9. "eta Psc". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2017-07-30.
  10. "Naming Stars". IAU.org. Retrieved 18 June 2018.
  11. Hessman, F. V.; Dhillon, V. S.; Winget, D. E.; Schreiber, M. R.; Horne, K.; Marsh, T. R.; Guenther, E.; Schwope, A.; Heber, U. (2010). "On the naming convention used for multiple star systems and extrasolar planets". arXiv:1012.0707 [astro-ph.SR].
  12. Allen, Richard Hinckley (1963) [1899], Star Names: Their Lore and Meaning (rep. ed.), New York, NY: Dover Publications Inc., pp. 328–29, ISBN 0-486-21079-0
  13. "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
  14. "WG Triennial Report (2015-2018) - Star Names" (PDF). p. 5. Retrieved 2018-07-14.
  15. (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 5 月 19 日
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