4. Constraints on the ejection process
The jet widths inferred for CW Tau and RW Aur compare remarkably well with the values derived from HST observations on similar spatial scale for HH 30 and HL Tau (Ray et al. 1996), two other active TTs. In addition, all five jets studied so far around TTs are resolved beyond 0.4" and show strikingly similar inner jet widths out to projected distances of 100 AU. Density collimation must therefore occur on spatial scales 56 AU, in quantitative agreement with predictions from current MHD wind models (Shang et al. 1998, Cabrit et al. 1999). The derived intrinsic jet widths of 25-35 AU on a 100 AU spatial scale seem to favor models with moderate opening of the inner streamlines (e.g. Model 1 in Cabrit et al. 1999). For younger, more embedded sources, driving HH flows, jet widths have been measured on the same spatial scale only in HH 34 (Ray et al. 1996). The derived HH 34 width of 110 AU at a projected distance of 270 AU is comparable to that observed in DG Tau, where strong contamination by the bow-shock wings seem to be present at these distances, but is typically 3 times larger than those derived in the other cTTs jets. Observations of a larger sample is obviously mandatory to test for significance.
In all three jets, the emission consists of bright knots with spacings in the range 1-5". The properties of the DG Tau bow-shaped knot strongly suggest an internal working surface produced by ejection variability: clear bow-shaped morphology, transverse velocity gradients (L97), high proper motion (this work), line ratios consistent with shock models (Lavalley et al. 2000). Strong velocity variability has been indeed observed in the high velocity wings of the emission profiles towards DG Tau (Solf, 1997). In addition, large velocity and ejection direction variations are indicated for the 4 outer knots (EM98). The inferred variability timescale is 10-20 yrs. Similar timescales have been identified in younger HH jets such as HH 34 (Raga & Noriega-Crespo 1998). Follow-up observations are however required to determine whether this model also holds for the other two sources.
The RW Aur case brings some additional insight on the accretion/ejection process in multiple systems. The detected jet originates from the component with highest accretion rate (Duchêne et al. 1999), as expected from the accretion-ejection correlation. In addition, the jet associated with the primary RW Aur-A appears similar (jet width, straightness) to that emanating from the single star CW Tau, indicating that the mass-loss process remains mostly unperturbed by the dynamical interaction with the companion. Indeed, according to the predictions of Terquem et al. (1999), the characteristic precession length induced by the secondary RW Aur-B should be 1 pc (assuming a primary mass of 0.85 , HEG95, a primary to secondary mass ratio of 1, an orbital to outer disk radius ratio of 3 and a minimum outer disk radius of 56 AU), largely in excess of the scale over which the jet is detected. The same calculation excludes companion induced precession as the origin of the DG Tau jet wiggling.
© European Southern Observatory (ESO) 2000
Online publication: June 5, 2000