The Cassiopeia Filament: A Blown Spur of the Local Arm

Abstract

We present wide-field and high-sensitivity CO(1-0) molecular line observations toward the Cassiopeia region, using the 13.7 m millimeter telescope of the Purple Mountain Observatory. The CO observations reveal a large-scale highly filamentary molecular cloud within the Galactic region of 132.°0 ≥ l ≥ 122.°0 and -1.°0 ≤ b ≤ 3.°0 and the velocity range from approximately +1 to +4 km s^-1. The measured length of the large-scale filament, referred to as the Cassiopeia Filament, is ~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 extends in the velocity range from roughly -1 to +7 km s^-1. 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^-1. Both the shell and finger-like structures outline a giant bubble with a radius of ~16 pc, which is likely produced by the stellar wind from the progenitor star of a supernova remnant. The observed spectral line widths suggest that the whole Cassiopeia Filament was quiescent initially until its west part was blown by the stellar wind and became supersonically turbulent.