Gamma Cassiopeiae variable

A Gamma Cassiopeiae variable (γ Cassiopeiae variable) is a type of variable star, named for its prototype γ Cassiopeiae.

## Contents

* 1 Variability
* 2 Mechanism
* 3 Examples
* 4 References
* 5 Further reading

## Variability

A light curve for Gamma Cassiopeiae, plotted from data published by Labadie-Bartz et al. (2021)[1]

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.[2]

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.[3]

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.[2]

## 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.[3]

## Examples

* 27 Canis Majoris
* Beta Canis Minoris
* Pleione

Designation (name) | Constellation | Discovery | Apparent magnitude (Maximum)[A] | Apparent magnitude (Minimum)[A] | Range of magnitude | Period | Spectral class | Comment

|
|
|
|
|
|
|
|

Gamma Cassiopeiae | Cassiopeia |  | 1m.6  | 3m.0  | 1.4 |  | B0.5 IVe  | Prototype
Kappa Canis Majoris | Canis Major |  | 3m.40  | 3m.97  | 0.57 |  | B1.5IVne  |
FW Canis Majoris  | Canis Major |  | 5m.00  | 5m.50  | 0.50 |  |  |
Lambda Pavonis | Pavo |  | 4m.00  | 4m.26  | 0.26 |  | B2II-IIIe  |
Phi Persei[4] | Perseus |  | 3m.96  | 4m.11  | 0.15 | 19.5 d  | B2Vpe  |
Psi Persei[4] | Perseus |  | 4m.17  | 4m.36  | 0.19 |  | B5III-Vne  |
X Persei | Perseus |  | 6m.03  | 7m.0  | 0.97 |  | B0Ve  | Also a High-mass X-ray binary
A (visual magnitude, unless marked (B) (= blue) or (p) (= photographic))

## References

1. ↑ Labadie-Bartz, Jonathan; Baade, Dietrich; Carciofi, Alex C.; Rubio, Amanda; Rivinius, Thomas; Borre, Camilla C.; Martayan, Christophe; Siverd, Robert J. (March 2021). "Short-term variability and mass loss in Be stars – VI. Frequency groups in γ Cas detected by TESS". Monthly Notices of the Royal Astronomical Society 502 (1): 242–259. doi:10.1093/mnras/staa3913. https://academic.oup.com/mnras/article/502/1/242/6043237#supplementary-data. Retrieved 23 July 2022.
2. ↑ 2.0 2.1 Samus, N. N. 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.
3. ↑ 3.0 3.1 Rivinius, 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. doi:10.1007/s00159-013-0069-0. Bibcode: 2013A&ARv..21...69R.
4. ↑ 4.0 4.1 Tur, 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. doi:10.1007/BF00636758. Bibcode: 1988Ap&SS.143...99T.

## 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. doi:10.1086/190820. Bibcode: 1982ApJS...50...55S.

* v
* t
* e

Variable stars

Pulsating|

| Cepheids and
cepheid-like|

* Classical
* Type II (BL Herculis, W Virginis, RV Tauri)
* RR Lyrae
* Delta Scuti
* Rapidly oscillating Ap
* SX Phoenicis

|

Blue-white with
early spectra|

* Alpha Cygni
* Beta Cephei
* Slowly pulsating B-type
* PV Telescopii
* Blue large-amplitude pulsator

Long-period|

* Mira
* Semiregular
* Slow irregular

Other|

* Gamma Doradus
* Solar-like oscillations
* White dwarf

Eruptive|

| Protostar and PMS|

* Herbig Ae/Be
* Orion
* FU Orionis
* T Tauri

|

Giants and
supergiants|

* Luminous blue variable
* R Coronae Borealis (DY Persei)
* Yellow hypergiant

Eruptive binary|

* Double periodic
* FS Canis Majoris
* RS Canum Venaticorum

Other|

* Flare
* Gamma Cassiopeiae
* Lambda Eridani
* Wolf–Rayet

Cataclysmic|

* AM Canum Venaticorum
* Dwarf nova
* Luminous red nova
* Nova
* Polar
* Intermediate polar
* Supernova
* Hypernova
* SW Sextantis
* Symbiotic
* Symbiotic nova
* Z Andromedae

Rotating|

| Non-spherical|

* Rotating ellipsoidal

|

Stellar spots|

* BY Draconis
* FK Comae Berenices

Magnetic fields|

* Alpha² Canum Venaticorum
* Pulsar
* SX Arietis

Eclipsing|

* Algol
* Beta Lyrae
* Planetary transit
* W Ursae Majoris

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