Recent ultraviolet (UV) observations suggest that there is a hot (Te≃ 80 000 K) and dense (Ne ≃ 1010 cm−3) wind associated with the large-scale jets observed in classical T Tauri stars (cTTSs). The observations of these rather evolved sources cannot be fitted with the classical cold disc wind solutions. This is not unexpected since the accretion rates are moderate (≤10−8 M⊙ yr−1) and the wind lighter than at earlier phases. Henceforth, X-ray radiation from the star and the star–disc interaction region is expected to modify the thermal structure of the inner disc, e.g. the base of the disc wind. Thus, thermal pressure is relevant to load gas on the field lines. In this work, we analyse whether warm disc winds can account for these UV observations.
To get a good hint on the physics, we have preferred to focus on analytical work. We have made use of the warm disc wind solution with pressure calculated by Vlahakis et al. The rich complexity of the magnetohydrodynamic (MHD) disc wind kinematics is analysed in detail. We show the following. (i) Warm disc flows departing from the inner disc radius (∼0.1 au) reach terminal velocities close to the observed (300 km s−1) and that the temperature at the base of the wind is ∼40 000 K. Thus, the spectral signature of warm disc winds is an enhancement of the chromospheric and transition region spectral indicators by several orders of magnitude, as observed in cTTSs. (ii) Warm disc winds rotate, but their rotation velocity is small since the wind dynamics is based in a transference of angular momentum from the star to the magnetic field. Henceforth, slow rotation plus high temperature (e.g. rotational broadening) show in very broad profiles at the base of the wind. (iii) The kinematics at the base of the wind is dominated by axial expansion; however, above z≃ 5 au, the dominant kinematical component is acceleration along the disc axis. (iv) The most often observed profile is a single-peaked line, slightly blueshifted and asymmetric with a tail to short wavelengths. The full width at half-maximum is clearly suprathermal and depends on the spectral tracer used.
Line profiles, fluxes and line ratios are calculated for the C iv[uv1], C iii]1908, C ii]2326, Si iii]1892 and [O ii]2471 UV lines. In edge-on systems, the profiles range from symmetric lines with suprathermal broadenings for the spectral tracers of the base of the wind (C iv[uv1] and Si iii]1892) to double-peaked profiles for spectral tracers more weighted to higher values of z ([O ii] and C ii]). In pole-on systems, the profiles are blueshifted (from few to 200 km s−1) and asymmetric exhibiting long tails towards the terminal velocity of the wind (Vrad= 300 km s−1). To reproduce the observed line fluxes and ratios, winds must be clumpy and extinction by the flaring disc and the wind itself ought to be taken into account.