Gamma Cassiopeiae

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Gamma Cassiopeiae
Cassiopeia constellation map.svg
Cercle rouge 100%.svg

γ Cassiopeiae (circled) is found in the middle of the "W"
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]
Characteristics
Spectral type B0.5 IVe[3]
U−B color index –1.08[2]
B−V color index –0.15[2]
Variable type Gamma Cas
Astrometry
Radial velocity (Rv) –6.8[4] 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
Distance 550 ± 10 ly
(168 ± 3 pc)
Details
Mass 19.3 ± 0.1[5] M
Radius 10[6] R
Luminosity 55,000[7] L
Surface gravity (log g) 3.50[6] cgs
Temperature 30,900[7] K
Rotational velocity (v sin i) 300[7] km/s
Age 8.0 ± 0.4[5] Myr
Other designations
Tsih, 27 Cassiopeiae, ADS 782, BD+59 144, FK5 32, HD 5394, HIP 4427, HR 264, SAO 11482, WDS 00567+6043.[8]

Gamma Cassiopeiae (γ Cas, γ Cassiopeiae) is the Bayer designation for a star in the northern circumpolar constellation of Cassiopeia. Although it is a fairly bright star with an apparent visual magnitude of 2.47,[2] it has no traditional Arabic or Latin name. However, in Chinese, it has the name Tsih, derived from the word 策 (), meaning "the whip". It is located at the center of the distinctive "W" shape that forms the constellation's asterism. American astronaut Virgil Ivan "Gus" Grissom nicknamed the star Navi after his own middle name spelled backwards.[9] The star was used as an easily identifiable navigational reference point during space missions.[10] Based upon parallax measurements made by the Hipparcos satellite, this star is located at a distance of roughly 550 light-years (170 parsecs) from Earth.[1]

Properties[edit]

This is an eruptive variable star, whose brightness changes irregularly between +2.15 mag and +3.40 mag. It is the prototype of the class of Gamma Cassiopeiae variable stars. The apparent magnitude of this star was +2.2 in 1937, +3.4 in 1940, +2.9 in 1949, +2.7 in 1965 and now it is +2.15. At maximum intensity, γ Cassiopeiae outshines both α Cassiopeiae (magnitude +2.25) and β Cassiopeiae (magnitude +2.3). It is a rapidly spinning star with a projected rotational velocity of 300 km s–1,[7] 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.[3] 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.[11] It has over 19[5] times the Sun's mass and is radiating as much energy as 55,000[7] 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.[5] The outer atmosphere has an intense effective temperature of 30,900,[7] which is causing it to glow with a blue-white hue.

X-ray emission[edit]

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 the 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.[12][13] Moreover, the X-ray emissions exhibit long-term cycles that correlate with the light curves in the visible wavelengths.[14]

One intriguing property is that 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.[15] 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.

Companions[edit]

Gamma Cassiopeiae is a spectroscopic binary with an orbital period of about 204 days and an eccentricity alternately reported as 0.26 and "near zero." The mass of the companion is believed to be about that of our Sun.[16][17]

Gamma Cassiopeiae is an optical double, with a faint magnitude 11 companion B about 2 arc seconds distant,[18][19] with the designation of ADS782AB,[8][20] and a further, fainter, optical companion C.[18]

See also[edit]

References[edit]

  1. ^ a b c d e f 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 
  2. ^ a b c d 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. ^ a b Lesh, Janet Rountree (December 1968), "The Kinematics of the Gould Belt: an Expanding Group?", Astrophysical Journal Supplement 17: 371, Bibcode:1968ApJS...17..371L, doi:10.1086/190179 
  4. ^ Wilson, Ralph Elmer (1953). General Catalogue of Stellar Radial Velocities. Washington: Carnegie Institution of Washington. Bibcode:1953QB901.W495..... 
  5. ^ a b c d Tetzlaff, N.; Neuhäuser, R.; Hohle, M. M. (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 
  6. ^ a b Sigut, T. A. A.; Jones, C. E. (October 2007), "The Thermal Structure of the Circumstellar Disk Surrounding the Classical Be Star γ Cassiopeiae", The Astrophysical Journal 668 (1): 481–491, arXiv:0706.4036, Bibcode:2007ApJ...668..481S, doi:10.1086/521209 
  7. ^ a b c d e f Balona, L. A.; Dziembowski, W. A. (October 1999), "Excitation and visibility of high-degree modes in stars", Monthly Notices of the Royal Astronomical Society 309 (1): 221–232, Bibcode:1999MNRAS.309..221B, doi:10.1046/j.1365-8711.1999.02821.x 
  8. ^ a b "CCDM J00567+6043AB -- Double or multiple star", SIMBAD (Centre de Données astronomiques de Strasbourg), retrieved 2009-04-13 
  9. ^ "Post-landing Activities", Apollo 15 Lunar Surface Journal (NASA)  commentary at 105:11:33
  10. ^ 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 
  11. ^ Kaler, James B., "Gamma Cassiopeiae", Stars (University of Illinois), retrieved 2012-03-31 
  12. ^ Smith, M. A.; Robinson, R. D. (1999), "A multiwavelength campaign on γ Cassiopeiae. III. The case for magnetically controlled circumstellar kinematics", The Astrophysical Journal 517 (2): 866–882, Bibcode:1999ApJ...517..866S, doi:10.1086/307216 
  13. ^ 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 
  14. ^ 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 
  15. ^ 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, Bibcode:2002ApJ...575..435R, doi:10.1086/341141 
  16. ^ Harmanec, P.; Habuda, P.; Štefl, S.; Hadrava, P.; Korčáková, D.; Koubský, P.; Krtička, J.; Kubát, J. et al. (2000), "Properties and nature of Be stars. XX. Binary nature and orbital elements of gamma Cas", Astronomy and Astrophysics 364: L85–L88, arXiv:astro-ph/0011516, Bibcode:2000A&A...364L..85H 
  17. ^ 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: 1226, Bibcode:2002PASP..114.1226M, doi:10.1086/342766 
  18. ^ a b VizieR Correlated Data, VizieR, retrieved 2009-04-13 
  19. ^ Norton, Arthur P. (1973), Norton's Star Atlas, p. 118, ISBN 0-85248-900-5 
  20. ^ VizieR Detailed Page. VizieR. Retrieved 2009-04-13. 

External links[edit]