Excellent reviews that taken together cover all aspects of T Tauri stars can be found in the literature [Appenzeller & Mundt 1989,Bertout 1989].
Here a brief overview is presented, paying particular attention to aspects directly relevant to the work presented in the chapters that follow.
T Tauri stars are among the youngest objects directly observed. The kinematic association of many T Tauri stars with dark clouds where stars are formed [Herbig 1977] and the presence of Li I Å in absorption [Bertout 1989] show that T Tauri stars are young. Comparing the position of these stars in the HR diagram with theoretical evolutionary tracks [D'Antona & Mazitelli 1994,Swenson et al. 1994] gives years as an upper limit to the estimated age. These evolutionary tracks do not take into account accretion and Siess, Forestini & Bertout (1997) show that they underestimate ages by a factor of 2 to 3. Nevertheless, the young age of T Tauri stars is not in question.
Observationally, T Tauri stars are identified by the following characteristics [Appenzeller & Mundt 1989, and references therein,]: they are stellar objects associated with regions of obscuration; their spectra show Balmer lines and Ca H and K lines, in emission; the photospheric absorption spectra are similar to those of stars with spectral-type later than late F.
Ubiquitous amongst T Tauri stars is variability. In fact variability is a defining characteristic of T Tauri stars. Photometric variations are seen to occur at all wavebands, from the X-ray to the infrared. Variability timescales range from a few minutes to at least a century [Appenzeller & Mundt 1989] and usually variations are very irregular, although some T Tauri stars show a quasi-periodic behaviour, most probably due to the presence of spots at the stellar surface [Bertout 1989, and references therein,]. Periods of a few days are found. Variations are not restricted to photometric observations, though. Emission lines are also seen to change in intensity and shape [Johns & Basri 1995a,Johns & Basri 1995b,Lago & Gameiro 1998, for example,] and polarimetric studies imply variations in the degree and position angle of polarization as well [Appenzeller & Mundt 1989].
These stars are the link between deeply embedded protostars [Lada 1986, Class I objects,] and low mass main sequence stars (M<3 ). Their spectral energy distributions (SED) are characterized by both ultraviolet and infrared excesses relative to the SED corresponding to a main sequence star of the same effective temperature.
According to the strength of the H Alpha emission line, T Tauri stars are classified as ``Weak Line T Tauri Stars'' (WTTS) or ``Classical T Tauri Stars'' (CTTS). The line strength is measured by its equivalent width and WTTS have W(H Alpha)<10 Å. It should be noted, however that this is not a rigid division. T Tauri stars can change spectroscopically such that the border W(H Alpha)=10 Å is crossed over [Bertout & Bouvier 1989]. When compared to CTTS, WTTS display no ultraviolet excess, little or no infrared excess and they show very weak, if any emission lines [Montmerle et al. 1993].
The origin of the infrared excess in T Tauri stars is attributed to the presence of a circumstellar disk [Beckwith & Sargent 1993,Strom et al. 1993, and references therein,]. This excess results both from reprocessing photospheric radiation and from heating of gas and dust by accretion [Lynden-Bell & Pringle 1974,Kenyon & Hartmann 1987,Adams et al. 1987,Bertout et al. 1988,Bertout 1989]. Beckwith et al. beckwithetal90 determine masses and sizes between and AU for the accretion disks.
The theory of accretion disks was set by Lynden-Bell & Pringle (1974). The disks envisioned by these authors are optically thick and quasi-Keplerian with viscosity allowing material to infall towards the star.
The old assumption was that an accretion disk extends right to the surface of the star where material has to loose half of its gravitational energy in order to accrete. This energy loss is supposed to occur in a very thin region close to the star, the boundary layer. The amount of energy released there accounts for the excess observed at ultraviolet wavelengths. The boundary layer is, according to Basri & Bertout (1989), also responsible for the Balmer continuum observed in T Tauri stars. Basri & Bertout (1989) estimate a mass accretion rate of . The, so called, boundary layer accretion model faces a few problems though. Material accreting through a quasi-Keplerian disk extending all the way to the stellar surface implies that a star should spin up to about half the break up speed, i.e. km/s, if no angular momentum is lost [Hartmann & Stauffer 1989]. However, rotational velocities in T Tauri stars are km/s, implying that substantial dissipation of angular momentum must occur. A natural explanation for this dissipation is not given by the boundary layer model. YY Orionis stars show inverse P Cygni profiles in the Balmer lines [Walker 1972]. The observed velocities of the redshifted absorption features are 300--400 km/s, which imply material falling onto the star at these velocities. Such velocities are irreconcilable with infalling velocities of a few km/s predicted by the boundary layer accretion model. Beckwith et al. (1990) find that for many T Tauri stars the near infrared emission is smaller than what is expected for a disk that extends to the stellar surface. These authors claim that this result requires the inner regions of disks to be devoid of material.
A more recent model for accretion that overcomes these problems is the magnetospheric accretion model [Camenzind 1990,Konigl 1991,Shu et al. 1994]. It invokes the presence of a dipolar stellar magnetic field which disrupts the circumstellar disk up to a few stellar radii from where material falls onto the star along magnetic columns (Figure 1.1).
Figure 1.1: Sketch of the Magnetospheric Accretion Geometry. From Camenzind (1990).
For accretion rates of the inner radius of the disk is inside 0.1 AU for magnetic fields at the stellar surface of the order G. [Camenzind 1990]. The gap alluded to by Beckwith et al. (1990) to explain the near infrared energy distributions requires an outer radius between 0.05-0.3 AU, which might be explained by this model.
Infall velocities inferred from inverse P Cygni profiles in YY Orionis stars, are predicted by the magnetospheric accretion model, if the material is free-falling from a few stellar radii.
Magnetospheric accretion also provides a natural explanation for the long standing problem of angular momentum dissipation in T Tauri stars, therefore keeping the star rotating well below break-up speed, as it is observed.
In this model the observed ultraviolet and optical excess result from the accretion shock taking place when the material hits the stellar surface at the region where the field lines are rooted [Konigl 1991].
The optical excess in T Tauri stars implies that the photospheric lines in these stars are not as deep as those of a main sequence star of the same spectral type. This effect, usually referred to as ``veiling'', as been studied mainly in the optical [Basri & Batalha 1990,Hartigan et al. 1991,Guenther & Hessman 1993a,Valenti et al. 1993,Hartigan et al.1995,Gullbring et al. 1998, for example,]. Models where the veiling continuum arises in the boundary layer (eg. Hartigan et al. 1991) and models where the veiling continuum arises from the accretion shock in the magnetospheric accretion picture (eg. Gulbring et al. 1998) result in similar estimates for the density and temperature of the emitting region, which are TK and n cm-3. Using this optical excess emission, mass accretion rates have been derived for many T Tauri stars. Hartigan, Edwards & Ghandour (1995) arrive at mass accretion rates of yr-1 while Valenti, Basri & Johns (1993) and Gullbring et al. (1998) obtain values typically one order of magnitude smaller, i.e. yr-1.
Winds and outflows are a common phenomenon in CTTS and they seem to be ultimately powered by accretion onto the star [Cabrit et al. 1990]. The observational evidence for their presence in T Tauri stars is overwhelming: forbidden lines, such as [O I] and [S II], often display a high velocity emission component at about -100 to -200 km/s [Hartigan et al. 1995]; images and long slit spectroscopy of the circumstellar region of some T Tauri stars clearly show jets emanating from these stars [Reipurth 1989, for example,] with velocities in the range 200-450 km/s; P Cygni profiles in H Alpha and in the Na I D resonance line imply outflowing velocities of km/s; CO observations have detected molecular outflows for which the expected sources are T Tauri stars [Fukui et al.1993]; radio continuum observations uncover the presence of an outflow of ionized gas from some T Tauri stars [Appenzeller & Mundt 1989, and references therein,]. Winds and jets are a feature of the magnetospheric accretion model developed by Camenzind (1990) and by Shu et al. (1994).