Epsilon Capricorni

Epsilon Capricorni, Latinized from ε Capricorni, is a quadruple star system[10][11] in the constellation Capricornus. It has the traditional star name Castra or Kastra, meaning "fort" or "military camp" in Latin. Based upon an annual parallax shift of 3.09 mas as seen from the Earth,[1] the star is located about 1,060 light years from the Sun. It can be seen with the naked eye, having a combined apparent visual magnitude of 4.62.[2]

"Kastra" redirects here. For the Late Roman fortified camp, see Castra ad Fluvio Frigido.
Epsilon Capricorni
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Capricornus
Right ascension 21h 37m 04.83068s[1]
Declination −19° 27′ 57.6464″[1]
Apparent magnitude (V) +4.62[2]
Characteristics
Spectral type B2.5 Vpe[3]
U−B color index −0.64[2]
B−V color index −0.19[2]
Variable type γ Cas[4]
Astrometry
Proper motion (μ) RA: +12.79[1] mas/yr
Dec.: +0.28[1] mas/yr
Parallax (π)3.09 ± 0.18[1] mas
Distance1,060 ± 60 ly
(320 ± 20 pc)
Absolute magnitude (MV)−3.03[5]
Details
ε Cap A
Mass7.6[6] M☉
Radius4.80[6] R☉
Luminosity4,649[5] L☉
Surface gravity (log g)4.0[6] cgs
Temperature18,800[6] K
Metallicity [Fe/H]−0.08[5] dex
Rotational velocity (v sin i)225[7] km/s
Age27.5±4.2[8] Myr
Other designations
Kastra, ε Cap, 39 Cap, BD−20° 6251/−20° 6253, FK5 3724, HD 205637, HIP 106723, HR 8260, SAO 164520, WDS J21371-1928A[9]
Database references
SIMBADdata

In Chinese, 壘壁陣 (Lěi Bì Zhèn), meaning Line of Ramparts, refers to an asterism consisting of ε Capricorni, κ Capricorni, γ Capricorni, δ Capricorni, ι Aquarii, σ Aquarii, λ Aquarii, φ Aquarii, 27 Piscium, 29 Piscium, 33 Piscium and 30 Piscium.[12] Consequently, the Chinese name for ε Capricorni itself is 壘壁陣二 (Lěi Bì Zhèn èr, English: the Second Star of Line of Ramparts.)[13]

The triple system comprises a close binary pair with an orbital period of 129 days, and a more distant, second companion star. The primary, component Aa, is a Be star that is surrounded by ionized gas that is producing the emission lines in the spectrum. This circumstellar shell is inclined by 80° to the line of sight from the Earth.[6] The system is undergoing both short term and long term variations in luminosity, with the short period variations showing a phase cycle of 1.03 days.[4] It is classified as a Gamma Cassiopeiae variable with an amplitude of 0.16 in magnitude.[4]

Epsilon Capricorni Aa is a blue-white hued B-type main sequence star with a stellar classification of B2.5 Vpe[3] and a visual magnitude of +4.62.[2] It has 7.6 times the mass of the Sun and 4.8 times the Sun's radius.[6] The star is spinning rapidly, with a projected rotational velocity of 225 km/s. This is giving it an oblate shape with an equatorial bulge that is 7% larger than the polar radius.[7]

The system has two visual companions. Component B is a visual magnitude 10.11 star at an angular separation of 65.8 arc seconds along a position angle of 46°, as of 2013. Component C with visual magnitude of 14.1 lies at an angular separation of 62.7 arc seconds along a position angle of 164°, as of 1999.[11]

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. Feinstein, A.; Marraco, H. G. (November 1979), "The photometric behavior of Be Stars", Astronomical Journal, 84: 1713–1725, Bibcode:1979AJ.....84.1713F, doi:10.1086/112600.
  3. Adelman, S. J.; et al. (December 2000), "On the Variability of O4-B5 Luminosity Class III-V Stars", Information Bulletin on Variable Stars, 5008 (5008): 1, Bibcode:2000IBVS.5008....1A.
  4. Balona, L. A. (December 2002), "Short period spectral variability in the Be stars I: eta Centauri and epsilon Capricorni", The Journal of Astronomical Data, 8: 1, Bibcode:2002JAD.....8....1B.
  5. 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.
  6. Silaj, J.; et al. (November 2014), "The Hα Profiles of Be Shell Stars", The Astrophysical Journal, 795 (1): 12, Bibcode:2014ApJ...795...82S, doi:10.1088/0004-637X/795/1/82, 82.
  7. Belle, G. T. (2012), "Interferometric observations of rapidly rotating stars", The Astronomy and Astrophysics Review, 20 (1): 51, arXiv:1204.2572, Bibcode:2012A&ARv..20...51V, doi:10.1007/s00159-012-0051-2, S2CID 119273474.
  8. Tetzlaff, N.; et al. (January 2011), "A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun", Monthly Notices of the Royal Astronomical Society, 410 (1): 190–200, arXiv:1007.4883, Bibcode:2011MNRAS.410..190T, doi:10.1111/j.1365-2966.2010.17434.x, S2CID 118629873.
  9. "eps Cap -- Be Star", SIMBAD Astronomical Database, Centre de Données astronomiques de Strasbourg, retrieved 2017-05-15.
  10. 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.
  11. Mason, B. D.; et al. (2014), "The Washington Visual Double Star Catalog", The Astronomical Journal, 122 (6): 3466, Bibcode:2001AJ....122.3466M, doi:10.1086/323920, retrieved 2015-07-22
  12. (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.
  13. (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 7 月 7 日
  • eps Cap, American Association of Variable Star Observers, retrieved 2017-05-17.
  • Kaler, James B. (October 9, 2015), "Epsilon Capricorni", Stars, University of Illinois, retrieved 2017-05-17.
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