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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.

Discovery

Many Be stars were discovered to have spectral peculiarities. One of these peculiarities was the presence of forbidden spectral lines of ionised iron and occasionally other elements.[1]

In 1973 a study of one of these stars, HD 45677 or FS CMa, showed an infrared excess as well as forbidden lines of [OI], [SII], [FeII], [NiII], and many more.[2]

In 1976 a study of Be stars with infrared excesses identified a subset of stars which showed forbidden emission lines from ionised iron and some other elements. These stars were all considered to be distinct from the classical main sequence Be stars, although they appeared to consist of a wide range of different types of star. The term B[e] star was coined to group these stars.[3]

One type of B[e] star was readily identified as being highly luminous supergiants. By 1985, eight dust-shrouded B[e] supergiants were known in the Magellanic Clouds.[4] Others were found to be definitely not supergiants. Some were binaries, others proto-planetary nebulae, and the term "B[e] phenomenon" was used to make it clear that different types of star could produce the same type of spectrum.[5]
Classification

Following the recognition that the B[e] phenomenon could occur in several distinct types of star, four sub-types were named:[6]

B[e] supergiants (sgB[e])
pre-main sequence B[e] stars (HAeB[e]), a subset of the Herbig Ae/Be star stars
compact planetary nebulae B[e] stars (cPNB[e])
symbiotic B[e] stars (SymB[e])

Around half of the known B[e] stars could not be placed in any of these groups and were called unclassified B[e] stars (unclB[e]). The unclB[e] stars have since been re-classified as FS CMa stars, a type of variable named for one of the earliest known B[e] stars.[7]
Nature

The forbidden emission, infrared excess, and other features indicative of the B[e] phenomenon, themselves provide strong hints at the nature of the stars. The stars are surrounded by ionised gas which produces intense emission lines in the same way as Be stars. The gas must be sufficiently extended to allow the formation of forbidden lines in the outer low density region, and also for dust to form which produces the infrared excess. These features are common to all the types of B[e] star.[8]
The Seagull Nebula is a roughly circular HII region centred on the Herbig Ae/Be star HD 53367.[9]

The sgB[e] stars have hot fast winds which produce extended circumstellar material, plus a denser equatorial disc. HAeB[e] are surrounded by the remains of the molecular clouds which are forming the stars. Binary B[e] stars can produce discs of material as it is transferred from one star to another through roche lobe overflow. cPNB[e] are post-AGB stars that have shed their entire atmospheres after reaching the end of their lives as actively fusing stars. The FS CMa stars appear to be binaries with a rapidly rotating mass-losing component.[8]
See also

Shell star

References

Burbidge, E. Margaret; Burbidge, G. R. (1954). "A Group of Peculiar Shell Stars". Astrophysical Journal. 119: 501. Bibcode:1954ApJ...119..501B. doi:10.1086/145856.
Swings, J. P. (1973). "Spectrographic observations of the peculiar Be star with infrared excess HD 45677". Astronomy and Astrophysics. 26: 443. Bibcode:1973A&A....26..443S.
Allen, D. A.; Swings, J. P. (1976). "The spectra of peculiar Be stars with infrared excesses". Astronomy and Astrophysics. 47: 293. Bibcode:1976A&A....47..293A.
Zickgraf, F.-J.; Wolf, B.; Leitherer, C.; Appenzeller, I.; Stahl, O. (1986). "B(e)-supergiants of the Magellanic Clouds". Astronomy and Astrophysics. 163: 119. Bibcode:1986A&A...163..119Z.
Cidale, L.; Zorec, J.; Tringaniello, L. (2001). "BCD spectrophotometry of stars with the B[e] phenomenon". Astronomy and Astrophysics. 368: 160–174. Bibcode:2001A&A...368..160C. doi:10.1051/0004-6361:20000409.
Lamers, Henny J. G. L. M.; Zickgraf, Franz-Josef; de Winter, Dolf; Houziaux, Leo; Zorec, Janez (1998). "An improved classification of B[e]-type stars". Astronomy and Astrophysics. 340: 117. Bibcode:1998A&A...340..117L.
Miroshnichenko, A.S.; Zharikov, S.V.; Danford, S.; Manset, N.; Korčáková, D.; KřÍček, R.; Šlechta, M.; Omarov, Ch.T.; Kusakin, A.V.; Kuratov, K.S.; Grankin, K.N. (2015). "Toward understanding the B[e] phenomenon. V. Nature and spectral variations of the MWC 728 binary system". The Astrophysical Journal. 809 (2): 129. arXiv:1508.00950. Bibcode:2015ApJ...809..129M. doi:10.1088/0004-637X/809/2/129. S2CID 27971806.
Miroshnichenko, A.S. (2007). "Toward Understanding the B[e] Phenomenon. I. Definition of the Galactic FS CMa Stars". The Astrophysical Journal. 667 (1): 497–504. Bibcode:2007ApJ...667..497M. CiteSeerX 10.1.1.548.81. doi:10.1086/520798.

"The Wings of the Seagull Nebula" (Press release). ESO. 6 February 2013. press release for Seagull Nebula image

External links

"Hot and Active Stars Research". Philippe Stee's homepage.
Thizy, Olivier. "Be stars". Archived from the original on 2012-06-19.


vte

Stars
Formation

Accretion Molecular cloud Bok globule Young stellar object
Protostar Pre-main-sequence Herbig Ae/Be T Tauri FU Orionis Herbig–Haro object Hayashi track Henyey track

Evolution

Main sequence Red-giant branch Horizontal branch
Red clump Asymptotic giant branch
super-AGB Blue loop Protoplanetary nebula Planetary nebula PG1159 Dredge-up OH/IR Instability strip Luminous blue variable Blue straggler Stellar population Supernova Superluminous supernova / Hypernova

Spectral classification

Early Late Main sequence
O B A F G K M Brown dwarf WR OB Subdwarf
O B Subgiant Giant
Blue Red Yellow Bright giant Supergiant
Blue Red Yellow Hypergiant
Yellow Carbon
S CN CH White dwarf Chemically peculiar
Am Ap/Bp HgMn Helium-weak Barium Extreme helium Lambda Boötis Lead Technetium Be
Shell B[e]

Remnants

White dwarf
Helium planet Black dwarf Neutron
Radio-quiet Pulsar
Binary X-ray Magnetar Stellar black hole X-ray binary
Burster

Hypothetical

Blue dwarf Green Black dwarf Exotic
Boson Electroweak Strange Preon Planck Dark Dark-energy Quark Q Black Gravastar Frozen Quasi-star Thorne–Żytkow object Iron Blitzar

Stellar nucleosynthesis

Deuterium burning Lithium burning Proton–proton chain CNO cycle Helium flash Triple-alpha process Alpha process Carbon burning Neon burning Oxygen burning Silicon burning S-process R-process Fusor Nova
Symbiotic Remnant Luminous red nova

Structure

Core Convection zone
Microturbulence Oscillations Radiation zone Atmosphere
Photosphere Starspot Chromosphere Stellar corona Stellar wind
Bubble Bipolar outflow Accretion disk Asteroseismology
Helioseismology Eddington luminosity Kelvin–Helmholtz mechanism

Properties

Designation Dynamics Effective temperature Luminosity Kinematics Magnetic field Absolute magnitude Mass Metallicity Rotation Starlight Variable Photometric system Color index Hertzsprung–Russell diagram Color–color diagram

Star systems

Binary
Contact Common envelope Eclipsing Symbiotic Multiple Cluster
Open Globular Super Planetary system

Earth-centric
observations

Sun
Solar System Sunlight Pole star Circumpolar Constellation Asterism Magnitude
Apparent Extinction Photographic Radial velocity Proper motion Parallax Photometric-standard

Lists

Proper names
Arabic Chinese Extremes Most massive Highest temperature Lowest temperature Largest volume Smallest volume Brightest
Historical Most luminous Nearest
Nearest bright With exoplanets Brown dwarfs White dwarfs Milky Way novae Supernovae
Candidates Remnants Planetary nebulae Timeline of stellar astronomy

Related articles

Substellar object
Brown dwarf Sub-brown dwarf Planet Galactic year Galaxy Guest Gravity Intergalactic Planet-hosting stars Tidal disruption event

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