Jet rotation: Launching region, angular momentum balance and magnetic properties in the bipolar outflow from RW Aur

Abstract

Abstract. Using STIS on board the HST we have obtained a spectroscopic map of the bipolar jet from RW Aur with the slit parallel to the jet axis and moved across the jet in steps of 0. 07. After applying a velocity correction due to uneven slit illumination we find signatures of rotation within the first 300 AU of the jet (1. 5 at the distance of RW Aur). Both lobes rotate in the same direction (i.e. with different helicities), with toroidal velocities in the range 5-30 km s −1 at 20 and 30 AU from the symmetry axis in the blueshifted and redshifted lobes, respectively. The sense of rotation is anti-clockwise looking from the tip of the blue lobe (PA 130 • north to east) down to the star. Rotation is more evident in the [OI] and [NII] lines and at the largest sampled distance from the axis. These results are consistent with other STIS observations carried out with the slit perpendicular to the jet axis, and with theoretical simulations. Using current magneto-hydrodynamic models for the launch of the jets, we find that the mass ejected in the observed part of the outflow is accelerated from a region in the disk within about 0.5 AU from the star for the blue lobe, and within 1.6 AU from the star for the red lobe. Using also previous results we estimate upper and lower limits for the angular momentum transport rate of the jet. We find that this can be a large fraction (two thirds or more) of the estimated rate transported through the relevant portion of the disk. The magnetic lever arm (defined as the ratio r A /r 0 between the Alfvèn and footpoint radii) is in the range 3.5-4.6 (with an accuracy of 20-25%), or, alternatively, the ejection index ξ = d ln(Ṁ acc )/dr is in the range 0.025-0.046 (with similar uncertainties). The derived values are in the range predicted by the models, but they also suggest that some heating must be provided at the base of the flow. Finally, using the general disk wind theory we derive the ratio B φ /B p of the toroidal and poloidal components of the magnetic field at the observed location (i.e. about 80-100 AU above the disk). We find this quantity to be 3.8 ± 1.1 at 30 AU from the axis in the red lobe and −8.9 ± 2.7 at 20 AU from the axis in the blue lobe (assuming cylindrical coordinates centred on the star and with positive z along the blue lobe). The toroidal component appears to be dominant, which would be consistent with magnetic collimation of the jet. The field appears to be more tightly wrapped on the blue side.

The excitation of planetary orbits by stellar jet variability and polarity reversal

“…Jet launching regions (JLRs) are confined to the inner part of the disk with estimates from 0.01 AU for the X-wind model to a few AU for disk-wind models (e.g. 1.6 AU for RW Auriga, Woitas et al 2005). The integrated momentum loss over the launching region accelerates the center of mass of the star-disk system which coincides with the star's center for axisymmetric star-disk systems.…”

HST/STIS observations of the RW Aurigae bipolar jet: mapping the physical parameters close to the source

“…Observational details and the basic steps of the data reduction have been described in Paper I and in Woitas et al (2005). Briefly, a slit of width 0.…”

“…In Woitas et al (2002) (hereafter Paper I) we presented reconstructed velocity channel maps of the bipolar jet in the forbidden lines, giving indications of the morphology and kinematics of the jet close to the star and at high resolution. In the second paper of the series, Woitas et al (2005), we reported signatures of rotation in both jet lobes within the first 1. 5 from the central source (about 200 AU at the distance of RW Aur).…”

“…During the past few years, high resolution studies of several jets from young stars on subarcsecond scales have become available -e.g HH 30 Hartigan & Morse 2007), DG Tau (Bacciotti et al 2000;Lavalley-Fouquet et al 2000;Pyo et al 2003), HL Tau (Pyo et al 2006), RW Aur (Pyo et al 2006), and LkHα 233 (Perrin & Graham 2007;Melnikov et al 2008). The present study is a continuation of the spectroscopic analysis of RW Aur performed with the Space Telescope Imaging Spectrograph (STIS) on-board the Hubble Space Telescope (HST), started by Woitas et al (2002, hereafter Paper I), and Woitas et al (2005).…”

Wide‐Angle Wind‐driven Bipolar Outflows: High‐Resolution Models with Application to Source I of the Becklin‐Neugebauer/Kleinmann‐Low OMC‐I Region

“…Thus the distinction between tightly collimated jets and wide-angle winds informs the debate about the nature of jet launching and accretion disk physics. Since it is difficult to directly observe the wind launching regions (see Woitas et al (2005) ), the properties of large scale bipolar outflows can aid in understanding the nature of their initial conditions. In addition to questions concerning the nature of the driving winds, bipolar outflows may play an important role in modifying their natal environments.…”