The Cassiopeia Filament: A Blown Spur of the Local Arm

TL;DR

This study uses CO observations to identify and analyze the Cassiopeia Filament, a large-scale molecular structure likely formed as a spur of the Local arm due to galactic shear, revealing its morphology, kinematics, and possible stellar wind influence.

Contribution

It provides the first detailed characterization of the Cassiopeia Filament as a galactic spur with associated shell and bubble structures, linking its formation to galactic shear and stellar wind effects.

Findings

The filament is approximately 390 pc long and consistent with inter-arm large-scale filaments.

A giant shell and bubble structure suggest stellar wind influence from a supernova progenitor.

The filament was initially quiescent before stellar wind-induced turbulence.

Abstract

We present wide-field and high-sensitivity CO(1-0) molecular line observations toward the Cassiopeia region, using the 13.7m millimeter telescope of the Purple Mountain Observatory (PMO). The CO observations reveal a large-scale highly filamentary molecular cloud within the Galactic region of 132\fdg0\,$\geq$\,$l$\,$\geq$\,122\fdg0 and -1\fdg0\,$\leq$\,$b$\,$\leq$\,3\fdg0 and the velocity range from approximately +1 to +4 km/s. The measured length of the large-scale filament, referred to as the Cassiopeia Filament, is about 390 pc. The observed properties of the Cassiopeia Filament, such as length, column density, and velocity gradient, are consistent with those synthetic large-scale filaments in the inter-arm regions. Based on its observed properties and location on the Galactic plane, we suggest that the Cassiopeia Filament is a spur of the Local arm, which is formed due to the galactic shear. The western end of the Cassiopeia Filament shows a giant arc-like molecular gas shell, which is extending in the velocity range from roughly -1 to +7 km/s. Finger-like structures, with systematic velocity gradients, are detected in the shell. The CO kinematics suggest that the large shell is expanding at a velocity of ~6.5 km/s. Both the shell and finger-like structures outline a giant bubble with a radius of ~16 pc, which is likely produced by stellar wind from the progenitor star of a supernova remnant. The observed spectral linewidths suggest that the whole Cassiopeia Filament was quiescent initially until its west part was blown by stellar wind and became supersonically turbulent.