T Tauri type stars

T Tauri stars (TTS ) are young stars at an age of less than a million years that hold the F to M spectral type and a mass 0.07 to 3 solar masses. They are located above the main sequence and thus in an early phase of development in which they are still contracting. In their cores no or only recently thermonuclear reactions take place. Such a star is not in hydrostatic equilibrium, which makes them tend to be more or less violent outbursts. Strong bipolar flows occur from a few hundred kilometers per second from its interior; where these jets form shock fronts and heat the interstellar gas, glowing nebula, known as Herbig - Haro objects can be observed.

T Tauri stars are found usually in the interior of dense interstellar clouds next to young stars of spectral types O and B. Despite their early stages of development are T Tauri stars more luminous than main sequence stars, which have the same temperature. Their spectra have sometimes some strong emission lines derived from a thin shell of gas, which is located around these stars around. In particular, in the Rho Ophiuchi cloud of dust was found due to their strong infrared emission, a large number of these stars. Local associations of T Tauri stars are referred to as T associations. Often, these stars are surrounded by a circumstellar disk, which are regarded as precursors of planetary systems.

From the inner region of the circumstellar disk formed along the magnetic field lines of the star gas streams flowing through the matter from the disk onto the star. Near the stellar surface reaches the incident matter nearly the speed of sound and is decelerated in a shock front, whereby one or more hot spots with temperatures of up to one million K arise.

The prototype of this class of stars, T Tauri, an irregular variables in a dark cloud of dust in the constellation Taurus.

Circumstellar disks

All T Tauri stars show an infrared excess due to a circumstellar disk with dimensions of a few hundred astronomical units. The disc is produced as a consequence of the angular momentum in the molecular cloud from which the star formed. Due to the spins in the contraction effect of the molecule through a cloud of the material plate and in that a part of the angular momentum is dissipated by friction, with the largest part of the angular momentum is carried away by a jet. During the development of the disc dissolves by

• Accretion of matter onto the T Tauri star
• Stellar winds
• Photoevaporation by radiation of the T Tauri star or nearby stars in the starburst region
• Dust formation
• Emergence of exoplanets or brown dwarfs in the circumstellar disk

This results in the circumstellar disks zones with low matter density. It is a central hole with a diameter of a few astronomical units growing and rings in which exoplanets have accreted matter. After a few million years ago, a pure dust disk remains as Vega and Beta Pictoris, which completely dissolves over time by radiation pressure.

The search for exoplanets around T Tauri stars is indeed favored by the fact that a young planet with an age of a few million years has a larger radius than after the completion of the contraction. But the pronounced photometric and spectroscopic variability of this star class increases the noise significantly. Maybe a hot Jupiter mt an orbital period of 0.44 days is photometrically around 7-10 million years old WTTS 2MASS J05250755 0134243 star in the Orion-OB1a/25-Ori region as has also been detected spectroscopically. However, the exoplanet would circulate within the Roche limit of the M3 - star and will be destroyed within a short time by tidal forces.

Variability

Almost all T Tauri stars show both a regular and an irregular variability in their light curves. The irregular brightness changes are a consequence of variations in the accretion rate of the pre-main sequence. The energy released by the accretion of matter onto the surface of the T Tauri star thermal radiation represents a significant share of the radiation budget. The cyclical component may be the result of a cycle of clumps of circumstellar material around the young star, which leads to a kind of cover change of light. A partially observed rotational light variation is, however, brought with dark star spots on the surface of the rapidly rotating young stars in conjunction. In addition, the magnetic activity of T Tauri stars also in the white light flares observed comparable lead to solar flares and eruptions of flare stars. The flares can also be detected in the field of X-ray radiation with a power of up to several 1032 erg / sec. As flares also events with increased accretion are called, which lead to an increase in the optical and X-ray brightness. Irregular brightness and spectral changes are associated with the Rayleigh-Taylor instability. This leads to a temporary training of tongues from the inner edge of the accretion disk to the magnetic poles of the star. These tongues are made probably only for a fraction of a stellar rotation period and lead to irregular brightness changes in their light curves.

T Tauri stars as other young stellar objects show also in the mid-infrared variability, the amplitude of the changes in brightness appears to decrease with age. In the mid-infrared radiation than most of the emission from the accretion disc should originate. The variability seems aperiodic may be carried in a characteristic period of 10 days with amplitudes of up to 0.5. These brightness changes are interpreted as changes in the accretion, structural changes in the inner pane, changes in the height of the disc, turbulence in the disc or variable absorption of the central star due to density variations in the disc.

Two classes of eruptive variable stars, FU Orionis stars and the EX Lupi star are closely related to the T Tauri stars. The FU Orionis stars are T Tauri stars before their outbreaks and develop during the outbreak to FG supergiants in the optical and in the infrared that of a Red Riesens. The outbreaks are ongoing for several decades and are interpreted as an illumination of the accretion disk similar to the dwarf nova eruptions. The EX Lupi star before and after the eruption indistinguishable from other T Tauri stars. They show a KM range, and the duration of the flares is of the order of months to years. During the resting phase, the accretion rate is 10-7 solar masses per year. Outbreak in the rate increases by a factor of 1000 and lead to the lighting up of the pseudo photosphere.

Closely related to the T Tauri stars and the Herbig-Ae/Be-Sternen are another group of variable young stars UX Orionis stars. The Uxor also called star show minima with depths of up to 2.5 mag in cycles lengths between 8 days and 11 years. The minima are caused by variable obscuration of the star by zirkumstellares material in the form of dust, planetesimals or cometary pile of bodies. The depth and shape of the minima is subjected to big changes from cycle to cycle. Surprisingly appears to be blue in the minima of UX Ori stars, the color index. This is interpreted to unusual properties of the dust, a self-shadowing of parts of the accretion disk or a pent-up wall made ​​of matter in the inner region of the disk. Besides are some T Tauri stars deep periodic minima with periods of longer than 10 days, the duration of the minima can be up to 2/3 of the period. This is interpreted as a light cover change in a binary system, the companion is surrounded by a disk of dust. The fluctuations in the Minimadauer and depth are a result of varying dust condensations in the disk and Präzessionseffekten third-body in a double star system.

Spectrum

T Tauri stars show a spectral type later than F. Furthermore, a high proportion of lithium characteristic for these young stars. The element lithium undergoes at temperatures below the limit for hydrogen burning thermonuclear reactions. Since the energy transport still takes place almost entirely in T Tauri stars by convection lithium is barely detected in the main sequence stage. This is an indication of the youth of the T Tauri stars.

Based on the optical spectrum of the T Tauri stars are divided into

• Classical T Tauri stars ( CTTS ) and
• Weak -line T Tauri stars ( "emission line weak" T Tauri stars, WTTS ).

In the former, the equivalent width of the Hα line is greater than about 10 Å, while the equivalent width lies with the weak- line T Tauri stars below this value. There is also the class of the naked T Tauri stars ( NTTS ). For them, no accretion disk in the near infrared can be detected. The inner part of the circumstellar disk has been cleared it by accretion, planet formation, stellar wind, photo-dissociation and radiation pressure.

The spectra of T Tauri stars show a high abundance of lithium in their atmospheres compared to main-sequence stars and young open clusters like the Pleiades. This observation applies to both CTTS and WTTS for. This is interpreted as an indication of the youth of the T Tauri stars, as lithium is destroyed by thermonuclear reactions at temperatures below the hydrogen burning. Since these stars are fully convective, in their first million years, the lithium is almost completely destroyed. The position of the T Tauri stars in the Hertzsprung -Russell diagram showing her young age, because the stars are placed between the Hayashi - line and the main sequence.

T Tauri stars show strong emission lines, as they can also be much weaker detected in the chromosphere of the sun. These emission lines are an indication of strong magnetic activity that could be detected directly by the Zeeman effect, and for the ionization of the inner edge of the accretion disk by the star. In addition, many T Tauri stars show directly to the emission lines blue-shifted absorption lines. From this property, the dynamic material flows have been derived, which were also detected by direct imaging as bipolar outflows and jets now. A subclass of T Tauri stars, the YY Orionis stars, show next outflowing gas by red-shifted absorption lines in the optical spectrum of the accretion of matter on the young star.

Characteristic of classical T Tauri stars is the low depth and width of the absorption lines in their spectra. This phenomenon is known in English as veiling ( " concealment "). The Veiling in the CTTS can be simulated by model spectra which assume a accretion along the field lines of a magnetic field with a flux density of 1000 to 3000 gauss. In this case, the gas is accelerated by the force of gravity to a speed of up to 300 km / s, and above the surface of the star, a shock wave is formed, in which the speed is slowed down by a factor of 4. In this case, the gas is heated to a temperature of the order of a million degrees and radiates its thermal energy in the area of ​​X-rays and UV radiation. Approximately half of the released energy flows down and forms on the star at the base of the accretion stream a hot spot. The Veiling is a consequence of the continuum spectrum of the hot spot, which is superimposed on the spectrum of the quiet zones of the classical T Tauri star.

Magnetic fields

The model of magnetospheric accretion was developed after the observation of magnetic fields on the order of a few thousand gauss at the surface of T Tauri stars based on the Zeeman effect. The magnetic field penetrates the circumstellar disk and dominates at a distance of some star radii, the co- rotational radius, the accretion of the material from the disc. This then flows along the magnetic field lines at the poles of the magnetic field. The detected absorption and emission lines of T Tauri stars can be well explained by the model of magnetospheric accretion. The strong magnetic field of T Tauri stars is associated with the high-speed rotation of the young stars in conjunction due to the accretion. After dissolution of the accretion disk and the stellar magnetic field decays within a few million years, and only reaches values ​​by a few Gauss.

The magnetic field is also the reason for the observed rotational velocities of T Tauri stars. If matter with an accretion rate of 10-7 solar masses per year over a million years a on a star, a rotational speed would consist of maintaining the torque near the critical ( where the star is no longer stable ) result. The measured rotational speeds are only 10-20 percent of this value. The stellar magnetic field provides two mechanisms for a braking torque at the CTTS:

• About a stellar wind that follows the stellar magnetic field lines and thus transported torque.
• About the Disk -Locking, in which the stellar magnetic field with the ionized gas in the Akkretionsakkretiionsscheibe interacts.

X-rays from T Tauri stars

Like other young stellar objects show T Tauri stars compared to main-sequence stars, a 1,000 to 10,000 - fold higher activity in the field of X-ray radiation. The X-ray radiation increases with age slowly and is not dependent on the speed of rotation of the young stars in contrast to main sequence stars. It is believed that X-ray radiation emanating from a magnetically confined plasma of the corona. The X-ray intensity is highly variable and outbreaks are likely associated with the accretion of gas from the circumstellar disk. These Akkretionsereignissen to form shock waves from the corona and heat up to several million Kelvin. The continuous X-ray radiation of the T Tauri stars is brought against it as in the sun with the magnetic activity in connection. The latter on the magnetic activity based X-rays, however, also occurs in the form of flares and is therefore also variable.

Some T Tauri stars in binary systems show a periodic modulation in the X-ray brightness, the period of the orbital motion duration of the double star equivalent. The intensity of the X-ray radiation in Periastron significantly increases. This phenomenon is also referred to as pulsed accretion. To each of the young star has formed an accretion disk that is placed in the periastron by the gravitational forces of the companion from the equilibrium and leads to an increase in the accretion rate. In addition to the X-ray intensity of the infrared brightness of the brightness increases.

Stellar winds

The T Tauri stars are three components of effluents, transfer the matter to the interstellar medium, observed:

• A continuous wind from the surface of the accretion disk whose temperature is too low to break up molecules and has a velocity of some 10 km / s
• An X - wind from the central hole in the accretion disk, the velocities of several 100 km / s.
• A stellar wind from the surface of CTTS. In some of the T Tauri stars of this wind is collimated by an interaction with the X - wind to a jet with an angle of a few degrees.

Brown dwarfs

The T Tauri stage is observed not only in stars that have enough material to ignite hydrogen burning. Also in brown dwarfs signs of chromospheric activity such as star spots, infrared emission of dust disks, strong Hα lines by accretion, evidence of dust formation and growth and bipolar outflows in the form of jets have been detected at the age of a few million years. These young brown dwarfs rotate extremely slowly, which is interpreted as an indication of the formation of a global magnetic field. In these low-mass T Tauri stars, the circumstellar disks only have masses of a few millionths of solar masses and are thus several orders of magnitude smaller than in normal T Tauri stars. This also applies to the accretion rates of some 10-12 to 10-10 solar masses per year. The spectral class of brown dwarfs in the T Tauri stage is later than M6 and decreases with increasing age further. It is observed no significant accretion more at brown dwarfs with an age of more than five million years ago.

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• Knowledge Centre for Astrophysics ( Dr. Andreas Müller, astrophysicist )