Gamma Cassiopeiae

Gamma Cassiopeiae, Latinized from γ Cassiopeiae, is a star at the center of the distinctive "W" asterism in the northern circumpolar constellation of Cassiopeia. Although it is a fairly bright star with an apparent visual magnitude that varies from 1.6 to 3.0, it has no traditional Arabic or Latin name. It sometimes goes by the informal name Navi.

γ Cassiopeiae
Location of γ Cassiopeiae (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Cassiopeia
Right ascension 00h 56m 42.50108s[1]
Declination +60° 43′ 00.2984″[1]
Apparent magnitude (V) 2.47[2] (1.6 - 3.0[3])
Characteristics
Spectral type B0.5IVe[4]
U−B color index –1.08[2]
B−V color index –0.15[2]
Variable type γ Cas[3]
Astrometry
Radial velocity (Rv)–6.8[5] km/s
Proper motion (μ) RA: +25.17[1] mas/yr
Dec.: –3.92[1] mas/yr
Parallax (π)5.94 ± 0.12[1] mas
Distance550 ± 10 ly
(168 ± 3 pc)
Absolute magnitude (MV)−3.98[6]
Details[7]
Mass17 M☉
Radius10 R☉
Luminosity34,000 L☉
Surface gravity (log g)3.50 cgs
Temperature25,000 K
Rotational velocity (v sin i)432[8] km/s
Age8.0 ± 0.4[8] Myr
Other designations
Tsih,[9] γ Cas, 27 Cassiopeiae, ADS 782, BD+59°144, FK5 32, HD 5394, HIP 4427, HR 264, SAO 11482, WDS 00567+6043, AAVSO 0050+60[10]
Database references
SIMBADdata

Gamma Cassiopeiae is a Be star, a variable star, and a binary star system. Based upon parallax measurements made by the Hipparcos satellite, it is located at a distance of roughly 550 light-years from Earth.

Physical properties
Amateur image of γ Cassiopeiae and the associated nebulae IC63 and IC59 (Neil Michael Wyatt)

Gamma Cassiopeiae is an eruptive variable star, whose apparent magnitude changes irregularly between +1.6 and +3.0. It is the prototype of the class of Gamma Cassiopeiae variable stars. In the late 1930s it underwent what is described as a shell episode and the brightness increased to above magnitude +2.0, then dropped rapidly to +3.4.[11] It has since been gradually brightening back to around +2.2.[12] At maximum intensity, γ Cassiopeiae outshines both α Cassiopeiae (magnitude +2.25) and β Cassiopeiae (magnitude +2.3).

Gamma Cassiopeiae is a rapidly spinning star with a projected rotational velocity of 472 km s−1, giving it a pronounced equatorial bulge. When combined with the star's high luminosity, the result is the ejection of matter that forms a hot circumstellar disk of gas. The emissions and brightness variations are apparently caused by this "decretion disk".

The spectrum of this massive star matches a stellar classification of B0.5 IVe. A luminosity class of IV identifies it as a subgiant star that has reached a stage of its evolution where it is exhausting the supply of hydrogen in its core region and transforming into a giant star. The 'e' suffix is used for stars that show emission lines of hydrogen in the spectrum, caused in this case by the circumstellar disk. This places it among a category known as Be stars; in fact, the first such star ever to be so designated.[13] It has 17 times the Sun's mass and is radiating as much energy as 34,000 Suns. At this rate of emission, the star has reached the end of its life as a B-type main sequence star after a relatively brief 8 million years. The outer atmosphere has an intense effective temperature of 25,000 K, which is causing it to glow with a blue-white hue.

X-ray emission

Gamma Cassiopeiae is the prototype of a small group of stellar sources of X-ray radiation that is about 10 times stronger than emitted from other B or Be stars. The character of the X-ray spectrum is Be thermal, possibly emitted from plasmas of temperatures up to least ten million kelvins, and shows very short term and long-term cycles. Historically, it has been held that these X-rays might be excited by matter originating from the star, from a hot wind or a disk around the star, accreting onto the surface of a degenerate companion, such as a white dwarf or neutron star. However, there are difficulties with either of these hypotheses. For example, it is not clear that enough matter can be accreted by a white dwarf, at the distance of the purported secondary star implied by the orbital period, sufficient to power an X-ray emission of nearly 1033 erg/s or 100 YW. A neutron star could easily power this X-ray flux, but X-ray emission from neutron stars is known to be non-thermal, and thus in apparent variance with the spectral properties.

Evidence suggests that the X-rays may be associated with the Be star itself or caused by some complex interaction between the star and surrounding decretion disk. One line of evidence is that the X-ray production is known to vary on both short and long time scales with respect to various UV line and continuum changes associated with a B star or with circumstellar matter close to the star.[14][15] Moreover, the X-ray emissions exhibit long-term cycles that correlate with the light curves in the visible wavelengths.[16]

Gamma Cassiopeiae exhibits characteristics consistent with a strong disordered magnetic field. No field can be measured directly from the Zeeman effect because of the star's rotation-broadened spectral lines. Instead, the presence of this field is inferred from a robust periodic signal of 1.21 days that suggests a magnetic field rooted on the rotating star's surface. The star's UV and optical spectral lines show ripples moving from blue to red over several hours, which indicates clouds of matter being held frozen over the star's surface by strong magnetic fields. This evidence suggests that a magnetic field from the star is interacting with the decretion disk, resulting in the X-ray emission. A disk dynamo has been advanced as a mechanism to explain this modulation of the X-rays. However, difficulties remain with this mechanism, among which is that there are no disk dynamos known to exist in other stars, rendering this behavior more difficult to analyze.[17]

Companions

Gamma Cassiopeiae has two faint optical companions, listed in double star catalogues as components B and C.[18][19] Star B is about 2 arc seconds distant and magnitude 11, and has a similar space velocity to the bright primary. Component C is magnitude 13, nearly an arc second distant.[20] and a further, fainter, optical companion C.[21]

Gamma Cassiopeiae A, the bright primary, is itself a spectroscopic binary with an orbital period of about 203.5 days and an eccentricity alternately reported as 0.26 and "near zero." The mass of the companion is believed to be about that of the Sun, but its nature is unclear. It has been proposed that it is a degenerate star or a hot helium star, but it seems unlikely that it is a normal star. Therefore, it is likely to be more evolved than the primary and to have transferred mass to it during an earlier stage of evolution.[4][22]

Names

The Chinese name Tsih, "the whip" (Chinese: 策; pinyin: cè), is commonly associated with this star.[23][24] The name however originally referred to Kappa Cassiopeiae,[25][26] and Gamma Cassiopeiae was just one of four horses pulling the chariot of legendary charioteer Wangliang.[25] This representation was later changed to make Gamma the whip.[25]

The star was used as an easily identifiable navigational reference point during space missions and American astronaut Virgil Ivan "Gus" Grissom nicknamed the star Navi after his own middle name spelled backwards.[27][28]

See also

References
  1. van Leeuwen, F. (November 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. Nicolet, B. (1978), "Photoelectric photometric Catalogue of homogeneous measurements in the UBV System", Astronomy and Astrophysics Supplement Series, 34: 1–49, Bibcode:1978A&AS...34....1N
  3. 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. 1: 02025. Bibcode:2009yCat....102025S.
  4. Nemravová, J.; Harmanec, P.; Koubský, P.; Miroshnichenko, A.; Yang, S.; Šlechta, M.; Buil, C.; Korčáková, D.; Votruba, V. (2012). "Properties and nature of Be stars. 29. Orbital and long-term spectral variations of γ Cassiopeiae". Astronomy & Astrophysics. 537: A59. arXiv:1111.3761. Bibcode:2012A&A...537A..59N. doi:10.1051/0004-6361/201117922. S2CID 34272401.
  5. Wilson, Ralph Elmer (1953). "General Catalogue of Stellar Radial Velocities". Carnegie Institute Washington D.C. Publication. Bibcode:1953GCRV..C......0W.
  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. Sigut, T. A. A.; Jones, C. E. (October 2007), "The Thermal Structure of the Circumstellar Disk Surrounding the Classical Be Star γ Cassiopeiae", Astrophysical Journal, 668 (1): 481–491, arXiv:0706.4036, Bibcode:2007ApJ...668..481S, doi:10.1086/521209, S2CID 14362961
  8. Zorec, J.; Frémat, Y.; Cidale, L. (2005). "On the evolutionary status of Be stars. I. Field Be stars near the Sun". Astronomy and Astrophysics. 441 (1): 235–248. arXiv:astro-ph/0509119. Bibcode:2005A&A...441..235Z. doi:10.1051/0004-6361:20053051. S2CID 17592657.
  9. Falkner, David E. (2011). "The Autumn Constellations". The Mythology of the Night Sky. Patrick Moore's Practical Astronomy Series. pp. 139–162. doi:10.1007/978-1-4614-0137-7_8. ISBN 978-1-4614-0136-0.
  10. "CCDM J00567+6043AB -- Double or multiple star", SIMBAD, Centre de Données astronomiques de Strasbourg, retrieved 2009-04-13
  11. Baldwin, Ralph B.; Torp-Smith, Robert (1941). "Light Curves of Gamma Cassiopeiae". Popular Astronomy. 49: 127. Bibcode:1941PA.....49..127B.
  12. Pollmann, E.; Vollmann, W.; Henry, G. W. (2014). "Long-term monitoring of Halpha emission strength and photometric V magnitude of gamma Cas". Information Bulletin on Variable Stars. 6109: 1. Bibcode:2014IBVS.6109....1P.
  13. Merrill, P. W.; Humason, M. L.; Burwell, C. G. (1925). "Discovery and Observations of Stars of Class Be". Astrophysical Journal. 61: 389. Bibcode:1925ApJ....61..389M. doi:10.1086/142899.
  14. Smith, M. A.; Robinson, R. D. (1999), "A multiwavelength campaign on γ Cassiopeiae. III. The case for magnetically controlled circumstellar kinematics", Astrophysical Journal, 517 (2): 866–882, Bibcode:1999ApJ...517..866S, doi:10.1086/307216
  15. Cranmer, S.; Smith, M.; Robinson, R. (2000), "A Multiwavelength Campaign on γ Cassiopeiae. IV. The Case for Illuminated Disk-enhanced Wind Streams", Astrophysical Journal, 537 (1): 433–447, Bibcode:2000ApJ...537..433C, doi:10.1086/309008
  16. Smith, Myron A.; Cohen, David H.; Gu, Ming Feng; Robinson, Richard D.; Evans, Nancy Remage; Schran, Prudence G. (2004), "High-Resolution Chandra Spectroscopy of γ Cassiopeiae (B0.5e)", Astrophysical Journal, 600 (2): 972–985, arXiv:astro-ph/0309293, Bibcode:2004ApJ...600..972S, doi:10.1086/379873, S2CID 166002
  17. Robinson, R. D.; Smith, M. A.; Henry, G. W. (2002), "X-Ray and Optical Variations in the Classical Be Star γ Cassiopeia: The Discovery of a Possible Magnetic Dynamo", Astrophysical Journal, 575 (1): 435–448, arXiv:astro-ph/0205278, Bibcode:2002ApJ...575..435R, doi:10.1086/341141
  18. Mason, Brian D.; Wycoff, Gary L.; Hartkopf, William I.; Douglass, Geoffrey G.; Worley, Charles E. (2001). "The 2001 US Naval Observatory Double Star CD-ROM. I. The Washington Double Star Catalog". The Astronomical Journal. 122 (6): 3466. Bibcode:2001AJ....122.3466M. doi:10.1086/323920.
  19. Aitken, Robert Grant; Doolittle, Eric (1932). New general catalogue of double stars within 120° of the North pole. Carnegie Institution of Washington. Bibcode:1932ngcd.book.....A.
  20. "VizieR Detailed Page". VizieR. Retrieved 2009-04-13. Cite journal requires |journal= (help)
  21. Dommanget, J.; Nys, O. (1994). "Catalogue des composantes d'etoiles doubles et multiples (CCDM) premiere edition - Catalogue of the components of double and multiple stars (CCDM) first edition". Com. De l'Observ. Royal de Belgique. 115: 1. Bibcode:1994CoORB.115....1D.
  22. Miroschnichenko, A. S.; Bjorkman, K. S.; Krugov, V. D. (2002), "Binary nature and long term nature of Gamma Cassiopeiae", Publications of the Astronomical Society of the Pacific, 114 (801): 1226, Bibcode:2002PASP..114.1226M, doi:10.1086/342766
  23. 唐山 (Tángshān) (1967). 天文學太空航空學辭典 [Dictionary of astronomy and astronautics] (in Chinese). 廣文書局. OCLC 22797568.
  24. Allen, Richard Hinckley (1963) [1899]. Star-names and their meanings. New York, NY: Dover Publications. p. 146. ISBN 978-1-931559-44-7.
  25. Ridpath, Ian (1988). "Cassiopeia". Star Tales. James Clarke & Co. ISBN 9780718826956.
  26. Sun, Xiaochun; Kistemaker, Jacob (1997). The Chinese Sky During the Han: Constellating Stars and Society. Leiden, New York, Köln: Koninklijke Brill. pp. 150, 168. ISBN 9789004107373.
  27. "Post-landing Activities", Apollo 15 Lunar Surface Journal, NASA commentary at 105:11:33
  28. Apollo 10 Flown CSM Star Chart Directly from the Personal Collection of Mission Command Module Pilot John Young, Heritage Auction Galleries, retrieved March 11, 2010
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