Eta Cephei

Eta Cephei (η Cep, η Cephei) is a star in the northern circumpolar constellation of Cepheus. It shares the name Al Kidr with θ Cep, although the meaning of this name is unknown. With an apparent visual magnitude of 3.4,[2] this is a third magnitude star that, according to the Bortle Dark-Sky Scale, is readily visible to the naked eye. Parallax measurements put it at a distance of 46.53 light-years (14.27 parsecs) from Earth.[1]

η Cephei
Location of η Cephei (circled)
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Cepheus
Right ascension  20h 45m 17.37555s[1]
Declination +61° 50′ 19.6167″[1]
Apparent magnitude (V) 3.426[2]
Characteristics
Spectral type K0 IV[3]
U−B color index +0.613[2]
B−V color index +0.918[2]
Astrometry
Radial velocity (Rv)–87.55 ± 0.11[4] km/s
Proper motion (μ) RA: +86.50[1] mas/yr
Dec.: +818.02[1] mas/yr
Parallax (π)70.10 ± 0.11[1] mas
Distance46.53 ± 0.07 ly
(14.27 ± 0.02 pc)
Absolute magnitude (MV)2.631[3]
Details
Mass1.6[5] M☉
Radius4.12 ± 0.07[6] R☉
Luminosity9.7 ± 0.5[6] L☉
Surface gravity (log g)3.41[6] cgs
Temperature4,950[6] K
Metallicity [Fe/H]–0.32[6] dex
Rotational velocity (v sin i)6.79[7] km/s
Age2.5 ± 0.3[5] Gyr
Other designations
3 Cephei, BD+61 2050, FK5 783, HD 198149, HIP 102422, HR 7957, SAO 19019.[8]
Database references
SIMBADdata

Etymology

Eta Cephei, along with α Cep (Alderamin) and β Cep (Alfirk), were identified as Al Kawākib al Firḳ (الكوكب الفرق), meaning "the Stars of The Flock" by Ulug Beg.[9][10]

In Chinese, 天鈎 (Tiān Gōu), meaning Celestial Hook, refers to an asterism consisting of η Cephei, 4 Cephei, HD 194298, θ Cephei, α Cephei, ξ Cephei, 26 Cephei, ι Cephei and ο Cephei.[11] Consequently, the Chinese name for η Cephei itself is 天鈎四 (Tiān Gōu sì, English: the Fourth Star of Celestial Hook.).[12]

Properties

Eta Cephei is a subgiant star with a stellar classification of K0 IV,[3] which indicates it is exhausting the supply of hydrogen at its core and is in the process of evolving into a giant star. With 1.6[5] times the Sun's mass, at an age of 2.5[5] billion years it has reached a radius four times larger than the Sun[6] and a luminosity ten times greater.[6] It is radiating this energy from its outer atmosphere at an effective temperature of 4,950 K,[6] giving it the orange-hued glow of a K-type star. Eta Cephei has a high proper motion across the celestial sphere[8] and a large peculiar velocity of 112 km s−1.[5]

Hunt for substellar objects

According to Nelson & Angel (1998),[13] Eta Cephei would show two significant periodicities of 164 days and 10 years respectively, hinting at the possible presence of one or more jovian planets in orbit around the subgiant. The authors have set an upper limit of 0.64 Jupiter masses for the putative inner planet and 1.2 Jupiter masses for the putative outer one. Also Campbell et al. (1988)[14] inferred the existence of planetary objects or even brown dwarfs less massive than 16.3 Jupiter masses.

However, more recent studies have not yet confirmed the existence of any substellar companion around Eta Cephei. McDonald Observatory team has set limits to the presence of one or more planets[15] with masses between 0.13 and 2.4 Jupiter masses and average separations spanning between 0.05 and 5.2 AU.

The Eta Cephei planetary system[13]
Companion
(in order from star)		 Mass Semimajor axis
(AU)		 Orbital period
(days)		 Eccentricity Inclination Radius
b (unconfirmed) ≥0.64 MJ 0.638 163.57 ?0.17 — —
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References
  1. van Leeuwen, Floor (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, arXiv:0708.1752v1, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357 Note: see VizieR catalogue I/311.
  2. Jennens, P. A.; Helfer, H. L. (September 1975), "A new photometric metal abundance and luminosity calibration for field G and K giants.", Monthly Notices of the Royal Astronomical Society, 172: 667–679, Bibcode:1975MNRAS.172..667J, doi:10.1093/mnras/172.3.667
  3. Soubiran, C.; et al. (2008), "Vertical distribution of Galactic disk stars. IV. AMR and AVR from clump giants", Astronomy and Astrophysics, 480 (1): 91–101, arXiv:0712.1370, Bibcode:2008A&A...480...91S, doi:10.1051/0004-6361:20078788
  4. Famaey, B.; et al. (January 2005), "Local kinematics of K and M giants from CORAVEL/Hipparcos/Tycho-2 data. Revisiting the concept of superclusters", Astronomy and Astrophysics, 430 (1): 165–186, arXiv:astro-ph/0409579, Bibcode:2005A&A...430..165F, doi:10.1051/0004-6361:20041272
  5. Affer, L.; et al. (April 2005), "Spectroscopic determination of photospheric parameters and chemical abundances of 6 K-type stars" (PDF), Astronomy and Astrophysics, 433 (2): 647–658, Bibcode:2005A&A...433..647A, doi:10.1051/0004-6361:20041308
  6. Piau, L.; et al. (February 2011), "Surface convection and red-giant radius measurements", Astronomy and Astrophysics, 526: A100, arXiv:1010.3649, Bibcode:2011A&A...526A.100P, doi:10.1051/0004-6361/201014442
  7. Martínez–Arnáiz, R.; et al. (September 2010), "Chromospheric activity and rotation of FGK stars in the solar vicinity. An estimation of the radial velocity jitter", Astronomy and Astrophysics, 520: A79, arXiv:1002.4391, Bibcode:2010A&A...520A..79M, doi:10.1051/0004-6361/200913725
  8. "LHS 3578 -- High proper-motion Star", SIMBAD Astronomical Object Database, Centre de Données astronomiques de Strasbourg, retrieved 2012-03-27
  9. Allen, R. H. (1963), Star Names: Their Lore and Meaning (Reprint ed.), New York: Dover Publications Inc, p. 157, ISBN 0-486-21079-0, retrieved 2010-12-12
  10. Davis Jr., G. A. (October 1944), "The Pronunciations, Derivations, and Meanings of a Selected List of Star Names", Popular Astronomy, LII (3): 16, Bibcode:1944PA.....52....8D
  11. (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.
  12. (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 7 月 6 日
  13. Nelson, A. F.; Angel, J. R. P. (June 1998), "The Range of Masses and Periods Explored by Radial Velocity Searches for Planetary Companions", Astrophysical Journal, 500 (2): 940–957, arXiv:astro-ph/9802194, Bibcode:1998ApJ...500..940N, doi:10.1086/305741
  14. Murdoch, Kaylene A.; Hearnshaw, J. B.; Clark, M. (August 1993), "A search for substellar companions to southern solar-type stars", Astrophysical Journal, Part 1, 413 (1): 349–363, Bibcode:1993ApJ...413..349M, doi:10.1086/173003
  15. Wittenmyer, Robert A.; et al. (July 2006), "Detection Limits from the McDonald Observatory Planet Search Program", The Astronomical Journal, 132 (1): 177–188, arXiv:astro-ph/0604171, Bibcode:2006AJ....132..177W, doi:10.1086/504942
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