B(e) star

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.

ESO- Reflection Nebula around HD 87643-phot-28a-09-fullres
Nebulosity around the B[e] star HD 87643

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]

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 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

The glowing cloud Sharpless 2-296, part of the Seagull Nebula01
The Seagull Nebula is a roughly circular HII region centred on the Herbig Ae/Be star HD 53367.

References

  1. ^ 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.
  2. ^ 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.
  3. ^ 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.
  4. ^ 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.
  5. ^ Cidale, L.; Zorec, J.; Tringaniello, L. (2001). "BCD spectrophotometry of stars with the B[e] phenomenon". Astronomy and Astrophysics. 368: 160. Bibcode:2001A&A...368..160C. doi:10.1051/0004-6361:20000409.
  6. ^ 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.
  7. ^ 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.
  8. ^ a b Miroshnichenko, A. S. (2007). "Toward Understanding the B[e] Phenomenon. I. Definition of the Galactic FS CMa Stars". The Astrophysical Journal. 667: 497. Bibcode:2007ApJ...667..497M. CiteSeerX 10.1.1.548.81. doi:10.1086/520798.

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3 Puppis

3 Puppis (3 Pup) is a supergiant star in the constellation Puppis. It is a very rare A[e] supergiant, sometimes referred to as a B[e] star despite its spectral classification, and its apparent magnitude is 3.93.

3 Puppis is surrounded by a disc of circumstellar dust, which is unusual for an A-type star. It is thought to be caused by a low mass companion. The companion is calculated to be a B8III - B6V star with a mass of 5 M☉, and its orbit has a semi major axis of 2.3 AU. Like most B[e] stars, 3 Pup rotates rapidly, at 30% - 60% of the speed at which it would start to break apart. The disc has its inner edge only 3.8 AU from the primary star and it is suspected that deceleration of the hot primary stellar wind by the companion allows the dust to form unusually close to such a luminous star.

B-type main-sequence star

A B-type main-sequence star (B V) is a main-sequence (hydrogen-burning) star of spectral type B and luminosity class V. These stars have from 2 to 16 times the mass of the Sun and surface temperatures between 10,000 and 30,000 K. B-type stars are extremely luminous and blue. Their spectra have neutral helium, which are most prominent at the B2 subclass, and moderate hydrogen lines. Examples include Regulus and Algol A.This class of stars was introduced with the Harvard sequence of stellar spectra and published in the Revised Harvard photometry catalogue. The definition of type B-type stars was the presence of non-ionized helium lines with the absence of singly ionized helium in the blue-violet portion of the spectrum. All of the spectral classes, including the B type, were subdivided with a numerical suffix that indicated the degree to which they approached the next classification. Thus B2 is 1/5 of the way from type B (or B0) to type A.Later, however, more refined spectra showed lines of ionized helium for stars of type B0. Likewise, A0 stars also show weak lines of non-ionized helium. Subsequent catalogues of stellar spectra classified the stars based on the strengths of absorption lines at specific frequencies, or by comparing the strengths of different lines. Thus, in the MK Classification system, the spectral class B0 has the line at wavelength 439 nm being stronger than the line at 420 nm. The Balmer series of hydrogen lines grows stronger through the B class, then peak at type A2. The lines of ionized silicon are used to determine the sub-class of the B-type stars, while magnesium lines are used to distinguish between the temperature classes.Type-B stars don't have a corona and lack a convection zone in their outer atmosphere. They have a higher mass loss rate than smaller stars such as the Sun, and their stellar wind has velocities of about 3,000 km/s. The energy generation in main-sequence B-type stars comes from the CNO cycle of thermonuclear fusion. Because the CNO cycle is very temperature sensitive, the energy generation is heavily concentrated at the center of the star, which results in a convection zone about the core. This results in a steady mixing of the hydrogen fuel with the helium byproduct of the nuclear fusion. Many B-type stars have a rapid rate of rotation, with an equatorial rotation velocity of about 200 km/s.

Beta Cephei variable

Beta Cephei variables, also known as Beta Canis Majoris stars, are variable stars that exhibit small rapid variations in their brightness due to pulsations of the stars' surfaces, thought due to the unusual properties of iron at temperatures of 200,000 K in their interiors. These stars are usually hot blue-white stars of spectral class B and should not be confused with Cepheid variables, which are named after Delta Cephei and are luminous supergiant stars.

CPD−57° 2874

CPD−57° 2874 is a B[e] supergiant in the constellation Carina. It is a rare star as it is hot but has dust which shows forbidden lines and IR emissions. In 2007 the high spatial and spectral resolution of the star's circumstellar envelope was studied. This was the first time such a study was carried out on a class B[e] supergiant.

HD 87643

HD 87643 is a B[e] class binary star embedded in a reflection nebula.

The system is described as having "one of the most extreme infrared excesses for this object class. It harbours a large amount of both hot and cold dust, and is surrounded by an extended reflection nebula." and is important for astronomers in their study of stellar formation.All the properties of HD 87643 are highly uncertain. Its distance has been estimated anywhere from one to six kpc. The General Catalogue of Variable Stars classifies it as an Orion variable, a pre-main sequence star, but other authors consider it to be a supergiant B[e] star. It has been confirmed to be a binary star system with the two stars separated by about 52 AU, but the nature of the companion is unknown.

List of stars in Carina

This is the list of notable stars in the constellation Carina, sorted by decreasing brightness.

This constellation's Bayer designations (Greek-letter star names) were given while it was still considered part of the constellation of Argo Navis. After Argo Navis was broken up into Carina, Vela, and Puppis, these Greek-letter designations were kept, so that Carina does not have a full complement of Greek-letter designations. For example, since Argo Navis's gamma star went to Vela, there is no Gamma Carinae.

List of stars in Puppis

This is the list of notable stars in the constellation Puppis, sorted by decreasing brightness.

This constellation's Bayer designations (Greek-letter star names) were given while it was still considered part of the constellation of Argo Navis. After Argo Navis was broken up into Carina, Vela, and Puppis, these Greek-letter designations were kept, so that Puppis does not have a full complement of Greek-letter designations. For example, since Argo Navis's alpha star went to Carina, there is no Alpha Puppis.

MWC 349

MWC 349 is a double (likely, triple) star system in the constellation Cygnus. Its properties are still debated and it may be a massive highly luminous star or a very young less luminous Herbig Ae/Be star. MWC 349 is also a variable star with the designation V1478 Cygni.

V1429 Aquilae

V1429 Aquilae is a candidate luminous blue variable multiple star system located in the constellation of Aquila. It is often referred to by its Mount Wilson Observatory catalog number as MWC 314. It is a hot luminous star with strong emission lines in its spectrum.

VV Cephei

VV Cephei, also known as HD 208816, is an eclipsing binary star system located in the constellation Cepheus, approximately 5,000 light years from Earth. It is both a B[e] star and shell star.

VV Cephei is an eclipsing binary with the second longest known period. A red supergiant fills its Roche lobe when closest to a companion blue star, the latter appearing to be on the main sequence. Matter flows from the red supergiant onto the blue companion for at least part of the orbit and the hot star is obscured by a large disk of material. The supergiant primary, known as VV Cephei A, is currently recognised as one of the largest stars in the galaxy although its size is not certain. It varies from 1,050 to 1,900 solar radii (4.9 to 8.8 au; 730,000,000 to 1.32×109 km). If placed in our solar system, VV Cephei A's photosphere would extend past the orbit of Jupiter and approach that of Saturn.

Westerlund 1

Westerlund 1 (abbreviated Wd1, sometimes called Ara Cluster) is a compact young super star cluster in the Milky Way galaxy, about 3.5–5 kpc away from Earth. It is one of the most massive young star clusters in the Milky Way, and was discovered by Bengt Westerlund in 1961 but remained largely unstudied for many years due to high interstellar absorption in its direction. In the future, it will probably evolve into a globular cluster.The cluster contains a large number of rare, evolved, high-mass stars, including: 6 yellow hypergiants, 4 red supergiants including Westerlund 1-26, one of the largest known stars, 24 Wolf-Rayet stars, a luminous blue variable, many OB supergiants, and an unusual supergiant sgB[e] star which has been proposed to be the remnant of a recent stellar merger. In addition, X-ray observations have revealed the presence of the anomalous X-ray pulsar CXO J164710.2-455216, a slow rotating neutron star that must have formed from a high-mass progenitor star. Westerlund 1 is believed to have formed in a single burst of star formation, implying the constituent stars have similar ages and compositions.

Aside from hosting some of the most massive and least-understood stars in our galaxy, Westerlund 1 is useful as a relatively nearby, easy to observe super star cluster that can help astronomers determine what occurs within extragalactic super star clusters.

Zack Godley

Zachary Thomas Godley (born April 21, 1990) is an American professional baseball pitcher for the Arizona Diamondbacks of Major League Baseball (MLB).

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