Beta Capricorni

Beta Capricorni (β Capricorni, abbreviated Beta Cap, β Cap) is a multiple star system in the constellation of Capricornus and located 328 light-years from the Sun. Because it is near the ecliptic, Beta Capricorni can be occulted by the Moon,[2] and also (rarely) by planets.

β¹/β² Capricorni
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Capricornus
Right ascension 20h21m00.7s
Declination −14°46′53″
Apparent magnitude (V) +3.05/+6.09
Characteristics
Spectral type K0II+B8V/A0III
U−B color index 0.28/−0.11
B−V color index 0.79/−0.02
Variable type none
Astrometry
Radial velocity (Rv)−19/−18 km/s
Proper motion (μ) RA: 48.42/42.64 mas/yr
Dec.: 14.00/0.37 mas/yr
Parallax (π)9.94 ± 1.00 mas
Distance328 ly
(101 pc)
Absolute magnitude (MV)−2.03/+1.03[1]
Other designations
Dabih, Dabikh, Dikhabda, 9 Capricorni, FK5 762, WDS J20210-1447AB
β1 Cap: BD−15° 5629, HD 193495, HIP 100345, HR 7776, WDS J20210-1447A
β2 Cap: BD−15° 5626, HD 193452, HIP 100325, HR 7775, WDS J20210-1447B
Database references
SIMBADβ Cap
β1 Cap
β2 Cap

The system is believed to consist of five stars.[3] With binoculars or a small telescope, Beta Capricorni can be resolved into a binary pair. The brighter of the two is designated Beta¹ Capricorni or Beta Capricorni A; the dimmer, Beta² Capricorni or Beta Capricorni B. Both are themselves made up of multiple stars. Beta¹ Capricorni has three components; a single star designated Beta Capricorni Aa (formally named Dabih /ˈdeɪbiː/, the traditional name of the system)[4][5] and a binary pair, Beta Capricorni Ab (whose two components are designated Beta Capricorni Ab1 and Ab2). Beta² Capricorni is also a binary pair, with components designated Beta Capricorni Ba and Bb.

Two other nearby stars were discovered by John Herschel. Sometimes referred to as Beta Capricorni D and E, it is unclear whether they are simply optical doubles or part of the Beta Capricorni system.

Nomenclature

β Capricorni (Latinised to Beta Capricorni) is the system's Bayer designation; β¹ and β² Capricorni those of its two constituents. The designations of the two constituents as Beta Capricorni A and B, and those of the sub-components - Beta Capricorni Aa, Ab, Ab1, Ab2, Ba and Bb - derive from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems, and adopted by the International Astronomical Union (IAU).[6]

Beta Capricorni bore the traditional name Dabih, deriving from the Arabic الذابح al-dhābiḥ "the butcher", with Beta¹ and Beta² subsequently named Dabih Major and Dabih Minor, respectively.[7] In 2016, the IAU organized a Working Group on Star Names (WGSN)[8] to catalogue and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems.[9] It approved the name Dabih for the component Beta Capricorni Aa on 21 August 2016 and it is now so included in the List of IAU-approved Star Names.[5]

In Chinese, 牛宿 (Niú Su), meaning Ox (asterism), refers to an asterism consisting of Beta Capricorni, Alpha² Capricorni, Xi² Capricorni, Pi Capricorni, Omicron Capricorni and Rho Capricorni.[10] Consequently, the Chinese name for Beta Capricorni itself is 牛宿一 (Niú Su yī, English: the First Star of Ox).[11]

Properties

Beta¹ Capricorni is the brighter of the two components with an apparent magnitude of +3.05, while the dimmer Beta² Capricorni has an apparent magnitude of +6.09. The two components are separated by 3.5 arcminutes on the sky, putting them at least 21,000 AU (0.34 light-years) apart. They take approximately 700,000 years to complete one orbit.

Aa
Sep. = 0.05″
Ab1
Period = 8.7d
Ab2
Ba
Sep. = 3″
Bb

Hierarchy of orbits in the β Capricorni system

Beta¹ Capricorni

Beta¹ Capricorni is the more complex of the pair and has a spectrum that is difficult to interpret. Its dominant pair of stars are the orange K-type bright giant Beta Capricorni Aa, with an apparent magnitude of +3.08, and the blue-white B-Type main sequence dwarf Beta Capricorni Ab1 with an apparent magnitude of +7.20. They are separated by 0.05 arcseconds (5 AU) and have an orbital period of 3.77 years.

The Aa component has a surface temperature of 4900 kelvins, a radius 35 times that of the Sun, and a luminosity 600 times that of the Sun. The Ab1 component's unseen companion, Beta Capricorni Ab2, orbits Ab1 with an orbital period of 8.7 days.

Beta¹ Capricorni is 4.6 degrees north of the ecliptic, so it can be occulted by the Moon.

Beta² Capricorni

Beta² Capricorni is simpler and more studied. Its brighter component, Beta Capricorni Ba, has a magnitude of 6.1 and is an A0-giant with 40 times the luminosity of the Sun. The companion, Beta Capricorni Bb, is approximately 3 arcseconds from Ba. Ba is unusual for having large amounts of mercury and manganese in its atmosphere.

Beta Capricorni D and E

These lie 112 arcseconds away from Beta¹ Capricorni.

References
  1. 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.
  2. White, Nathaniel M.; Feierman, Barry H. (September 1987), "A Catalog of Stellar Angular Diameters Measured by Lunar Occultation", Astronomical Journal, 94: 751, Bibcode:1987AJ.....94..751W, doi:10.1086/114513.
  3. "Displaying next number in catalog HIP => 100345". Multiple Star Catalog. Retrieved 2018-07-26.
  4. Kunitzsch, Paul; Smart, Tim (2006). A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations (2nd rev. ed.). Cambridge, Massachusetts: Sky Pub. ISBN 978-1-931559-44-7.
  5. "Naming Stars". IAU.org. Retrieved 18 June 2018.
  6. 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].
  7. Allen, Richard Hinckley (1963) [1899]. Star Names: Their Lore and Meaning (Reprint ed.). New York, NY: Dover Publications Inc. p. 140. ISBN 0-486-21079-0.
  8. IAU Working Group on Star Names (WGSN), International Astronomical Union, retrieved 22 May 2016.
  9. "WG Triennial Report (2015-2018) - Star Names" (PDF). p. 5. Retrieved 2018-07-14.
  10. (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.
  11. (in Chinese) 香港太空館 - 研究資源 - 亮星中英對照表 Archived 2009-09-29 at the Wayback Machine, Hong Kong Space Museum. Accessed on line November 23, 2010.
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