Definitions

decretion

Gamma Cassiopeiae

Gamma Cassiopeiae (γ Cas / γ Cassiopeiae) is an eruptive variable star, whose brightness changes irregularly between +2.20 mag and +3.40 mag. It is the prototype of the Gamma Cassiopeiae variable stars. Although it is a fairly bright star, it has no traditional Arabic or Latin name. In Chinese, however, it has the name Tsih, meaning "the whip". It is located at the center of the distinctive "W" shape that forms the Cassiopeia constellation. American astronaut Virgil Ivan "Gus" Grissom nicknamed the star Navi after his own middle name spelled backwards. The star was used as an easily identifiable navigational reference point during space missions.

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).

This is a rapidly spinning star that bulges outward along the equator. When combined with the high luminosity, the result is mass loss that forms a disk around the star. The emissions and brightness variations are apparently caused by this "decretion" disk.

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 (Harmanec et al. 2000, Miroschnichenko et al. 2002)

Gamma Cas is also the prototype of a small group of stellar sources of X-ray radiation that is about 10 times higher that emitted from other B or Be stars, which shows very short term and long-term cycles. The character of the X-ray spectrum is be "thermal" and is possibly emitted from plasmas of temperatures up to least ten million kelvins. 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 It is now realized that there are interpretational difficulties with either of these pictures. For example, it is not clear that enough matter can be accreted by the white dwarf at the distance of the secondary star (whose nature is not known), implied by the orbital period, is sufficient to power the X-rays (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 nonthermal, and thus in apparent variance with the spectral properties.

Recent evidence suggests that the X-rays may be associated with the Be star itself or in 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 diagnostics associated with a B star or with circumstellar matter close to the star (see Smith and Robinson 1999, Cranmer et al. 2000). Moreover, the X-ray emissions exhibit long-term cycles that correlate with the visible wavelength light curves (Smith et al. 2006). One intriguing property is that gamma cas exhibits characteristics consistent with a strong, disordered field (although no field can be measured directly by zeeman techniques because of its broad spectral lines). This inference comes from a coherent signature giving rise to robust period of 1.21 days suggesting a rooted magnetic field on its surface. The star's UV and optical spectral lines also show ripples moving from blue to red over several hours, which is indicative of clouds of matter frozen over the star's surface by strong magnetic fields. This evidence suggests that a magnetic field from the star interacting with the decretion disk are responsible for the X-rays. A disk dynamo has been advanced as a mechanism to explain the modulation of the X-rays (Robinson et al. 2002). However, difficulties remain with this mechanism, among which is that there are no disk dynamos are known to exist in other stars, rendering their behavior somewhat speculative.

This is also a visual double star system, with the designation of ADS782AB.

References

Notes

General reference

  • Cranmer, S., Smith, M., and Robinson, R., "A multiwavelength campaign on gamma Cassiopeiae IV. The case for illuminated disk-enhanced wind streams," 2002, "ApJ," 537, 433
  • Miroschnichenko, A. S., Bjorkman, K. S., and Krugov, V. D. 2002, "Binary nature and long term nature of gamma Cassiopeiae," "Pub. Astron. Soc. Pacific," 114, 1226
  • P. Harmanec, P. et al, "Binary Nature and Orbital Elements of Gamma Cas", 2000, Astronomy and Astrophysics, 364, L85.
  • Robinson, R. D., Smith, M. A., and Henry, G. W., "X-ray and optical variations in the classical Be star gamma Cas," 2002, "ApJ", 575, 435
  • Smith, M. A., Cohen, D. H., Gu, M., "et al", "High resolution Chandra spectroscopy of gamma Cas (B0.5e)", 2004, "ApJ", 600, 972
  • Smith, M. A. and Robinson, R. D., "A multiwavelength campaign on gamma Cas III. The case for magnetically controlled circumstellar kinematics, 1999, "ApJ", 517, 866

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