B(e) star
B-type stars with forbidden emission lines and infrared excess.
Wikipedia / Wikimedia Commons
A B[e] star—also known as a B[e]-type star—is a B-class star whose spectrum shows distinctive forbidden emission lines from neutral or low-ionization elements. Its name comes from the spectral class B, the lowercase “e” for emission, and square brackets that mark forbidden lines. While these stars often have strong hydrogen emission lines too, that feature alone isn’t enough to identify a B[e] object, since it appears in many other star types. Other telltale signs include optical linear polarization and an infrared excess—much stronger infrared radiation than typical B-class stars produce. The B[e] nature is not permanent; a star may sometimes show a normal B-type spectrum, and an ordinary B star can later turn into a B[e]-type star.
**Discovery**
Many Be stars were found to have unusual spectral traits, including forbidden lines from ionized iron and occasionally other elements. In 1973, a study of HD 45677 (FS CMa) revealed both an infrared excess and forbidden lines of [OI], [SII], [FeII], [NiII], and many more. Three years later, a survey of Be stars with infrared excesses picked out a group that displayed forbidden emission lines from ionized iron and a few other elements. These stars were clearly different from classical main-sequence Be stars, even though they seemed to include a wide variety of stellar types. The term “B[e] star” was introduced to gather them together. One type was quickly identified as highly luminous supergiants; by 1985, eight dust-shrouded B[e] supergiants were known in the Magellanic Clouds. Others turned out not to be supergiants at all—some were binaries, others proto-planetary nebulae. The phrase “B[e] phenomenon” was adopted to stress that different kinds of stars could produce the same spectral signature.
**Classification**
Once it was clear that the B[e] phenomenon could appear in several distinct stellar types, four subcategories were named: B[e] supergiants (sgB[e]), pre-main-sequence B[e] stars (HAeB[e], a subset of Herbig Ae/Be stars), compact planetary nebula B[e] stars (cPNB[e]), and symbiotic B[e] stars (SymB[e]). About half of all known B[e] stars didn’t fit any of these groups and were labeled unclassified B[e] stars (unclB[e]). Those unclassified stars were later reclassified as FS CMa stars, a variable-star type named after one of the earliest B[e] stars.
**Nature**
The forbidden emission, infrared excess,
- discovery_year
- 1976
- spectral_type
- B-type
- defining_feature
- Forbidden emission lines of neutral or low ionisation elements
- subtypes
- sgB[e], HAeB[e], cPNB[e], SymB[e], unclB[e] (later FS CMa stars)
- known_for
- B[e] phenomenon: forbidden lines, infrared excess, and transient nature
Lore & Background
Many Be stars were discovered to have spectral peculiarities, including forbidden spectral lines of ionised iron and occasionally other elements. In 1973 a study of HD 45677 (FS CMa) showed an infrared excess as well as forbidden lines of [OI], [SII], [FeII], [NiII], and many more. 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 distinct from 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.
One type of B[e] star was readily identified as highly luminous supergiants. By 1985, eight dust-shrouded B[e] supergiants were known in the Magellanic Clouds. 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. Following the recognition that the B[e] phenomenon could occur in several distinct types of star, four sub-types were named: B[e] supergiants (sgB[e]), pre-main sequence B[e] stars (HAeB[e]), compact planetary nebulae B[e] stars (cPNB[e]), and 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]), later re-classified as FS CMa stars.
Reader's Guide
The B[e] phenomenon represents a significant class of stellar objects whose spectra reveal complex circumstellar environments. The forbidden emission, infrared excess, and other features indicate that these stars are surrounded by ionised gas sufficiently extended to allow 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 types of B[e] star. The sgB[e] stars have hot fast winds producing extended circumstellar material plus a denser equatorial disc. HAeB[e] are surrounded by the remains of molecular clouds forming the stars. Binary B[e] stars can produce discs of material 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. A proposed binary-interaction mechanism for producing structured circumstellar material around some sgB[e] is the Lizard Autotomy Effect, suggested as a possible origin for spiral, ring-like, or fragmented circumstellar structures around some sgB[e] systems.
Did You Know?
- The term B[e] star was coined in 1976 to group Be stars with infrared excesses and forbidden emission lines.
- Around half of known B[e] stars could not be placed in any of the four named sub-types and were called unclassified B[e] stars, later re-classified as FS CMa stars.
- The B[e] nature is transient; such stars might exhibit a normal B-type spectrum at times, and normal B-type stars may become B[e]-type stars.
- A proposed mechanism for producing structured circumstellar material around some sgB[e] stars is the Lizard Autotomy Effect.
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