Atomic T Tauri disk winds heated by ambipolar diffusion - I. Thermal structure

Motivated by recent subarcsecond resolution observations of jets from T Tau ri stars, we extend the work of Safier (1993a,b) by computing the thermal a nd ionization structure of self-similar, magnetically-driven, atomic disk w inds heated by ambipolar diffusion. Improvements over his work include: (1) new magnetized cold jet solutions consistent with the underlying accretion disk (Ferreira 1997); (2) a more accurate treatment of ionization and ion- neutral momentum exchange rates; and (3) predictions for spatially resolved forbidden line emission (maps, long-slit spectra, and line ratios), presen ted in a companion paper, Garcia et al. (2001). As in Safier (1993a), we ob tain jets with a temperature plateau around 10(4) K, but ionization fractio ns are revised downward by a factor of 10-100. This is due to previous omis sion of thermal speeds in ion-neutral momentum-exchange rates and to differ ent jet solutions. The physical origin of the hot temperature plateau is ou tlined. In particular we present three analytical criteria for the presence of a hot plateau, applicable to any given MHD wind solution where ambipola r diffusion and adiabatic expansion are the dominant heating and cooling te rms. We finally show that, for solutions favored by observations, the jet t hermal structure remains consistent with the usual approximations used for MHD jet calculations (thermalized, perfectly conducting, single hydromagnet ic cold fluid calculations).