Millimeter-wave Molecular Line Observations of the Tornado Nebula
  We report the results of millimeter-wave molecular line observations of the
Tornado Nebula (G357.7--0.1), which is a bright radio source behind the
Galactic Center region. A 15'x15' area was mapped in the J=1--0 lines of CO,
13CO, and HCO+ with the Nobeyama Radio Observatory 45-m telescope. The VLA
archival data of OH at 1720 MHz were also reanalyzed. We found two molecular
clouds with separate velocities, V_LSR=-14 km/s and +5 km/s. These clouds show
rough spatial anti-correlation. Both clouds are associated with OH 1720 MHz
emissions in the area overlapping with the Tornado Nebula. The spatial and
velocity coincidence indicates violent interaction between the clouds and the
Tornado nebula. Modestly excited gas prefers the position of the Tornado "head"
in the -14 km/s cloud, also suggesting the interaction. Virial analysis shows
that the +5 km/s cloud is more tightly bound by self-gravity than the -14 km/s
cloud. We propose a formation scenario for the Tornado Nebula; the +5 km/s
cloud collided into the -14 km/s cloud, generating a high-density layer behind
the shock front, which activates a putative compact object by
Bondi-Hoyle-Lyttleton accretion to eject a pair of bipolar jets.
