Editing B(e) star

{{Short description|B-type star with distinctive forbidden neutral or low ionisation emission lines in its spectrum}}
{{hatnote group|
{{Distinguish|Be star}}
{{correct title|B[e] star|reason=bracket}}
}}
[[File:ESO- Reflection Nebula around HD 87643-phot-28a-09-fullres.jpg|thumb|right|upright=1.2|Nebulosity around the B[e] star [[HD 87643]]]]
A '''B[e] star''', frequently called a '''B[e]-type star''', is a [[Stellar classification|B-type star]] with distinctive [[forbidden mechanism|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 star]]s were discovered to have spectral peculiarities.  One of these peculiarities was the presence of [[Forbidden line|forbidden spectral lines]] of ionised iron and occasionally other elements.<ref name=burbr1954/>

In 1973 a study of one of these stars, [[HD 45677]] or FS CMa, showed an [[infrared excess]] as well as forbidden lines of [O<sub>I</sub>], [S<sub>II</sub>], [Fe<sub>II</sub>], [Ni<sub>II</sub>], and many more.<ref name=swings1973/>

In 1976 a study of [[Be star]]s with [[infrared excess]]es 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.<ref name=allen1976/>

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]].<ref name=zickg1986/> 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.<ref name=cidale2001/>

==Classification==
[[File:The glowing cloud Sharpless 2-296, part of the Seagull Nebula01.jpg|thumb|upright=1.2|The [[Seagull Nebula]] is a roughly circular [[HII region]] centred on the [[Herbig Ae/Be star]] [[HD 53367]].<ref name=ESO_seagull/>]]
Following the recognition that the B[e] phenomenon could occur in several distinct types of star, four sub-types were named:<ref name=lamers1998/>
* B[e] [[supergiant]]s (sgB[e])
* [[Pre-main-sequence star|pre-main sequence]] B[e] stars (HAeB[e]), a subset of the [[Herbig Ae/Be star]]s
* compact [[planetary nebula]]e B[e] stars (cPNB[e])
* [[Symbiotic star|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 Canis Majoris variable|FS CMa]] stars, a type of variable named for one of the earliest known B[e] stars.<ref name=miro2015/>

==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.<ref name=miro2007/>

The sgB[e] stars have hot fast [[stellar wind|wind]]s which produce extended circumstellar material, plus a denser equatorial disc. HAeB[e] are surrounded by the remains of the [[molecular cloud]]s 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-[[asymptotic giant branch|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.<ref name=miro2007/>

== See also ==
* [[Shell star]]

== References ==
{{reflist|25em |refs=
<ref name=allen1976>{{cite journal |bibcode=1976A&A....47..293A |title=The spectra of peculiar Be stars with infrared excesses|journal=Astronomy and Astrophysics |volume=47 |pages=293 |last1=Allen |first1=D. A. |last2=Swings |first2=J. P. |year=1976}}</ref>

<ref name=burbr1954>{{cite journal |bibcode=1954ApJ...119..501B |title=A Group of Peculiar Shell Stars |journal=Astrophysical Journal |volume=119 |pages=501 |last1=Burbidge |first1=E. Margaret |last2=Burbidge |first2=G. R. |year=1954 |doi=10.1086/145856}}</ref>

<ref name=cidale2001>{{cite journal |doi=10.1051/0004-6361:20000409 |title=BCD spectrophotometry of stars with the B[e] phenomenon |journal=Astronomy and Astrophysics |volume=368 |pages=160–174 |year=2001 |last1=Cidale |first1=L. |last2=Zorec |first2=J. |last3=Tringaniello |first3=L. |bibcode=2001A&A...368..160C|doi-access=free }}</ref>

<ref name=ESO_seagull>{{cite press release |publisher=ESO |url=http://www.eso.org/public/news/eso1306 |title=The Wings of the Seagull Nebula |date=6 February 2013}} press release for Seagull Nebula image</ref>

<ref name=lamers1998>{{cite journal |bibcode=1998A&A...340..117L |title=An improved classification of B[e]-type stars |journal=Astronomy and Astrophysics |volume=340 |pages=117 |last1=Lamers |first1=Henny J. G. L. M. |last2=Zickgraf |first2=Franz-Josef |last3=de Winter |first3=Dolf |last4=Houziaux |first4=Leo |last5=Zorec |first5=Janez |year=1998}}</ref>

<ref name=miro2007>{{cite journal |doi=10.1086/520798 |title=Toward Understanding the B[e] Phenomenon. I. Definition of the Galactic FS CMa Stars |journal=The Astrophysical Journal |volume=667 |pages=497–504 |year=2007 |last1=Miroshnichenko |first1=A.S. |issue=1 |bibcode=2007ApJ...667..497M |citeseerx=10.1.1.548.81|s2cid=40320323 }}</ref>

<ref name=miro2015>{{cite journal |doi=10.1088/0004-637X/809/2/129 |title=Toward understanding the B[e] phenomenon. V. Nature and spectral variations of the MWC&nbsp;728 binary system  |journal=The Astrophysical Journal |volume=809 |issue=2 |pages=129 |year=2015 |last1=Miroshnichenko |first1=A.S. |last2=Zharikov |first2=S.V. |last3=Danford |first3=S. |last4=Manset |first4=N. |last5=Korčáková |first5=D. |last6=KřÍček |first6=R. |last7=Šlechta |first7=M. |last8=Omarov |first8=Ch.T. |last9=Kusakin |first9=A.V. |last10=Kuratov |first10=K.S. |last11=Grankin |first11=K.N. |arxiv=1508.00950 |bibcode=2015ApJ...809..129M|s2cid=27971806 }}</ref>

<ref name=swings1973>{{cite journal |bibcode=1973A&A....26..443S |title=Spectrographic observations of the peculiar Be star with infrared excess HD 45677 |journal=Astronomy and Astrophysics |volume=26 |pages=443 |last1=Swings |first1=J. P. |year=1973}}</ref>

<ref name=zickg1986>{{cite journal |bibcode=1986A&A...163..119Z |title=B(e)-supergiants of the Magellanic Clouds |journal=Astronomy and Astrophysics |volume=163|pages=119 |last1=Zickgraf |first1=F.-J. |last2=Wolf |first2=B. |last3=Leitherer |first3=C. |last4=Appenzeller |first4=I. |last5=Stahl |first5=O. |year=1986}}</ref>
}}

== External links ==
* {{cite web |website=Philippe Stee's homepage |url=http://www.oca.eu/stee/page1/page3/page3.html |title=Hot and Active Stars Research}}
* {{cite web |author=Thizy, Olivier |url=http://www.shelyak.com/dossier.php?id_dossier=24&lang=2 |archive-url=https://web.archive.org/web/20120724022606/http://www.shelyak.com/dossier.php?id_dossier=24&lang=2 |url-status=dead |archive-date=2012-07-24 |title=Be stars}}


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{{DEFAULTSORT:Be Star}}
[[Category:Star types]]
[[Category:B(e) stars| ]]