Neutral hydrogen in the CAS OB6 association.

The present investigation of neutral hydrogen distribution and kinematics around the H II regions W 3, W 4, and W 5, as well as the SNR HB3 (which belongs to the Cas OB6 association), has discovered the development of an expanding neutral shell around W 4 which suggests that W 4 is the oldest H II region in the association. The neutral shell expansion is of the blister type, and on a scale of 50-100 pc, due to its location at the edge of a large neutral and molecular cloud complex. The shell has reached W 3 in the west, and may have initiated star formation, while tunnels direct away from the W 4 cavity towards W 5 in the east, where an expanding shell has begun to form.