T-Tauri star

An unknown event disturbed the equilibrium of the interstellar cloud, and it collapsed. This process may have been caused by shock waves from a supernova explosion, or by a density wave of a spiral arm of the galaxy. The gas molecules and the particles were compressed, and with increasing compression, both temperature and pressure increased. It is possible that the centrifugal forces due to the rotation of the system prevented a spherical contraction. The result was a relatively flat, rotating disc of matter, in the centre of which was the primeval sun. Analogues of the early solar system, i.e., protoplanetary discs, have been identified from the radiation emitted by T Tauri stars (Koerner, 1997). 

Theory doesn t tell us what initial Li a star has, only what depletion it suffers. An accurate estimate of the initial Li abundance is therefore a pre-requisite before observations and models can be compared. The Sun is a unique exception, where we know the present abundance, A(Li) = 1.1 0.1 (where A(Li)= log[AT(Li)/AT(H)] + 12) and the initial abundance of A(Li)= 3.34 is obtained from meteorites. For recently born stars, the initial Li abundance is estimated from photospheric measurements in young T-Tauri stars, or from the hotter F stars of slightly older clusters, where theory suggests that no Li depletion can yet have taken place. Results vary from 3.0 < A(Li) < 3.4, somewhat dependent on assumed atmospheres, NLTE corrections and TeS scales [23,33]. It is of course quite possible that the initial Li, like Fe abundances in the solar neighbourhood, shows some cosmic scatter. Present observations certainly cannot rule this out, leading to about a 0.2 dex systematic uncertainty when comparing observations with Li depletion predictions. 

At the centre of the cloud is the young stellar object destined to become the Sun. It accounts for approximately 99.9 per cent of the mass of the nebula and there are various examples of this in the heavens, including the classic pre-main sequence T-Tauri star. The star continues to evolve, blowing off bipolar jets (see Figure 4.5) and beginning a solar wind of particles. Of course, the star does not reach its full luminous intensity and the best theories suggest that the Sun was some 30 per cent less luminous when the Earth began to form. 

T-Tauri stars Stars early in their evolution life cycle that throw off a dust jacket in the form of polar jets and begin to shine. 

The lithium resonance doublet line X 6707 is fairly easy to observe in cool stars of spectral types F and later, and it has also been detected in diffuse interstellar clouds. There is thus an abundance of data, although in the ISM the estimation of an abundance is complicated by ionization and depletion on to dust grains. The youngest stars (e.g. T Tauri stars that are still in the gravitational contraction phase before reaching the main sequence) have a Li/H ratio that is about the same as the Solar System ratio derived from meteorites, Li/H = 2 x 10-9, which is thus taken as the Population I standard. 

Artistic rendering of four observed stages of star formation, (a) Class 0 object a deeply embedded hydrostatic core surrounded by a dense accretion disk. Strong bipolar jets remove angular momentum, (b) Class I object protostar in the later part of the main accretion phase, (c) Class II object or T Tauri star pre-main-sequence star with optically thick protoplanetary disk, (d) Class III object or naked T Tauri star star has an optically thin disk and thus can be directly observed. Some may have planets. 

Class I obj ects also have bipolar outflows, but they are less powerful and less well collimated than those of Class 0 objects. This stage lasts 100 000 to 200 000 years. Class //objects, also known as classical T Tauri stars, are pre-main-sequence stars with optically thick proto-planetary disks. They are no longer embedded in their parent cloud, and they are observed in optical and infrared wavelengths. They still exhibit bipolar outflows and strong stellar winds. This stage lasts from 1-10 million years. Class ///objects are the so-called weak line or naked T-Tauri stars. They have optically thin disks, perhaps debris disks in some cases, and there are no outflows or other evidence of accretion. They are observed in the visible and near infrared and have strong X-ray emission. These stars may have planets around them, although they cannot be observed. 

Class II. Sources with spectral index —1.5 < cyir < 0. These are pre-main-sequence stars with observable accretion discs (classical T Tauri stars). 

The initial evolutionary stages (Classes 0 and I) are optically hidden by the dust of the collapsing envelope. These phases can be only observed in the far-infrared. Optically visible are the Class II and III objects, that form the long-known class of T Tauri stars. They have reached almost their final mass ( 90% see Beckwith et al. 1990) at the transition from Class I to Class II, which occurs for solar-mass stars approximately 2 x 105 yr after the onset of collapse (see Table 2.2). 

After a few million years of evolution, where most of the remaining disk mass is accreted to the T Tauri star, the residual disk is dispersed (see Chapter 8) and the star continues its further evolution to the main sequence as a Class III object. The processes going on in these disks, that are usually called protoplanetary during this final phase of protostar evolution, are the subject of the following chapters of this book. 

In heavily obscured regions with ongoing star formation one observes the so-called Herbig-Haro (HH) objects thin collimated jets of matter rapidly flowing (up to several hundred kilometers per second) out from young stellar objects. An example is shown in Fig. 2.11. These jets are mainly associated with Class 0 and I objects but sometimes are also observed for T Tauri stars. The outflows interact with... 

The sensitivity of the Infrared Array Camera (IRAC) camera on board the Spitzer Space Telescope (Fazio et al. 2004) recently allowed to characterize in detail the decrease in disk frequency with stellar age and trace dust slightly cooler than that observed in the L-band, out to about 1AU from T Tauri stars. Figure 9.2 shows the fraction of T Tauri stars (mostly K and M stars) with infrared excess at IRAC wavelengths (3.6, 4.5, 5.8, and 8 pm, full circles). In addition to the data (and references) presented in Hernandez et al. (2008) we have included the disk statistics... 

T Tauri stars very young, low-mass stars, less than 10 million years old and under 3 solar-masses, that are still undergoing gravitational contraction. T Tauri stars represent an intermediate stage between a protostar and a low-mass main-sequence star like the Sun. The prototype for this class of stars is T Tau. 

Fig. 12. Contour map of the visual extinction in the Taurus cloud. The shaded areas represent dark clouds of small diameter with visual extinction ranging from 5m to 8m. Also indicated are the positions of T-Tauri stars and quasi-thermal OH emission...

Fig. 4. The pre-main sequence evolution of 0.5 to 1.5 solar mass stars in the Hertzsprung-Russell diagram. The lines represent theoretical evolutionary tracks, while the dots represent observational data for T Tauri stars (Cohen and Kuhi, 1979). The corresponding ages and ratios of UV flux to present solar UV flux are indicated for a solar-mass star.

Observations of star-forming regions have advanced our understanding of the star-formation process considerably in the last few decades. We now can study examples of nearly aU phases of the evolution of a dense molecular cloud core into a nearly fully formed star (i.e., the roughly solar-mass T Tauri stars). As a result, the theory of star formation is relatively mature, with fumre progress expected to center on defining the role played by binary and multiple stars and on refining observations of known phases of evolution. 

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