B(e) star

The B(e)-type star Achernar, flattened by extremely rapid rotation

A B[e] star, frequently called a B(e)-type star, is a B-type star with distinctive forbidden neutral or low ionisation emission lines in its spectrum. The designation results from combining the spectral class B, the lowercase e denoting emission in the spectral classification system, and the surrounding square brackets signifying forbidden lines. These stars frequently also show strong hydrogen emission lines, but this feature is present in a variety of other stars and is not sufficient to classify a B(e) object. Other observational characteristics include optical linear polarization and often infrared radiation that is much stronger than in ordinary B-class stars, called infrared excess. As the B(e) nature is transient, B(e)-type stars might exhibit a normal B-type spectrum at times, and hitherto normal B-type stars may become B(e)-type stars. The term Be-type star is also used, for B class stars showing emission but not necessarily meeting the other criteria for a B(e)-type star, and sometimes just for typographical reasons. For clarity, the term classical Be-type star is used for B class stars with emission lines but not having forbidden lines, strong polarisation, or infrared excesses.^[1]

While most B(e)-type stars are on the main sequence, the identifier actually refers to a heterogeneous group of objects including pre main-sequence stars, supergiant stars, protoplanetary nebulae, and others.^[2] They may be subclassed into B[e] supergiants (notation sgB[e]), Herbig Be-type stars, compact planetary nebula B[e], symbiotic B[e], and a catch-all "unclear" category.

The first star recognized as a B(e)-type star was Gamma Cassiopeiae, observed 1866 by Angelo Secchi, the first star ever observed with emission lines. With the understanding of the processes of emission line formation in the early 20th century it became clear that these lines must come from the circumstellar environment, not from the star itself. Nowadays, all the observational characteristics are explained with a gaseous disk that is formed of material ejected from the star. The infrared excess and the polarization result from the scattering of stellar light in the disk, while the line emission is formed by re-processing stellar ultraviolet light in the gaseous disc.

The forbidden emission, infrared excess, and other features indicative of the B(e) phenomenon, themselves provide strong hints at the nature of the stars. These are produced from circumstellar discs of material, most likely ejected or stripped from the stars themselves. B(e)-type stars frequently rotate rapidly, which has been confirmed by interferometric measurements of the rotational distortion of Achernar.^[3] However, rotation alone is probably not sufficient to form the disk, but an additional ejection mechanism is required, such as a magnetic field or nonradial stellar pulsation. The transient nature of the B(e) phenomenon is most likely connected to the nature of that secondary process, but the details are currently still being discussed. The heterogeneous nature of the group of B(e)-type stars suggest a heterogeneous mechanism for formation of the discs. Many supergiant B(e)-type stars have been shown to be involved in binary systems where the companion may contribute to periodic ejection of material, or are unstable highly luminous stars which have continuum driven winds.

B(e)-type stars are typically variable and can either be classified as Gamma Cassiopeiae variable due to the transient nature of the disk and the scattering processes, or as Lambda Eridani variable on account of their pulsational nature.

See also

The Seagull Nebula is a roughly circular HII region centered on the Herbig Ae/Be-type star HD 53367.

• Shell star

References

Porter J., Rivinius Th.: Classical Be stars, 2003 PASP 115, 1153

External links

• Philippe Stee's homepage: Hot and Active Stars Research
• Article from Olivier Thizy: Be Stars
• ESO press release for Seagull Nebula image The Wings of the Seagull Nebula

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