Be star

Depiction of Achernar, the brightest Be star

Be Stars are a heterogeneous set of stars with B spectral types and emission lines. A narrower definition, sometimes referred to as Classical Be Stars, is non-supergiant stars with Balmer emission lines in their spectra.

Definition and classification

Many stars have B-type spectra and show hydrogen emission lines, including many supergiants, Herbig Ae/Be stars, mass-transferring binary systems, and B[e] stars. It is preferred to restrict usage of the term Be star to non-supergiant stars showing one or more Balmer series lines in emission. These are sometimes referred to as classical Be stars. The emission lines may be present only at certain times.^[1]

Although the Be type spectrum is most strongly produced in class B stars, it is also detected in O and A shell stars, and these are sometimes included under the "Be star" banner. Be stars are primarily considered to be main sequence stars, but a number of subgiants and giant stars are also included.^[2]

Discovery

The first star recognized as a Be star was Gamma Cassiopeiae, observed 1866 by Angelo Secchi, the first star ever observed with emission lines.^[3] Many other bright stars were found to show similar spectra, although many of these are no longer considered to be classical Be stars.^[4] The brightest is Achernar, although it was not recognised as a Be star until 1976.^[5][6]

Model

With the understanding of the processes of emission line formation in the early 20th century it became clear that these lines in Be stars must come from circumstellar material ejected by the rapid rotation of the star.^[7] All the observational characteristics of Be stars can now be 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.^[2]

Shell stars

Some Be stars exhibit spectral features that are interpreted as a detached "shell" of gas surrounding the star, or more accurately a disc or ring. These shell features are thought to be caused when the disc of gas that is present around many Be stars is aligned edge on to us so that it creates very narrow absorption lines in the spectrum.

Variability

Be stars are often visually and spectroscopically variable. Be stars can be classified as Gamma Cassiopeiae variables when a transient or variable disk is observed. Be stars that show variability without clear indication of the mechanism are listed simply as BE in the General Catalogue of Variable Stars. Some of these are thought to be pulsating stars and are sometimes called Lambda Eridani variables.

References

1. Porter, John M.; Rivinius, Thomas (2003). "Classical Be Stars". Publications of the Astronomical Society of the Pacific 115 (812): 1153. Bibcode:10.1086/378307. doi:10.1086/378307.
2. Rivinius, Thomas; Carciofi, Alex C.; Martayan, Christophe (2013). "Classical Be stars". The Astronomy and Astrophysics Review 21 (1): 69. arXiv:1310.3962. Bibcode:2013A&ARv..21...69R. doi:10.1007/s00159-013-0069-0. ISSN 0935-4956.
3. Secchi, A. (1867). "Schreiben des Herrn Prof.Secchi, Dir. Der Sternwarte des Collegio Romano, an den Herausgeber". Astronomische Nachrichten 68 (4): 63. Bibcode:1866AN.....68...63S. doi:10.1002/asna.18670680405.
4. Merrill, P. W.; Humason, M. L.; Burwell, C. G. (1925). "Discovery and Observations of Stars of Class Be". Astrophysical Journal 61: 389. Bibcode:1925ApJ....61..389M. doi:10.1086/142899.
5. Snow, T. P.; Marlborough, J. M. (1976). "Evidence for mass loss at moderate to high velocity in Be stars". Astrophysical Journal 203: L87. Bibcode:1976ApJ...203L..87S. doi:10.1086/182025.
6. Massa, D. (1975). "The influence of rotation and stellar winds upon the Be phenomenon". Astronomical Society of the Pacific 87: 777. Bibcode:1975PASP...87..777M. doi:10.1086/129842.
7. Struve, Otto (1931). "On the Origin of Bright Lines in Spectra of Stars of Class B". Astrophysical Journal 73: 94. Bibcode:1931ApJ....73...94S. doi:10.1086/143298.

Further reading

• Be and Shell Stars. 1976. doi:10.1007/978-94-010-1498-4. ISBN 978-94-010-1498-4.

External links

• Philippe Stee's homepage: Hot and Active Stars Research
• Article from Olivier Thizy: Be Stars