Letter to the Editor

Predicted rotation signatures in MHD disc winds and comparison to DG Tau observations

¹ Laboratoire d'Astrophysique, UMR 5571, Observatoire de Grenoble, BP 53, 38041 Grenoble Cedex 9, France
² LERMA, UMR 8112, Observatoire de Paris, 61 avenue de l'Observatoire, 75014 Paris, France
³ Institute for Plasma Physics Rijnhuizen, PO Box 1207, 3430 BE Nieuwegein, The Netherlands
⁴ Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal

Corresponding author: N. Pesenti, Nicolas.Pesenti@obs.ujf-grenoble.fr

Received: 6 October 2003
Accepted: 28 January 2004

Abstract

Motivated by the first detections of rotation signatures in the DG Tau jet (Bacciotti et al. [CITE]), we examine possible biases affecting the relation between detected rotation signatures and true azimuthal velocity for self-similar MHD disc winds, taking into account projection, convolution as well as excitation gradients effects. We find that computed velocity shifts are systematically smaller than the true underlying rotation curve. When outer slower streamlines dominate the emission, we predict observed shifts increasing with transverse distance to the jet axis, opposite to the true rotation profile. Determination of the full transverse rotation profile thus requires high angular resolution observations (<5 AU) on an object with dominant inner faster streamlines. Comparison of our predictions with HST/STIS observations of DG Tau clearly shows that self-similar, warm MHD disc wind models with and an outer radius of the disc 3 AU are able to reproduce detected velocity shifts, while cold disc wind models () are ruled out for the medium-velocity component in the DG Tau jet.