T-Tauri stars ( TTS , named after the prototype of this star class , T Tauri , an irregular variable in a dark cloud of dust in the constellation of Taurus ) are young stars with an age of less than a million years, the spectral class F to M and a mass between 0.07 and 3 solar masses . They are located above the main sequence and therefore in an early phase of their development in which they are still contracting. Thermonuclear reactions have not yet taken place in their nuclei or have only recently taken place.
Such stars are not yet in hydrostatic equilibrium , which means that they tend to erupt more or less violently. Strong bipolar currents emerge from inside at a few hundred kilometers per second; Glowing nebulae, called Herbig-Haro objects , can be observed where these jets form shock fronts and heat the interstellar gas .
T-Tauri stars are usually found inside dense interstellar clouds next to young stars of spectral classes O and B. Despite their early developmental phase, they are more luminous than main sequence stars, which are at the same temperature. Their spectra sometimes show some strong emission lines that come from a thin gas envelope that surrounds these stars. In the Rho-Ophiuchi cloud in particular , a large number of these stars have been found due to their strong infrared emission . Local associations of T Tauri stars are known as T associations .
T-Tauri stars are often surrounded by a circumstellar disk , which are considered to be the precursors of planetary systems. From the inner area of this disk, gas flows form along the magnetic field lines of the star, through which matter flows from the disk onto the star ( accretion ). Near the star's surface, the incident matter almost reaches the speed of sound and is slowed down in a shock front, creating one or more hot spots with temperatures of up to a million Kelvin .
All T-Tauri stars show an infrared excess due to a circumstellar disk with dimensions of a few hundred astronomical units . The disk is created as a result of the angular momentum in the molecular cloud from which the star was formed. Due to the pirouette effect when the cloud contracts, the matter passes through a disk. In this, part of the angular momentum is reduced by friction, with the largest part of the angular momentum being transported away via a jet . In the course of development, the disc dissolves through:
- Accretion of matter onto the T-Tauri star
- Stellar winds
- Photoevaporation through radiation from the T-Tauri star or neighboring stars in the surrounding star formation area
- Dust formation
- Formation of exoplanets or brown dwarfs .
This creates zones with a low density of matter in the circumstellar disks : a central hole with an increasing diameter of a few astronomical units and rings in which exoplanets have accreted matter. After a few million years, a pure disk of dust remains, as in Wega and Beta Pictoris , which completely dissolves over time due to radiation pressure .
The search for exoplanets around T-Tauri stars is favored by the fact that a young planet with an age of a few million years has a larger radius than after the end of the contraction. But the pronounced photometric and spectroscopic variability of this class of stars increases the noise considerably.
It is possible that around the 7-10 million year old WTTS star 2MASS J05250755 + 0134243 in the Orion-OB1a / 25-Ori region a hot Jupiter with an orbital period of 0.44 days has been detected photometrically and spectroscopically. However, this exoplanet would orbit within the Roche boundary of the M3 star and would be destroyed by tidal forces within a short time .
Almost all T-Tauri stars show both a cyclical and an irregular variability of their light curves :
- the cyclical changes in brightness may come from some kind of occlusion light change caused by clumps of circumstellar material orbiting around the young star.
- a partially observed change of rotating light (is always cyclical), however, is associated with dark star spots on the surface of the rapidly rotating young T-Tauri stars.
- the irregular changes in brightness are a result of fluctuations in the accretion rate of the T-Tauri stars. The thermal radiation that is released when matter hits the surface of the star represents a considerable proportion of the total radiation budget.
- In addition, the magnetic activity of the T-Tauri stars can lead to (irregular) flares that can be observed in white light , comparable to solar flares and the outbursts of flare stars . The flares can also be detected in the X-ray range with an output of up to a few 10 32 ergs / second. Flares are also events with increased accretion that lead to an increase in optical and X-ray brightness.
- irregular brightness and spectral changes are associated with the Rayleigh-Taylor instability . This leads to a temporary formation of tongues from the inner edge of the accretion disk to the magnetic poles of the star. These tongues are likely to exist for only a fraction of a stellar period of rotation .
- like other young stellar objects, T-Tauri stars also exhibit variability in the mid- infrared , where most of the radiation should come as emission from the accretion disk. This variability appears to be non-periodic with amplitudes of up to 0.5 mag over a characteristic period of 10 days, and the amplitude seems to decrease with age. This variability is interpreted as changes in the accretion rate, structural changes in the inner disk, turbulence in the disk, or as changes in the thickness or density of the disk, which in each case changes the extinction of the central star.
Two classes of eruptive variable stars are closely related to the T-Tauri stars:
- the FU Orionis stars are T Tauri stars before their eruptions and develop into FG supergiants in the optical and red giants in the infrared during the outbreak . The eruptions last for several decades and are interpreted as a lighting up of the accretion disk similar to the dwarf nova eruptions .
- The EX Lupi stars ( EXors ) are also indistinguishable from other T Tauri stars before and after the eruption. They show a KM spectrum , the duration of the eruptions is on the order of months to years. During the resting phase, the accretion rate is 10 −7 solar masses per year. In the outbreak, the rate increases by a factor of 1000 and causes the pseudo photosphere to light up .
Closely related to the T-Tauri stars and the Herbig-Ae / Be stars is another group of variable young stars, the UX Orionis stars . These stars, also known as UXOR, show minima in their light curves with depths of up to 2.5 magnitudes with cycle lengths between 8 days and 11 years. The minima are caused by a variable darkening of the star by circumstellar material in the form of dust , planetesimals or clusters of cometary bodies. The depth and shape of the minima are subject to great changes from cycle to cycle. Surprisingly, the color index of the UX Orionis star minima appears to be bluer . This is attributed to unusual properties of the dust, a self-shadowing of parts of the accretion disc or a pent-up wall of matter in the inner area of the disc.
In addition, some T-Tauri stars have deep periodic minima with periods of more than 1 day, whereby the duration of the minima can be up to 2/3 of the period. This is interpreted as an eclipsing light change in a binary star system, with the companion being surrounded by a disk of dust. The fluctuations in the minimum duration and depth are a result of variable dust condensation in the disk and of precession effects through third bodies in the binary star system.
T-Tauri stars show one spectral class later than F.
Based on the optical spectrum, the T-Tauri stars are divided into
- classical T-Tauri stars ( cTTS ) with an equivalent width of the Hα line greater than about 10 Å
- Weak-line T-Tauri stars ( wTTS ; " emission-line weak" T-Tauri stars) with an equivalent width of the Hα line smaller than about 10 Å.
There is also the class of the naked T-Tauri stars ( nTTS ). With them is no accretion disk near infrared detectable. The inner part of the circumstellar disk has been cleared by accretion, planet formation , stellar wind , photodissociation or radiation pressure.
The spectra of the T-Tauri stars (both cTTS and wTTS) show a high abundance of lithium in their atmospheres compared to main sequence stars and young open star clusters like the Pleiades . This is interpreted as an indication of the low age of the T-Tauri stars, since lithium is destroyed by thermonuclear reactions at temperatures below hydrogen burning . Since the energy transport in T-Tauri stars is still almost entirely by convection , the lithium is almost completely destroyed in their first million years. The position of the T-Tauri stars in the Hertzsprung-Russell diagram also shows their early age, since they are placed between the Hayashi line and the main series.
T-Tauri stars show pronounced emission lines , as they can also be detected much weaker in the sun's chromosphere . These emission lines are an indication of strong magnetic activity, which could be directly detected by the Zeeman effect , and of the ionization of the inner edge of the accretion disk by the star. In addition, many T-Tauri stars show blue-shifted absorption lines directly at the emission lines . The dynamic flows of matter were derived from this property, which in the meantime could also be detected as bipolar outflows and jets by means of direct imaging.
A subclass of the T-Tauri stars, the YY-Orionis stars, show - in addition to outflowing gas - through red-shifted absorption lines also in the optical spectrum the incidence of matter on the young star.
The narrow depth and width of the absorption lines in their spectra are characteristic of classic T-Tauri stars . This phenomenon is referred to in English as veiling (" covering up "). The ceiling at the cTTS can be simulated by model spectra that assume accretion along the field lines of a magnetic field with a flux density of 1000 to 3000 Gauss . The gas is accelerated to a speed of up to 300 km / s by gravitational forces , and a shock wave forms above the surface of the star , in which the speed is decelerated by a factor of 4. The gas heats up to a temperature in the order of one million degrees and emits its thermal energy in the range of X-rays and UV radiation . About half of the energy released flows downwards and forms a hot spot on the star at the base of the accretion current. The veiling is a consequence of the continuum spectrum from the hot spot, which is superimposed on the spectrum from the quiet zones of the classic T-Tauri star.
The model of magnetospheric accretion was developed after observing magnetic fields in the order of a few thousand Gauss on the surface of T-Tauri stars using the Zeeman effect . The magnetic field penetrates the circumstellar disk and dominates the accretion of matter from the disk at a distance of a few star radii, the co-rotation radius. This flows along the magnetic field lines onto the poles of the magnetic field. The proven absorption and emission lines of the T-Tauri stars can be explained well by the model of magnetospheric accretion. The strong magnetic field of the T-Tauri stars is associated with the high speed of rotation of the young stars due to accretion. After the accretion disk has dissolved , the stellar magnetic field also decays within a few million years and only reaches values around a few Gauss.
The magnetic field is also the reason for the observed rotational speeds of the T-Tauri stars. If matter falls on a star with an accretion rate of 10 −7 solar masses per year for over a million years, the preservation of the angular momentum would result in a rotational speed close to the critical one (at which the star is no longer stable). The measured rotation speeds are only 10–20 percent of this value. The stellar magnetic field reduces the angular momentum in the cTTS via two mechanisms:
- Via a stellar wind that follows the stellar magnetic field lines and thus removes angular momentum.
- Via disk locking , in which the stellar magnetic field interacts with the ionized gas in the accretion disk.
Like other young stellar objects, T-Tauri stars show a 1,000 to 10,000-fold higher activity in the range of X-rays compared to main sequence stars . X-rays slowly decrease with age and, unlike main sequence stars, are not dependent on the speed of rotation. It is assumed that X-rays emanate from a magnetically enclosed plasma of the corona .
The intensity of the X-rays varies widely, and outbreaks are likely to be related to accretion of gas from the circumstellar disk. During these accretion events, shock waves form in the corona and heat up to several million Kelvin . The permanent X-rays of the T-Tauri stars, on the other hand, are associated with magnetic activity, as is the case with the sun. The latter, based on the magnetic activity, also occurs in the form of flares and is therefore also variable.
Some T Tauri stars in binary systems show a periodic modulation of the X-ray brightness, the period of the track encircling period corresponds to the binary stars. The intensity of the X-rays in the periastron increases significantly. This phenomenon is also known as pulsed accretion . An accretion disk has formed around each of the young stars, which is unbalanced in the periastron by the gravitational forces of the companion and leads to an increase in the accretion rate. In addition to the X-ray brightness, the intensity of the infrared brightness also increases .
In the case of the T-Tauri stars, three components of outflows that transfer matter to the interstellar medium are observed:
- a continuous wind from the surface of the accretion disk, the temperature of which is too low to break up molecules and which only reaches speeds of a few 10 km / s;
- an X-wind from the central hole of the accretion disc reaching speeds of a few 100 km / s;
- a stellar wind from the surface of the CTTS. In the case of some of the T-Tauri stars, this wind is collimated through an interaction with the X-wind to form a jet with an angle of only a few degrees.
The T-Tauri stage is not only observed in stars that have enough matter to ignite hydrogen burning . Even in brown dwarfs , signs of chromospheric activity have been demonstrated at an age of several million years, such as
- Star spots
- Infrared emission from dust discs
- pronounced Hα lines due to accretion
- Signs of dust formation
- Growth and bipolar outflows in the form of jets.
These young brown dwarfs rotate extremely slowly, which is interpreted as an indication of the formation of a global magnetic field. With these low-mass T-Tauri stars, the circumstellar disks only have a few millionths of the solar mass and are therefore several orders of magnitude smaller than with normal T-Tauri stars. This also applies to accretion rates of some 10 −12 to 10 −10 solar masses per year. The spectral class of the brown dwarfs in the T-Tauri stage is later than M6 and continues to decrease with age. No significant accretion is observed in brown dwarfs older than five million years.
- Knowledge portal for astrophysics. By Andreas Müller, astrophysicist
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