Hubble Space Telescope/STIS spectroscopy of the optical outflow from DG Tauri:: Indications for rotation in the initial jet channel

We have carried out a kinematical, high angular resolution (similar to0.1) study of the optical blueshifted flow from DG Tau within 0.5 from the source (i.e., 110 AU when deprojected along this flow). We analyzed optical emission line profiles extracted from a set of seven long-slit spectra taken with the Space Telescope Imaging Spectrograph ( STIS) on board the Hubble Space Telescope, obtained by maintaining the slit parallel to the outflow axis while at the same time moving it transversely in steps of 0.07. For the spatially resolved flow of moderate velocity (peaking at -70 km s(-1)), we have found systematic differences in the radial velocities of lines from opposing slit positions, i.e., on alternate sides of the jet axis. The results, obtained using two independent techniques, are corrected for the spurious wavelength shift due to the uneven illumination of the STIS slit. Other instrumental effects are shown to be either absent or unimportant. The derived relative Doppler shifts range from 5 to 20 km s(-1). Assuming that the flow is axially symmetric, the velocity shifts are consistent with the southeastern side of the flow moving toward the observer faster than the corresponding northwestern side. If this finding is interpreted as rotation, the flow is then rotating clockwise looking from the jet toward the source and the derived toroidal velocities are in the range 6-15 km s(-1), depending on position. Combining these values with recent estimates of the mass-loss rate, one would obtain an angular momentum flux, for the low-to moderate-velocity regime of the flow, of (J)over dot(w,lm) similar to3.8 x 10(5) M-circle dot yr(-1) AU km s(-1). Our findings may constitute the first detection of rotation in the initial channel of a jet flow. The derived values appear to be consistent with the predictions of popular magnetocentrifugal jet-launching models, although we cannot exclude the possibility that the observed velocity differences are due to some transverse outflow asymmetry other than rotation.