Shell star

Pleione in the Pleiades is a shell star.

A shell star is a star having a spectrum that exhibits features indicating a circumstellar disc of gas surrounding the star at the equator. They may exhibit irregular variations in their luminosity due to the outflow of matter. The shell stars are fast rotators, giving a partial explanation on the mechanism, but shell stars are still considered enigmatic. Shell stars belong to spectral types O7.5 to F5, but their spectra are characterized by enormously widened absorption lines due to fast rotation and the disc that contributes also to other spectral peculiarities. Rotation velocities are about 200–250 km/s, not far from the point when the rotational acceleration would disrupt the star.

Subtypes

Shell stars are subdivided in four categories:^[1]

• early Be stars of spectral types O7.5 to B2.5
• middle Be stars of types B3 to B6.5,
• late Be stars of types B7 to B9.5, and
• A-F shell stars ranging from A0 to F5.

The spectrum is variable in the long term, and the early type shell stars may switch from and to exhibiting a Be type spectrum or an ordinary B spectrum. The earlier types usually exhibit He I and usually Fe II emission, while later types show Ca II and Ti II emission. It is believed that the shell stars belong to somewhere in the range of main sequence up to giant star size, although the exact luminosity class is uncertain due to rotational line broadening.

Variability

Shell stars often show variability in their spectra and brightness. The shell features may come and go, with the star changing from a shell star to a normal B star or Be star. Shell stars which show irregular variability due to changes in, or the disappearance of, the "shell" are called Gamma Cassiopeiae variables.^[2] Pleione and Gamma 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.

Examples

• 27 Canis Majoris
• Beta Canis Minoris
• Pleione

This is a dynamic list and may never be able to satisfy particular standards for completeness. You can help by expanding it with reliably sourced entries.

Designation (name)	Constellation	Discovery	Apparent magnitude (Maximum)[3]	Apparent magnitude (Minimum)[4]	Range of magnitude	Period	Spectral class	Comment
Gamma Cassiopeiae	Cassiopeia		1^m.6	3^m.0	1.4		B0.5 IVe	Prototype
Kappa Canis Majoris	Canis Major		3^m.40	3^m.97	0.57		B1.5IVne
FW Canis Majoris	Canis Major		5^m.00	5^m.50	0.50
Lambda Pavonis	Pavo		4^m.00	4^m.26	0.26		B2II-IIIe
Phi Persei[5]	Perseus		3^m.96	4^m.11	0.15	19.5 d	B2Vpe
Psi Persei[5]	Perseus		4^m.17	4^m.36	0.19		B5III-Vne
X Persei	Perseus		6^m.03	7^m.0	0.97		B0Ve	Also a High-mass X-ray binary

References

1. Slettebak, A. (1982). "Spectral types and rotational velocities of the brighter Be stars and A-F type shell stars, page 80". Astrophysical Journal Supplement Series 50: 55. Bibcode:1982ApJS...50...55S. doi:10.1086/190820.
2. vartype.txt in Combined General Catalog of Variable Stars GCVS, Samus N.N., Durlevich O.V., et al.
3. (visual magnitude, unless marked (B) (= blue) or (p) (= photographic))
4. (visual magnitude, unless marked (B) (= blue) or (p) (= photographic))
5. 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. Bibcode:1988Ap&SS.143...99T. doi:10.1007/BF00636758.

Further reading

• Porter, John M. (1996). "On the rotational velocities of Be and Be-shell stars". Monthly Notices of the Royal Astronomical Society 280: L31. Bibcode:1996MNRAS.280L..31P. doi:10.1093/mnras/280.3.31L (inactive 2016-03-23).
• Quirrenbach, A.; Buscher, D. F.; Mozurkewich, D.; Hummel, C. A.; Armstrong, J. T. (1994). "Maximum-entropy maps of the Be shell star zeta Tauri from optical long-baseline interferometry". Astronomy and Astrophysics (ISSN 0004-6361) 283: L13. Bibcode:1994A&A...283L..13Q.
• 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. Bibcode:1982ApJS...50...55S. doi:10.1086/190820.