Atomic T Tauri disk winds heated by ambipolar diffusion

I. Thermal structure

¹ Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
² CRAL/Observatoire de Lyon, CNRS UMR 5574, 9 avenue Charles André, 69561 St. Genis-Laval Cedex, France
³ Laboratoire d'Astrophysique de l'Observatoire de Grenoble, BP 53, 38041 Grenoble Cedex, France
⁴ Observatoire de Paris, DEMIRM, UMR 8540 du CNRS, 61 avenue de l'Observatoire, 75014 Paris, France
⁵ Instituto de Astronomía, UNAM, Ap. 70-264, 04510 D. F., México

Corresponding author: P. J. V. Garcia, pgarcia@astro.up.pt

Received: 21 March 2001
Accepted: 3 August 2001

Abstract

Motivated by recent subarcsecond resolution observations of jets from T Tauri stars, we extend the work of [CITE] by computing the thermal and ionization structure of self-similar, magnetically-driven, atomic disk winds heated by ambipolar diffusion. Improvements over his work include: (1) new magnetized cold jet solutions consistent with the underlying accretion disk [CITE]; (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), presented in a companion paper, [CITE]. As in Safier (1993a), we obtain jets with a temperature plateau around 10⁴ K, but ionization fractions are revised downward by a factor of 10-100. This is due to previous omission of thermal speeds in ion-neutral momentum-exchange rates and to different jet solutions. The physical origin of the hot temperature plateau is outlined. In particular we present three analytical criteria for the presence of a hot plateau, applicable to any given MHD wind solution where ambipolar diffusion and adiabatic expansion are the dominant heating and cooling terms. We finally show that, for solutions favored by observations, the jet thermal structure remains consistent with the usual approximations used for MHD jet calculations (thermalized, perfectly conducting, single hydromagnetic cold fluid calculations).