摘要：Aims. Recently, differences in Doppler shifts across the base of four
close classical T Tauri star jets have been detected with the HST in optical and
near-ultraviolet (NUV) emission lines, and these Doppler shifts were interpreted as
rotation signatures under the assumption of steady state flow. To support this
interpretation, it is necessary that the underlying disks rotate in the same sense.
Agreement between disk rotation and jet rotation determined from optical lines has been
verified in two cases and rejected in one case. Meanwhile, the near-ultraviolet lines,
which may trace faster and more collimated inner spines of the jet than optical lines,
either agree or show no clear indication. We propose to perform this test on the fourth
system, Th 28.
Methods. We present ALMA high angular resolution Band 7 continuum,
12CO(3−2) and 13CO(2−1) observations of the circumstellar disk around the T Tauri star
Th 28.
Results. The sub-arcsecond angular resolution (0.46′′× 0.37′′)
and high sensitivity reached enable us to detect, in CO and continuum, clear signatures of
a disk in Keplerian rotation around Th 28. The
12CO emission is
clearly resolved, allowing us to derive estimates of disk position angle and inclination.
The large velocity separation of the peaks in 12CO, combined with the resolved extent of the emission,
indicate a central stellar mass in the range 1−2 M⊙. The rotation sense of the disk is
well detected in both 13CO and 12CO emission lines, and this direction is opposite to
that implied by the transverse Doppler shifts measured in the optical lines of the
jet.
Conclusions. The Th 28 system
is now the second system, among the four investigated so far, where counter-rotation
between the disk and the optical jet is detected. These findings imply either that optical
transverse velocity gradients detected with HST do not trace jet rotation or that modeling
the flow with the steady assumption is not valid. In both cases jet rotation studies that
rely solely on optical lines are not suitable to derive the launching radius of the
jet.
