On the source of dense outflows from T Tauri stars - II. Warm disc winds

Abstract

Recent ultraviolet (UV) observations suggest that there is a hot (T e 80 000 K) and dense (Ne 10 10 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 (V rad = 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.

Where are the hot ion lines in classical T Tauri stars formed?

“…Undoubtedly CTTS also show outflows (a review is given by Bally et al 2007), however, the precise origin and the physical driving mechanism(s) are uncertain. Theoretical models propose a variety of collimated stellar outflows and disk winds to remove angular momentum from the system (Shu et al 1994;Matt et al 2002;Gómez de Castro & Ferro-Fontán 2005;Pudritz et al 2007). The outflows can take the form of winds observed from radio to the UV (e.g.…”

Pre main sequence stars as UV sources for the World Space Observatory-UV mission

“…Flares associated with reconnection events would naturally be produced leading to the formation of a hot disc corona. Warm disk winds, centrifugally launched from the disc corona, have been shown to be able to reproduce the observed UV lines fluxes and lines ratios provided that the winds are clumpy (with filling factors about 1%) by Gómez de Castro and Ferro-Fontán (2005). Warm disc winds have also been shown to be able to reproduce larger scale jet observations (see i.e.…”

“…Dark blue is used to represent the predictions of the "control model", i.e., the outflow launched from the interaction by a stellar dipolar field of 1-kG and a passive accretion disk that it is not dynamo-active. Finally, purple squares at the bottom right represent predictions from the warm disc wind models by Gómez de Castro and Ferro-Fontán (2005) The diagram in Fig. 2, plots the velocity dispersion (σ ) versus the velocity centroid ( Ṽ ) for theoretical and observed Si III] profiles of CTTSs.…”

The formation of planetary disks and winds: an ultraviolet view

“…These lines can be generically described as broad, asymmetric emission lines with typical full widths at 10 % intensity of few hundreds km/s. The broad blueward shifted absorption component characteristic of mass-loss was detected in few sources (Penston & Lago, 1983, Imhoff & Appenzeller, 1989 but not in all of them; the degree of absorption (the asymmetry of the line) varies from not absorption & Ferro-Fontán (2005). Left: Velocity field in the flow.…”