γ Cassiopeiae variables show irregular changes in brightness on a timescale of decades. These typically have amplitudes of the order of a magnitude. For example, γ Cassiopeiae is usually about magnitude 2.5 and has varied between magnitudes 1.6 and 3.0. The variations are associated with changes in the spectrum between normal absorption spectra and Be star spectra, often also including shell star characteristics.[1]
Pleione and γ Cassiopeiae itself are both variable stars that have intermittent shell episodes where strong shell features appear in the spectrum and the brightness increases or decreases significantly. At other times the shell is not detectable in the spectrum, and even the emission lines may disappear.[2]
The General Catalogue of Variable Stars (GCVS) categorises γ Cassiopeiae stars as eruptive variables and describes them as rapidly-rotating B class giants or subgiants, although many of them are main sequence stars. It distinguishes them from those Be stars that only show smaller amplitude brightness variations. The GCVS uses the code GCAS to denote γ Cassiopeiae variables.[1]
Mechanism[]
γ Cassiopeiae variables are understood to be hot stars which have equatorial decretion disks which periodically disappear and reform, or possibly just change dramatically in scale. They are probably all very rapid rotators and most can be classified as Be stars. They are often also shell stars at least part of the time, where the disk is seen edge-on and produces very narrow absorption lines in addition to the broader photospheric lines and possible emission lines. Regardless of whether they are shell stars by the most narrow definition, the periods when they produce strong disks and increase in brightness are known as shell events.[2]
^ abSamus, 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. Originally Published in: 2009yCat....102025S. 1: B/gcvs. Bibcode:2009yCat....102025S.
^ abRivinius, Thomas; Carciofi, Alex C.; Martayan, Christophe (2013). "Classical be stars. Rapidly rotating B stars with viscous Keplerian decretion disks". Astronomy and Astrophysics Review. 21: 69. arXiv:1310.3962. Bibcode:2013A&ARv..21...69R. doi:10.1007/s00159-013-0069-0. S2CID118652497.
^ abTur, N. S.; Goraya, P. S. (April 1988). "Scanner observations of shell stars Phi Persei and Psi Persei". Astrophysics and Space Science. 143 (1): 99–105. Bibcode:1988Ap&SS.143...99T. doi:10.1007/BF00636758. S2CID121741933.
Further reading[]
Bohlender, D. (2016). "Searching for and Monitoring Ae and a Shell Stars at the DAO". Bright Emissaries: Be Stars as Messengers of Star-Disk Physics. 506: 275. Bibcode:2016ASPC..506..275B.
Slettebak, A. (1982). "Spectral types and rotational velocities of the brighter Be stars and A-F type shell stars". Astrophysical Journal Supplement Series. 50: 55–83. Bibcode:1982ApJS...50...55S. doi:10.1086/190820.
