Millimeter-wave Molecular Line Observations of the Tornado Nebula

TL;DR

This study presents millimeter-wave molecular line observations of the Tornado Nebula, revealing interacting molecular clouds and proposing a formation scenario involving cloud collision and jet ejection from a compact object.

Contribution

First detailed millimeter-wave molecular line study of the Tornado Nebula, identifying cloud interactions and proposing a new formation mechanism involving cloud collision and jet activity.

Findings

Two molecular clouds with distinct velocities are spatially anti-correlated.

Both clouds are associated with OH 1720 MHz emissions, indicating interaction.

The +5 km/s cloud is more gravitationally bound than the -14 km/s cloud.

Abstract

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.