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.[1][2][3][4] 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.

See also

References

  1. ^ Richard B. Larson (10 September 2003). "The physics of star formation" (PDF). Reports on Progress in Physics. 66 (10): 1669–73. arXiv:astro-ph/0306595. Bibcode:2003RPPh...66.1651L. doi:10.1088/0034-4885/66/10/r03.
  2. ^ Neil F. Comins; William J. Kaufmann III (2011). Discovering the Universe. p. 350. ISBN 978-1429255202.
  3. ^ Derek Ward-Thompson; Anthony P. Whitworth (2011). An Introduction to Star Formation. Cambridge University Press. p. 119. ISBN 978-1107627468.
  4. ^ Stahler, S. W. & Palla, F. (2004). The Formation of Stars. Weinheim: Wiley-VCH. ISBN 3-527-40559-3.
1RXS J160929.1−210524

1RXS J160929.1−210524 (also known as GSC 6213-1358 or PZ99 J160930.3−210459) is a pre-main-sequence star approximately 456 light-years away in the constellation of Scorpius.

The star was identified as a member of the Upper Scorpius subgroup of the Scorpius–Centaurus Association by Thomas Preibisch and coauthors in 1998, and originally assigned an age of 5 million years old based on its group membership.

A more recent analysis of the ages of the stars in the Upper Scorpius group pegs its average age at 11 million years.

1 Boötis

1 Boötis (1 Boo) is a binary star system in the northern constellation of Boötes, located 318 light years away from the Sun. It is visible to the naked eye as a dim, white-hued star with a combined apparent visual magnitude of 5.71. The pair had an angular separation of 4.660″ as of 2008. It is moving closer to the Earth with a heliocentric radial velocity of −26 km/s.The magnitude 5.78 primary component is an A-type main-sequence star with a stellar classification of A1 V. This star has 2.5 times the mass of the Sun and is radiating 56 times the Sun's luminosity from its photosphere at an effective temperature of 9,863 K. It is 323 million years old and is spinning with a projected rotational velocity of 60 km/s.The system is a source for X-ray emission, which is most likely coming from the companion star. This magnitude 9.60 component is a possible pre-main sequence star with a mass similar to the Sun. It is radiating 76% of the Sun's luminosity at an effective temperature of 6,370 K.

AP Columbae

AP Columbae also known as AP Col is a pre-main-sequence star in the constellation of Columba, which has been studied for the last 15 years, but was recently discovered to be very young and close to Earth. It has been recognized as the closest young star to the Earth.

EPIC 204278916

EPIC 204278916 is a pre-main-sequence star, about five million years old with a spectral type of M1, implying a red dwarf. It is part of the Upper Scorpius sub-group of the Scorpius–Centaurus Association, and is in the constellation Scorpius. The star is approximately the size of the Sun at 0.97 R☉, but is only half its mass (0.50 M☉) and a fraction of its luminosity (0.15 L☉).This stellar object was first characterized by the 2nd USNO CCD Astrograph Catalog and the Two Micron All-Sky Survey, and was further studied during the Kepler space telescope's extended K2 mission Campaign 2 between 23 August and 13 November 2014.

FU 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 207129

HD 207129 is a G-type pre-main-sequence star in the constellation of Grus. It has an apparent visual magnitude of approximately 5.58. This is a Sun-like star with the same stellar classification G2V and a similar mass. It is roughly the same age as the Sun, but has a lower abundance of elements other than hydrogen and helium; what astronomers term the star's metallicity.A debris disk has been imaged around this star in visible light using the ACS instrument on the Hubble Space Telescope; it has also been imaged in the infrared (70 μm) using the MIPS instrument on the Spitzer Space Telescope. Based on the ACS image, the disk appears to have a radius of about 163 astronomical units and to be about 30 AU wide, and to be inclined at 60° to the plane of the sky.Another star, CCDM J21483-4718B (also designated CD−47 13929 or WDS J21483-4718B), of apparent visual magnitude 8.7, has been observed 55 arcseconds away from this star, but based on comparison of proper motions, it is believed to be an optical double and not physically related to its companion.

HD 35984

HD 35984 is star in the northern constellation Auriga. It has an apparent magnitude of 6.20, which, according to the Bortle scale, indicates it is faintly visible to the naked eye from dark rural skies. Parallax measurements by the Hipparcos satellite indicates it lies at a distance of roughly 290 light years away.A stellar classification of F6III suggests that this is an evolved giant star that has consumed the supply of hydrogen at its core. However, X-ray emission, variations in luminosity, and levels of lithium may indicate that this is instead a weak-lined T Tauri star. That is, a low mass pre-main sequence star that is relatively poor in circumstellar matter.

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.

HD 95086 b

HD 95086 b is a confirmed, directly imaged exoplanet orbiting the young, 17 Myr A-class pre-main-sequence star HD 95086. It is roughly 5 times as massive as Jupiter and orbits 56–61 AU away from the parent star. It was detected at thermal infrared wavelengths (3.8 µm) through direct imaging, using the NACO instrument on the VLT. A debris disk has been detected in this system at submillimeter wavelengths and has been resolved in the far-infrared from data obtained with the Herschel Space Observatory.The planet was initially detected in data taken in 2012 at a separation from the parent star of 623.9 ± 7.4 mas (~56 AU) and position angle of 151.8 ± 0.8 degrees. The planet was recovered at a high signal-to-noise ratio in June 2013. Astrometry for the planet in January 2012 and June 2013, and from a marginal detection in March 2013 confirm that it is bound to the parent star, not a background star. HD 95086 b's brightness at 3.8 µm when combined with sensitive upper limits on its brightness at shorter wavelengths is consistent with trends seen for other young, directly imaged planets like those around HR 8799. In particular, the lower limit of its 1.65–3.8 µm color of 3.1 magnitudes excludes background stars and most brown dwarfs.

The host star is a highly probable member of the Lower Centaurus Crux star forming region. The star has a mass of 1.6 solar masses making it a late A type star. It is located approximately 90 parsecs away in the constellation of Carina.HD-95086b is a massive planet that exists at a large orbital radius, therefore it is a viable candidate for a planet that formed close to its host star and was later scattered outwards via gravitational interactions with other massive particles in orbit around the star. In a debris disk, these particles are of the form of other orbiting planets or planetesimals. This scattering mechanism has been investigated for particles of the form of over-dense fluid elements like those found in a protoplanetary disk containing a planet. It has been proposed that scattering events could explain observed massive planets at large orbital radii.

Spatially resolved images of the system from the Herschel Space Observatory show evidence for a possible 2 belt system with a large clearing between the belts, similar to HR 8799. HD-95086b is probably responsible for carving the sharp inner edge of the outer disk, but additional planets in this system occupying the rest of the gap may be a strong possibility.

Henyey track

The Henyey track is a path taken by pre-main-sequence stars with masses >0.5 Solar mass in the Hertzsprung–Russell diagram after the end of Hayashi track. The astronomer Louis G. Henyey and his colleagues in the 1950s, showed that the pre-main-sequence star can remain in radiative equilibrium throughout some period of its contraction to the main sequence.

The Henyey track is characterized by a slow collapse in near hydrostatic equilibrium. They are approaching the main sequence almost horizontally in the Hertzsprung–Russell diagram (i.e. the luminosity remains almost constant).

Herbig Ae/Be star

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:

Spectral type earlier than F0 (in order to exclude T Tauri stars),

Balmer emission lines in the stellar spectrum (in order to be similar to T Tauri stars),

Projected location within the boundaries of a dark interstellar cloud (in order to select really young stars near their birthplaces),

Illumination of a nearby bright reflection nebula (in order to guarantee physical link with star formation region).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:

Spectral type earlier than F0,

Balmer emission lines in the stellar spectrum,

Infrared radiation excess (in comparison with normal stars) due to circumstellar dust (in order to distinguish from classical Be stars, which have infrared excess due to free-free emission).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.

K2-33

K2-33 is an extremely young pre-main-sequence star located about 456 light-years (140 pc) away from the Earth in the constellation of Scorpius. It is known to host one planet, a super-Neptune, named K2-33b. It is also notable for its young age.

K2-33b

K2-33b (also known by its EPIC designation EPIC 205117205.01) is a very young super-Neptune exoplanet, orbiting the pre-main-sequence star K2-33. It was discovered by NASA's Kepler spacecraft on its "Second Light" mission. It is located about 456 light-years (140 parsecs) away from Earth in the constellation of Scorpius. The exoplanet was found by using the transit method, in which the dimming effect that a planet causes as it crosses in front of its star is measured. It is mostly notable for its extremely young age − a mere 9.3 million years old, only one other exoplanet is even younger with an age of 2 Myr (V830 Tau b).

NGC 6729

NGC 6729 is a reflection/emission nebula in the constellation Corona Australis. It was discovered by Johann Friedrich Julius Schmidt in 1861.This fan-shaped nebula opens from the star R Coronae Australis toward the star T CrA to the south-east. R CrA is a pre-main-sequence star in the Corona Australis molecular complex, one of the closer star-forming regions of the galaxy at a distance of 130 pc.

Nu Pavonis

Nu Pavonis (ν Pav) is a slowly pulsating B-type star of class B7III (blue giant) in the constellation Pavo. Its apparent magnitude varies from 4.60 to 4.64 over a period of 0.85584 days and it is approximately 439 light years away based on parallax.

It has one reported companion, probably a pre-main-sequence star, at magnitude 13.7 and separation 3.1", estimated at 0.15 solar masses and an effective temperature of 3,192 K.

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.

SU Aurigae

SU Aurigae is a T Tauri-type variable star in the constellation Auriga. It is located about 500 light-years (150 parsecs) away in the Taurus-Auriga Star Forming Region. Its apparent magnitude is 9.30, which is dim enough that it cannot be seen with the unaided eye.

SU Aurigae's spectral type of G2IIIne means that it is a G-type star with an effective temperature similar to the Sun. The III in the spectral type refers to its luminosity, which is much higher than normal G-type main sequence stars and would put it in the giant star class. However, it is only about 4 million years old, which is relatively young for a star - young protostars like SU Aurigae are luminous because they are larger, not condensing into a normal size until they are older.SU Aurigae is known to have a circumstellar protoplanetary disk surrounding it, which is typical of many T Tauri stars. SU Aurigae's disk has a high inclination of 62° and is nearly perpendicular to the plane of sky, so orbiting protoplanets or comets may be the cause of why there are drops in the amount of light detected. SU Aurigae's proper motion and distance is similar to AB Aurigae, a better known pre-main-sequence star, meaning that the two may form a very wide binary system; if not, they are still in the same star association.

V4046 Sagittarii

V4046 Sagittarii is a young binary consisting of two K-type main-sequence stars. The two stars are about 271 light-years (83 parsecs) away from the Earth. The two stars orbit each other every 2.42 days on a circular orbit.V4046 Sagittarii is surrounded by a massive protoplanetary disk. The disk has a radius of about 370 astronomical units (au) with about 40 Earth masses of dust in the disk. V4046 Sagittarii is one of four pre-main-sequence star systems within 100 parsecs with protoplanetary disks, the others being TW Hydrae, HD 141569, and 49 Ceti. The two stars are still accreting matter from the disk, and gas giant planets may be forming in the disk as well.The red dwarf binary GSC 07396-00759 is separated about 2.82″ from V4046 Sagittarii. Since it has a similar motion throughout space with V4046 Sagittarii, GSC 07396-00759 is assumed to be gravitationally bound (although weakly) to V4046 Sagittarii. The two systems are separated by at least 12,350 astronomical units (0.1953 ly) away, and the orbital period would be on the order of 100,000 years.

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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