T Tauri star

T Tauri stars (TTS) are a class of variable stars associated with youth. They are less than about ten million years old.[1] This class is named after the prototype, T Tauri, a young star in the Taurus star-forming region. They are found near molecular clouds and identified by their optical variability and strong chromospheric lines. T Tauri stars are pre-main-sequence stars in the process of contracting to the main sequence along the Hayashi track, a luminosity–temperature relationship obeyed by infant stars of less than 3 solar masses (M) in the pre-main-sequence phase of stellar evolution. It ends when a star of 0.5 M develops a radiative zone, or when a larger star commences nuclear fusion on the main sequence.

TTauriStarDrawing
Artist's impression of a T Tauri star with a circumstellar accretion disc

History

While T Tauri itself was discovered in 1852, the T Tauri class of stars were initially defined by Alfred Harrison Joy in 1945.[2]

Characteristics

T Tauri stars comprise the youngest visible F, G, K and M spectral type stars (<2 M). Their surface temperatures are similar to those of main-sequence stars of the same mass, but they are significantly more luminous because their radii are larger. Their central temperatures are too low for hydrogen fusion. Instead, they are powered by gravitational energy released as the stars contract, while moving towards the main sequence, which they reach after about 100 million years. They typically rotate with a period between one and twelve days, compared to a month for the Sun, and are very active and variable.

There is evidence of large areas of starspot coverage, and they have intense and variable X-ray and radio emissions (approximately 1000 times that of the Sun). Many have extremely powerful stellar winds; some eject gas in high-velocity bipolar jets. Another source of brightness variability are clumps (protoplanets and planetesimals) in the disk surrounding T Tauri stars.

Hs-2000-32-a-animated gif
The ejection of a bubble of hot gas from XZ Tauri, a binary system of T Tauri stars. The scale is much larger than that of the Solar System.

Their spectra show a higher lithium abundance than the Sun and other main-sequence stars because lithium is destroyed at temperatures above 2,500,000 K. From a study of lithium abundances in 53 T Tauri stars, it has been found that lithium depletion varies strongly with size, suggesting that "lithium burning" by the P-P chain, during the last highly convective and unstable stages during the later pre–main sequence phase of the Hayashi contraction may be one of the main sources of energy for T Tauri stars. Rapid rotation tends to improve mixing and increase the transport of lithium into deeper layers where it is destroyed. T Tauri stars generally increase their rotation rates as they age, through contraction and spin-up, as they conserve angular momentum. This causes an increased rate of lithium loss with age. Lithium burning will also increase with higher temperatures and mass, and will last for at most a little over 100 million years.

The P-P chain for Lithium burning is as follows


p
 
6
3
Li
 
→  7
4
Be
 
7
4
Be
 

e
 
→  7
3
Li
 

ν

p
 
7
3
Li
 
→  8
4
Be
 
  (unstable)
    8
4
Be
 
→  4
2
He
 
+ energy

It will not occur in stars with less than sixty times the mass of Jupiter (MJ). In this way, the rate of lithium depletion can be used to calculate the age of the star.

Types

Several types of TTSs exist:[3]

  • Classical T Tauri star (CTTS)
  • Weak-line T Tauri star (WTTS)
    • Naked T Tauri star (NTTS), which is a subset of WTTS.

Roughly half of T Tauri stars have circumstellar disks, which in this case are called protoplanetary discs because they are probably the progenitors of planetary systems like the Solar System. Circumstellar discs are estimated to dissipate on timescales of up to 10 million years. Most T Tauri stars are in binary star systems. In various stages of their life, they are called Young Stellar Objects (YSOs). It is thought that the active magnetic fields and strong solar wind of Alfvén waves of T Tauri stars are one means by which angular momentum gets transferred from the star to the protoplanetary disc. A T Tauri stage for the Solar System would be one means by which the angular momentum of the contracting Sun was transferred to the protoplanetary disc and hence, eventually to the planets.

Analogs of T Tauri stars in the higher mass range (2–8 solar masses)—A and B spectral type pre–main-sequence stars, are called Herbig Ae/Be-type stars. More massive (>8 solar masses) 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

Planets around T Tauri stars include:

See also

References

  1. ^ Appenzeller, I; Mundt, R (1989). "T Tauri stars". The Astronomy and Astrophysics Review. 1 (3–4): 291. Bibcode:1989A&ARv...1..291A. doi:10.1007/BF00873081.
  2. ^ Joy, Alfred H. (1945). "T Tauri Variable Stars". The Astrophysical Journal. 102: 168–195. Bibcode:1945ApJ...102..168J. doi:10.1086/144749.
  3. ^ Scott J. Wolk (1996). "T Tauri Stars, Naked and Otherwise". Retrieved 2018-03-14.
BM Andromedae

BM Andromedae (BM And) is a T Tauri star in the constellation Andromeda. Its apparent visual magnitude has irregular variations between a maximum of 11.63 and a minimum of 14.02.

CT Chamaeleontis

CT Chamaeleontis (CT Cha) is a T Tauri star in the constellation of Chamaeleon. It has an apparent visual magnitude which varies between 12.31 and 12.43.In 2006 and 2007, a faint companion was observed 2.7 arcseconds away from CT Chamaeleontis, using the Very Large Telescope at the European Southern Observatory. Since the object shares common proper motion with CT Chamaleontis, it is believed to be physically close to the star, with a projected separation of approximately 440 astronomical units. It is estimated to have a mass of approximately 17 Jupiter masses and is probably a brown dwarf or a planet. The companion has been designated CT Chamaeleontis b.

CVSO 30

CVSO 30 (PTFO 8-8695) is a T Tauri star, located in constellation Orion at 1200 light years from Earth away with two candidate planets (CVSO 30 b and CVSO 30 c). Both candidate planets are gas giants. It is the first star around which potential planets have been found both by the transit method and by direct imaging.

CVSO 30 b is calculated to have a period of 10.76 hours (0.008 AU) and CVSO 30 c a period of 27,000 years (660 AU). It is a hot Jupiter candidate planet orbiting the T Tauri star, with 6.2 Jupiter masses.Direct imaging of CVSO 30 c, with 4.7 Jupiter masses, has been achieved through photometric and spectroscopic high contrast observations carried out with the Very Large Telescope located in Chile, the Keck Observatory in Hawaii and the Calar Alto Observatory in Spain.

CoKu Tau/4

CoKu Tau/4 is a pre-main-sequence binary T Tauri star system in the constellation Taurus. The stars are surrounded by a circumbinary disc with a central cavity of radius 10 astronomical units. Before its binary nature was known, the central cavity in the system's disc was thought to have been cleared out by a planet of at least 10 Jupiter masses, a rare example of a so-called "transitional disc". This model was disproven in 2008 when the star was resolved using adaptive optics as a system of two near-equal-mass stars with a projected separation of 8 AU. The central cavity is thus cleared out by the stars, not by the gravitational influence of a planet.

DG Tauri B

DG Tauri B, a faint companion of the T Tauri star DG Tauri, is a young stellar object located 450 light-years (140 parsecs) from Earth, within the Taurus constellation. Observations of DG Tauri B were first made in October, and later December 1995 at the 6 element Owens Valley millimeter wave array. Its most notable characteristics are its bipolar jets of molecular gas and dust emanating from either side of the object. Red-shifted carbon monoxide emissions extend out 6,000 au to the northwest of the object from the undetermined source, and are symmetrically distributed about the jet, while blue-shifted CO emissions are confined to a region with a roughly 500 au radius.

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.

Glossary of astronomy

This glossary of astronomy is a list of definitions of terms and concepts relevant to astronomy and cosmology, their sub-disciplines, and related fields. Astronomy is concerned with the study of celestial objects and phenomena that originate outside the atmosphere of Earth. The field of astronomy features an extensive vocabulary and a significant amount of jargon.

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.

HL Tauri

HL Tauri (abbreviated HL Tau) is a very young T Tauri star in the constellation Taurus, approximately 450 light-years (140 pc) from Earth in the Taurus Molecular Cloud. The luminosity and effective temperature of HL Tauri imply that its age is less than 100,000 years. At apparent magnitude 15.1, it is too faint to be seen with the unaided eye. It is surrounded by a protoplanetary disk marked by dark bands visible in submillimeter radiation that may indicate a number of planets in the process of formation. It is accompanied by the Herbig–Haro object HH 150, a jet of gas emitted along the rotational axis of the disk that is colliding with nearby interstellar dust and gas.

KH 15D

KH 15D (V582 Monocerotis), described as a winking star because of its unusual dips in brightness, is a binary T Tauri star system embedded in a circumbinary disk. It is a member of the young open cluster NGC 2264, located about 2,500 light-years (770 pc) from the Sun in the constellation of Monoceros.

LkCa 15

LkCa 15 is a T Tauri star in the Taurus-Auriga Star Forming Region. These types of stars are relatively young pre-main-sequence stars that show irregular variations in brightness. It has a mass that is about 97% of the Sun, an effective temperature of 4370 K, slightly cooler than the Sun. Its apparent magnitude is 11.91, meaning it is not visible to the naked eye.

LkCa 15 is surrounded by a protoplanetary disk, typical of many T Tauri stars. The disk around the star is about 55 times more massive than Jupiter. Small changes in the observed brightness of the disk may be due to a planetary companion; the star likely has a protoplanetary object or exoplanet orbiting it, known as LkCa 15 b This name stems from an older survey.

Orion variable

An Orion variable is a variable star which exhibits irregular and eruptive variations in its luminosity and is typically associated with diffuse nebulae. It is thought that these are young stars which will later become regular, non-variable stars on the zero-age main sequence. Brightness fluctuations can be as much as several magnitudes.

PDS 70

PDS 70 (V1032 Centauri) is a low-mass T Tauri star in the constellation Centaurus. Located approximately 370 light-years from Earth, it has a mass of 0.82 M☉, and is approximately 10 million years old. The star has a protoplanetary disk containing two nascent exoplanets, named PDS 70b and PDS 70c, which have been directly imaged by the European Southern Observatory's Very Large Telescope. PDS 70b was the first confirmed protoplanet to be directly imaged.

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.

TW Hydrae association

The TW Hydrae association is a group of very young low-mass stars and substellar objects located approximately 25–75 parsecs (80–240 light years) from Earth. They share a common motion and appear to all be roughly the same age, 10±3 million years old. It is the youngest such association within 100 pc from Earth.As of 2017, 42 objects (in 23 systems) are assigned to the association confidently, and several dozens — uncertainly. Masses of its known members vary from 5 Jupiter masses to 2 solar masses, and their spectral types vary from A0 to L7.Some of the best studied members of this stellar association are TW Hydrae (nearest known accreting T Tauri star to the Earth), HR 4796 (an A-type star with resolved dusty debris disk; the most massive known group member), HD 98800 (a quadruple star system with debris disk), and 2M1207 (accreting brown dwarf with remarkable planetary-mass companion 2M1207b).

Included in the association is WISEA 1147, which is a brown dwarf.

TW Hydrae b

TW Hydrae b is a likely extrasolar planet orbiting the young T Tauri star TW Hydrae approximately 176 light-years (54 parsecs, or nearly 1.665×1016 km) away in the constellation of Hydra. It is likely a Neptune-like planet orbiting at a distance of nearly 22 AU from its star.

V830 Tauri

V830 Tauri is a star located 427 light-years (or 131 parsecs) away from the Sun in the constellation Taurus. This star is very young, with an age of only 2 million years, compared to the Sun's age, which is 4.6 billion years. The star has an exoplanet orbiting around it.

V830 Tauri b

V830 Tauri b is an exoplanet orbiting around the young T Tauri star V830 Tauri, about 427 light-years (131 pc) away from Earth in the constellation of Taurus. The exoplanet has a young age of only about 2 million years.

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