  One of the outstanding problems in star-formation theory concerns the
transfer of angular momentum such that mass can accrete onto a newly born young
stellar object (YSO). From a theoretical standpoint, outflows and jets are
predicted to play an essential role in angular momentum transfer and their
rotation motions have been reported for both low- and high-mass YSOs. However,
little quantitative discussion on outflow launching mechanisms have been
presented for high-mass YSOs due to a lack of observational data. Here we
present a clear signature of rotation in the bipolar outflow driven by Orion
Source I, a high-mass YSO candidate, using the Atacama Large
Millimeter/Submillimeter Array (ALMA). A rotational transition of silicon
monoxide (Si18O) reveals a velocity gradient perpendicular to the outflow axis
which is consistent with that of the circumstellar disk traced by a
high-excitation water (H2O) line. The launching radii and outward velocity of
the outflow are estimated to be >10 au and 10 km s-1, respectively. These
parameters rule out a possibility that the observed outflow is produced by
entrainment of a high-velocity jet, and that contribution from stellar-wind or
X-wind which have smaller launching radii are significant in the case of Source
I. Thus, present results provide a convincing evidence of a rotating outflow
directly driven by the magneto-centrifugal disk wind launched by a high-mass
YSO candidate.
