T-Tauri

On September 20, 2003, all subsystems finally came together to reveal the unique capability of the Keck LGS/AO system. The system locked on HK Tau, a 15 magnitude well-known T Tauri binary, revealing details of the cir-cumstellar disk of its companion (Fig. 25). This was the first demonstration of a LGS/AO system on a large telescope. The FWHM is 50 milli-arcsec, compared to 183 milli- arcsec for the uncorrected image. While locked on a 14 magnitude star, the LGS/AO system recorded Strehls 0.36 (at 2.1/rm), 30-sec exposure time, compared to 0.04 for uncorrected images. 

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). 

Fig. 2.2 The state of the incipient solar system during the T Tauri phase of the young sun. The central region around the sun was blown free from the primeval dust cloud. Behind the shock front is the disc with the remaining solar nebula, which contained the matter formed by the influence of the solar wind on the primeval solar nebula. From Gaffey (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. 

The uncertainty in the age of pre main sequence stars is therefore of the order of the thermal timescale at the luminosity of D-burning smaller than a few times 105 yr for normal T Tauri, and larger than 106 yr for very low mass stars and brown dwarfs (BD). In fact, comparing observations spanning a wide range of masses we could even constrain the models, for example we can ascertain whether the Stahler et al. (1986) picture of collapse is valid also in the BD regime, or... 

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... 

---