The fragmenting superbubble associated with the H II region W4.
WEST J.L., ENGLISH J., NORMANDEAU M. and LANDECKER T.L.
Abstract (from CDS):
New observations at high latitudes above the H II region W4 show that the structure formerly identified as a chimney candidate, an opening to the Galactic halo, is instead a superbubble in the process of fragmenting and possibly evolving into a chimney. Data at high Galactic latitudes (b>5°) above the W3/W4 star-forming region at 1420 and 408 MHz Stokes I (total power) and 1420 MHz Stokes Q and U (linear polarization) reveal an egg-shaped structure with morphological correlations between our data and the Hα data of Dennison, Topasna, and Simonetti. Polarized intensity images show depolarization extending from W4 up the walls of the superbubble, providing strong evidence that the radio continuum is generated by thermal emission coincident with the Hα emission regions. We conclude that the parts of the H II region hitherto known as W4 and the newly revealed thermal emission are all ionized by the open cluster OCl 352. At an assumed distance of 2.35 kpc, the ovoid structure is 164 pc wide and extends 246 pc above the midplane of the Galaxy. The shell's emission decreases in total intensity and polarized intensity in various locations, appearing to have a break at its top and another on one side. Using a geometric analysis of the depolarization in the shell's walls, we estimate that a magnetic field line-of-sight component of 3-5 µG exists in the shell. We explore the connection between W4 and the Galactic halo, considering whether sufficient radiation can escape from the fragmenting superbubble to ionize the kiloparsec-scale Hα loop discovered by Reynolds, Sterling and Haffner.