A Herbig Ae/Be star (HAeBe) is a pre-main-sequence star – a young (<10Myr) star of spectral types A or B. These stars are still embedded in gas-dust envelopes and are sometimes accompanied by circumstellar disks. Hydrogen and calcium emission lines are observed in their spectra. They are 2-8 Solar mass (M☉) objects, still existing in the star formation (gravitational contraction) stage and approaching the main sequence (i.e. they are not burning hydrogen in their center). In the Hertzsprung–Russell diagram these stars are located to the right of the main sequence. They are named after the American astronomer George Herbig, who first distinguished them from other stars in 1960. The original Herbig criteria were:
There are now several known isolated Herbig Ae/Be stars (i.e. not connected with dark clouds or nebulae). Thus the most reliable criteria now can be:
Sometimes Herbig Ae/Be stars show significant brightness variability. They are believed to be due to clumps (protoplanets and planetesimals) in the circumstellar disk. In the lowest brightness stage the radiation from the star becomes bluer and linearly polarized (when the clump obscures direct star light, scattered from disk light relatively increases – it is the same effect as the blue color of our sky).
Analogs of Herbig Ae/Be stars in the smaller mass range (<2 M☉) – F, G, K, M spectral type pre-main-sequence stars – are called T Tauri stars. More massive (>8 M☉) stars in pre-main-sequence stage are not observed, because they evolve very quickly: when they become visible (i.e. disperses surrounding circumstellar gas and dust cloud), the hydrogen in the center is already burning and they are main-sequence objects.
Planets around Herbig Ae/Be stars include:
51 Ophiuchi (51 Oph) is a star located approximately 410 light years away in the constellation Ophiuchus, northwest of the center of the Milky Way. It is notable for being "a rare, nearby example of a young planetary system just entering the last phase of planet formation". There is uncertainty about the stellar classification of this star. It has the nominal classification of B9.5IIIe, a B-type giant star with emission lines. However, it has also been classified as an A0 II-IIIe star and as a Herbig Ae/Be star.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.Chamaeleon complex
The Chamaeleon complex is a large star forming region (SFR) that includes the Chamaeleon I, Chamaeleon II, and Chamaeleon III dark clouds. It occupies nearly all of the constellation Chamaeleon and overlaps into Apus, Musca, Carina and Octans. The mean density of X-ray sources is about one source per square degree.Dark nebula
A dark nebula or absorption nebula is a type of interstellar cloud that is so dense that it obscures the light from objects behind it, such as background stars and emission or reflection nebulae. The extinction of the light is caused by interstellar dust grains located in the coldest, densest parts of larger molecular clouds. Clusters and large complexes of dark nebulae are associated with Giant Molecular Clouds. Isolated small dark nebulae are called Bok globules. Like other interstellar dust or material, things it obscures are only visible using radio waves in radio astronomy or infrared in infrared astronomy.
Dark clouds appear so because of sub-micrometre-sized dust particles, coated with frozen carbon monoxide and nitrogen, which effectively block the passage of light at visible wavelengths. Also present are molecular hydrogen, atomic helium, C18O (CO with oxygen as the 18O isotope), CS, NH3 (ammonia), H2CO (formaldehyde), c-C3H2 (cyclopropenylidene) and a molecular ion N2H+ (diazenylium), all of which are relatively transparent. These clouds are the spawning grounds of stars and planets, and understanding their development is essential to understanding star formation.The form of such dark clouds is very irregular: they have no clearly defined outer boundaries and sometimes take on convoluted serpentine shapes. The largest dark nebulae are visible to the naked eye, appearing as dark patches against the brighter background of the Milky Way like the Coalsack Nebula and the Great Rift. These naked-eye objects are sometimes known as dark cloud constellations and take on a variety of names.
In the inner outer molecular regions of dark nebulae, important events take place, such as the formation of stars and masers.Emission-line star
An emission-line star is a star whose spectrum exhibits emission lines. Common types include:
Herbig Ae/Be star
Wolf–Rayet starFU Orionis star
In stellar evolution, an FU Orionis star (also FU Orionis object, or FUor) is a pre–main-sequence star which displays an extreme change in magnitude and spectral type. One example is the star V1057 Cyg, which became 6 magnitudes brighter and went from spectral type dKe to F-type supergiant. These stars are named after their type-star, FU Orionis.
The current model developed primarily by Lee Hartmann and Scott Jay Kenyon associates the FU Orionis flare with abrupt mass transfer from an accretion disc onto a young, low mass T Tauri star. Mass accretion rates for these objects are estimated to be around 10−4 solar masses per year. The rise time of these eruptions is typically on the order of 1 year, but can be much longer. The lifetime of this high-accretion, high-luminosity phase is on the order of decades. However, even with such a relatively short timespan, as of 2015 no FU Orionis object had been observed shutting off. By comparing the number of FUor outbursts to the rate of star formation in the solar neighborhood, it is estimated that the average young star undergoes approximately 10–20 FUor eruptions over its lifetime.
The prototypes of this class are: FU Orionis, V1057 Cygni, V1515 Cygni, and the embedded protostar V1647 Orionis, which erupted in January 2004.HD 141569
HD 141569 is an isolated Herbig Ae/Be star of spectral class A2Ve approximately 320 light-years away in the constellation of Libra. The primary star has two red dwarf companions (orbiting each other) at about nine arcseconds. In 1999, a protoplanetary disk was discovered around the star. A gap in the disk speculate a possible extrasolar planet forming in the disk.HD 97048
HD 97048 or CU Chamaeleontis is a Herbig Ae/Be star 603 ly away in the constellation Chamaeleon. It is a variable star embedded in a dust cloud containing a stellar nursery, and is itself surrounded by a dust disk.
HD 97048 is a young star still contracting towards the main sequence. Its brightness varies between magnitudes 8.38 and 8.48 and it is classified as an Orion variable. It was given the variable star designation CU Chamaeleontis in 1981. Its spectrum is also variable. The spectral class is usually given as A0 or B9, sometimes with a giant luminosity class, sometimes main sequence. The spectrum shows strong variable emission lines indicative of a shell surrounding the star.HD 97048 is a mamber of the Chamaeleon T1 stellar association and is still embedded within the dark molecular cloud that it is forming from. It illuminates a small reflection nebula against the dark cloud. This young star has a substantial dust disk having a central cavity with a 40−46 AU radiusKelvin–Helmholtz mechanism
The Kelvin–Helmholtz mechanism is an astronomical process that occurs when the surface of a star or a planet cools. The cooling causes the pressure to drop, and the star or planet shrinks as a result. This compression, in turn, heats the core of the star/planet. This mechanism is evident on Jupiter and Saturn and on brown dwarfs whose central temperatures are not high enough to undergo nuclear fusion. It is estimated that Jupiter radiates more energy through this mechanism than it receives from the Sun, but Saturn might not. The latter process causes Jupiter to shrink at a rate of two centimetres each year.The mechanism was originally proposed by Kelvin and Helmholtz in the late nineteenth century to explain the source of energy of the Sun. By the mid-nineteenth century, conservation of energy had been accepted, and one consequence of this law of physics is that the Sun must have some energy source to continue to shine. Because nuclear reactions were unknown, the main candidate for the source of solar energy was gravitational contraction.
However, it soon was recognized by Sir Arthur Eddington and others that the total amount of energy available through this mechanism only allowed the Sun to shine for millions of years rather than the billions of years that the geological and biological evidence suggested for the age of the Earth. (Kelvin himself had argued that the Earth was millions, not billions, of years old.) The true source of the Sun's energy remained uncertain until the 1930s, when it was shown by Hans Bethe to be nuclear fusion.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.MWC 480
MWC 480 is a young star about twice the mass of the Sun located 455 light-years away in the Taurus star-forming region. The name refers to the Mount Wilson Catalog of B and A stars with bright hydrogen lines in their spectra.MWC 480 has X-ray emissions typical of a pre-main-sequence Herbig Ae/Be star but with an order of magnitude more photoelectric absorption. It has a gas-dust envelope and is surrounded by a protoplanetary disc. Astronomers using the ALMA (Atacama Large Millimeter/submillimeter Array) have found that the protoplanetary disc surrounding MWC 480 contains large amounts of methyl cyanide (CH3CN), a complex carbon-based molecule. Hydrogen cyanide (HCN) has also been detected in the disc. No signs of planet formation have yet been detected.Musca
Musca (Latin for "the fly") is a small constellation in the deep southern sky. It was one of 12 constellations created by Petrus Plancius from the observations of Pieter Dirkszoon Keyser and Frederick de Houtman, and it first appeared on a celestial globe 35 cm (14 in) in diameter published in 1597 (or 1598) in Amsterdam by Plancius and Jodocus Hondius. The first depiction of this constellation in a celestial atlas was in Johann Bayer's Uranometria of 1603. It was also known as Apis (Latin for "the bee") for 200 years. Musca remains below the horizon for most Northern Hemisphere observers.
Many of the constellation's brighter stars are members of the Scorpius–Centaurus Association, a loose group of hot blue-white stars that appears to share a common origin and motion across the Milky Way. These include Alpha, Beta, Gamma, Zeta2 and (probably) Eta Muscae, as well as HD 100546, a blue-white Herbig Ae/Be star that is surrounded by a complex debris disk containing a large planet or brown dwarf and possible protoplanet. Two further star systems have been found to have planets. The constellation also contains two cepheid variables visible to the naked eye. Theta Muscae is a triple star system, the brightest member of which is a Wolf–Rayet star.Pre-main-sequence star
A pre-main-sequence star (also known as a PMS star and PMS object) is a star in the stage when it has not yet reached the main sequence. Earlier in its life, the object is a protostar that grows by acquiring mass from its surrounding envelope of interstellar dust and gas. After the protostar blows away this envelope, it is optically visible, and appears on the stellar birthline in the Hertzsprung-Russell diagram. At this point, the star has acquired nearly all of its mass but has not yet started hydrogen burning (i.e. nuclear fusion of hydrogen). The star then contracts, its internal temperature rising until it begins hydrogen burning on the zero age main sequence. This period of contraction is the pre-main sequence stage. An observed PMS object can either be a T Tauri star, if it has fewer than 2 solar masses (M☉), or else a Herbig Ae/Be star, if it has 2 to 8 M☉. Yet more massive stars have no pre-main-sequence stage because they contract too quickly as protostars. By the time they become visible, the hydrogen in their centers is already fusing and they are main-sequence objects.
The energy source of PMS objects is gravitational contraction, as opposed to hydrogen burning in main-sequence stars. In the Hertzsprung–Russell diagram, pre-main-sequence stars with more than 0.5 M☉ first move vertically downward along Hayashi tracks, then leftward and horizontally along Henyey tracks, until they finally halt at the main sequence. Pre-main-sequence stars with less than 0.5 M☉ contract vertically along the Hayashi track for their entire evolution.
PMS stars can be differentiated empirically from main-sequence stars by using stellar spectra to measure their surface gravity. A PMS object has a larger radius than a main-sequence star with the same stellar mass and thus has a lower surface gravity. Although they are optically visible, PMS objects are rare relative to those on the main sequence, because their contraction lasts for only 1 percent of the time required for hydrogen fusion. During the early portion of the PMS stage, most stars have circumstellar disks, which are the sites of planet formation.Protoplanetary disk
A protoplanetary disk is a rotating circumstellar disk of dense gas and dust surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star. The protoplanetary disk may also be considered an accretion disk for the star itself, because gases or other material may be falling from the inner edge of the disk onto the surface of the star. This process should not be confused with the accretion process thought to build up the planets themselves. Externally illuminated photo-evaporating protoplanetary disks are called proplyds.
In July 2018, the first confirmed image of such a disk, containing a nascent exoplanet, named PDS 70b, was reported.Protostar
A protostar is a very young star that is still gathering mass from its parent molecular cloud. The protostellar phase is the earliest one in the process of stellar evolution. For a low mass star (i.e. that of the Sun or lower), it lasts about 500,000 years The phase begins when a molecular cloud fragment first collapses under the force of self-gravity and an opaque, pressure supported core forms inside the collapsing fragment. It ends when the infalling gas is depleted, leaving a pre-main-sequence star, which contracts to later become a main-sequence star at the onset of Hydrogen fusion.V380 Orionis
V380 Ori is a young multiple star system located near the Orion Nebula in the constellation Orion, thought to be somewhere between 1 and 3 million years old. It lies at the centre of NGC 1999 and is the primary source lighting up this and other nebulae in the region.W75N(B)-VLA2
W75N(B)-VLA2 is a massive protostar some 4,200 light-years from Earth, about 8 times more massive and 300 times brighter than our Sun, observed in 1996 and 2014 by the Karl G. Jansky Very Large Array (VLA). In 2014 its stellar wind had changed from a compact spherical form to a larger thermal, ionized elliptical one outlining collimated motion, giving critical insight into the very early stages of the formation of a massive star. Being able to observe its rapid growth as it happens (in real time in an astronomical context) is unique, according to Huib van Langevelde of Leiden University, one of the authors of a study of the object.
The authors of the study believe W75N(B)-VLA2 is forming in a dense, gaseous environment, surrounded by a dusty torus. The star intermittently ejects a hot, ionized wind for several years. Initially the wind can expand in all directions and forms a spherical shell; later it hits the dusty torus, which slows it. There is less resistance along the poles of the torus, so the wind moves more quickly there, giving rise to an elongated shape.Young stellar object
Young stellar object (YSO) denotes a star in its early stage of evolution. This class consists of two groups of objects: protostars and pre-main-sequence stars.Z Canis Majoris
Z Canis Majoris (Z CMa) is a B-type star in the constellation of Canis Major. It has an average apparent visual magnitude of approximately 9.85, though has brightened by 1-2 magnitudes in irregular outbursts in 1987, 2000, 2004 and 2008.The star is a complex binary system only 300,000 years old with two main components separated by an estimated 100 astronomical units (AU) or 0.1" as seen from Earth. The southeast component is an FU Orionis star (a type of pre-main-sequence star in a phase of very high mass accretion resulting in an accretion disk which dominates the optical spectrum) that is 1300 times as luminous as the Sun, has 3 times its mass and 13 times its diameter and a surface temperature of 10,000 K. The northwest component is a Herbig Ae/Be star that has been calculated to be 12 times as massive as the Sun with 1690 times its diameter, and shining with 2400 times its luminosity, though there is some uncertainty about its properties. It is enveloped in an irregular roughly spherical cocoon of dust that has an inner diameter of 20 and outer diameter of 50 AU. The cocoon has a hole in it through which light shines that covers an angle of 5 to 10 degrees of its circumference. Both stars are surrounded by a large envelope of in-falling material that left over from the original cloud that formed the system. Both stars are emitting jets of material, that of the Herbig Ae/Be star being much larger - up to 11.7 light-years (3.6 parsecs) long.It is unclear whether the most recent (and brightest) brightening in 2008 was due to the Herbig Ae/Be star increasing in luminosity or a hole appearing in the cocoon.
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