Progresses in long slit differential astrometry techniques and high angular resolution imaging from Adaptive Optics and the Hubble Space Telescope have shown that the high velocity forbidden emission observed in Classical T Tauri Stars (CTTSs) is related to collimated (micro-)jets (e.g. Solf 1989; Solf & Böhm 1993; Ray et al. 1996; Hirth et al. 1997; Lavalley-Fouquet et al. 2000; Dougados et al. 2000; Bacciotti et al. 2000). Although outflow activity is known to decrease with age (Bontemps et al. 1996), CTTSs still harbor considerable activity (e.g. Mundt & Eislöffel 1998) and present the advantage of not being embedded. It is now commonly believed that such jets are magnetically self-confined, by a "hoop stress'' due to a non-vanishing poloidal current (Chan & Henriksen 1980; Heyvaerts & Norman 1989). The main reason lies in the need to produce highly supersonic unidirectional flows. Indeed, this requires an acceleration process that is closely related to the confining mechanism. The most promising models of jet production rely therefore on the presence of large scale magnetic fields, extracting energy and mass from a rotating object. However, we still do not know precisely what the jet driving sources are. Moreover, observed jets harbor time-dependent features, with time-scales ranging from tens to thousands of years. Such time-scales are much longer than those involving the protostar or the inner accretion disk. Therefore, although the possibility remains that jets have a non-stationary origin (e.g. Ouyed & Pudritz 1997; Goodson et al. 1999), only steady-state models will be addressed here.
Stationary stellar wind models have been developed (e.g. Sauty & Tsinganos 1994), however observed correlations between signatures of accretion and ejection clearly show that the disk is an essential ingredient in jet formation (Cohen et al. 1989; Cabrit et al. 1990; Hartigan et al. 1995). Therefore we expect accretion and ejection to be interdependent, through the action of magnetic fields. There are mainly two classes of stationary magnetized disk wind models, depending on the radial extent of the wind-producing region in the disk. In the first class (usually referred to as "disk winds''), a large scale magnetic field threads the disk on a large region (Blandford & Payne 1982; Wardle & Koenigl 1993; Ferreira & Pelletier 1993,1995; Li 1995; Li 1996; Ferreira 1997; Krasnopolsky et al. 1999; Casse & Ferreira 2000a,b; Vlahakis et al. 2000). Such a field is assumed to arise from both advection of interstellar magnetic field and local dynamo generation (Rekowski et al. 2000). In the second class of models (referred to as "X-winds''), only a tiny region around the disk inner edge produces a jet (Camenzind 1990; Shu et al. 1994; Shu et al. 1995; Shu et al. 1996; Lovelace et al. 1999). The magnetic field is assumed to originally come from the protostar itself, after some eruptive phase that linked the disk inner edge to the protostellar magnetosphere. Note that in both models, jets extract angular momentum and mass from the underlying portion of the disk. However, by construction, "disk-winds'' are produced from a large spread in radii, while "X-winds'' arise from a single annulus. Apart from distinct disk physics, the difference in size and geometry of the ejection regions should also introduce some observable jet features. Another scenario has been proposed, where the protostar produces a fast collimated jet surrounded by a slow uncollimated disk wind or disk corona (Kwan & Tademaru 1988,1995; Kwan 1997), but such a scenario still lacks detailed calculations.
So far, all disc-driven jet calculations used a "cold'' approximation, i.e. negligible thermal pressure gradients. Therefore, each magnetic surface is assumed either isothermal or adiabatic. But to test which class of models is at work in T Tauri stars, reliable observational predictions must be made and the thermal equilibrium needs then to be solved along the flow. Such a difficult task is still not addressed in a fully self-consistent way, namely by solving together the coupled dynamics and energy equations. Thus, no model has been able yet to predict the gas excitation needed to generate observational predictions.
One first possibility is to use a posteriori a simple parameterization for the temperature and ionization fraction evolution along the flow. This was done by Shang et al. (1998) and Cabrit et al. (1999) for X-winds and disk winds respectively. These approaches are able to predict the rough jet morphology, but do not provide reliable flux and line profile predictions, since the thermal structure lacks full physical consistency.
The second possibility is to solve the thermal evolution a posteriori, with the difficulty of identifying the heating sources (subject to the constraint of consistency with the underlying dynamical solution). Several heating sources are indeed possible: (1) planar shocks (e.g. Hartigan et al. 1987,1994); (2) oblique magnetic shocks in recollimating winds (Ouyed & Pudritz 1993,1994); (3) turbulent mixing layers (e.g. Binette et al. 1999); and (4) current dissipation by ion-neutral collisions, referred to as ambipolar diffusion heating (Safier 1993a,1993b). A further heating scenario (not yet explored in the context of MHD jets and only valid in some environments) is photoionization from OB stars (Reipurth et al. 1998; Raga et al. 2000; Bally & Reipurth 2001). Of all these previous mechanisms only ambipolar diffusion heating allows "minimal'' thermal solutions, in the sense that the same physical process - non-vanishing currents - is responsible for jet dynamics and heating. As a consequence no additional tunable parameter is invoked for the thermal description. Furthermore, Safier (1993b) was able to obtain fluxes and profiles in reasonable agreement with observations. In this paper, we extend the work of Safier (1993a,1993b) by (1) using magnetically-driven jet solutions self-consistently computed with the underlying accretion disk, and (2) a more accurate treatment of ionization using the Mappings Ic code and ion-neutral momentum exchange rates which include the thermal contribution. In a companion paper (Garcia et al. 2001, hereafter Paper II), we generate predictions for spatially resolved orbidden line emission maps, long-slit spectra, and line ratios.
This article is structured as follows: in Sect. 2 we introduce the dynamical structure of the disk wind under study, and present physical values of the density, velocity, magnetic field, and Lorentz force along streamlines; in Sect. 3 we describe the physical processes taken into account in the thermal evolution computations, whose results are presented and discussed in Sect. 4. Conclusions are presented in Sect. 5. Some important derivations, dust description and consistency checks of our calculations are presented in the appendices.
Copyright ESO 2001