EVIDENCE FOR AN IONIZED ROTATING STRUCTURE AROUND RW AUR * ANA I. G ´ OMEZ DE CASTRO 1 and EVA VERDUGO 2 1 Instituto de Astronom´ ıa y Geodesia (CSIC-UCM), Facultad de Ciencias Matem´ aticas, Universidad Complutense de Madrid, Spain; E-mail: aig@mat.ucm.es 2 ISO Data Center, VILSPA-ESA, P.O. Box 50727, Madrid, Spain; E-mail: everdugo@iso.vilspa.esa.es Abstract. Profiles of the UV semiforbidden lines of C III] 1908 and Si III] 1892 of RW Aur have been obtained with the HST/STIS. The C III] 1908 profile shows two high velocity components at v = ± 170 km s −1 and a central one. The Si III] 1892 profile is very broad (FWHM = 293 km s −1 ) and the high velocity components are unresolved. It is shown that the high velocity components are most probably produced in a rotating belt alike the detected in other sources of bipolar outflows. A radius between 2.7 R ∗ and the corotation radius (6.1 R ∗ ) is derived and a log T e (K) ≃ 4.7 and log n e (cm −3 ) = 11.6 are estimated. The belt is clumpy and the most likely source of heating is local X-rays radiation, probably associated with the release of magnetic energy. Keywords: stars: pre-main-sequence, stars: T Tauri, stars: individual: RW Aur 1. Introduction Determining the precise physical mechanism that connects accretion to outflow is one of the key issues in star formation. The T Tauri phase is the most interesting for this purpose since at this stage bipolar flows, often detected as optical jets, are still active but the central engine can be observed at optical and ultraviolet wavelengths allowing a detailed spectroscopic diagnosis. The most suitable spectral indica- tors are in the ultraviolet (UV) range where the semiforbidden intercombination lines of abundant species such as the O III], Si III] and C III] are observed. These lines are optically thin and are strong in the spectra of the T Tauri stars (TTSs) (G´ omez de Castro and Franqueira, 1997). The hot plasma (T e ≃ 5 × 10 4 K) traced by these lines, is expected to be produced in the star-disk magnetic interaction region and the stellar atmosphere. Moreover, recent UV observations have shown that there is also a contribution from a hot jet (G´ omez de Castro and Verdugo, 2001, 2002). In this work, we analyze C III] 1908 and Si III] 1892 lines of RW Aur A, a TTS which has been extensively studied so the main physical parameters are well known. Also, the spectral signatures of the jet (Dougados et al., 2000, Hirth et al., 1994, Haman, 1994, Hirth et al., 1997), accretion flow (Petrov et al., 2001, hereafter P2001) and the stellar atmosphere (P2001) are well known. ∗ Based on observations made with the NASA/ESA Hubble Space Telescope. Astrophysics and Space Science 292: 691–696, 2004. C  2004 Kluwer Academic Publishers. Printed in the Netherlands.