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B. D. Moore, J. J. Hester, P. A. Scowen (Arizona State University)
NGC 7635 is the bright inner component of the large emission region S162. The most prominent emission from the nebula is a roughly spherical bubble 3' in diameter, earning NGC 7635 the popular name of the ``Bubble Nebula.'' We present images of the NE sector of the nebula taken with the HST WF/PC in the light of emission from H\alpha, [\ion{O}{3}], and [\ion{S}{2}]. We also present ground based images and spectroscopy. Assuming a distance to the nebula of 3.6 kpc, our linear resolution is 5.4 \times 1015 cm, which allows us to investigate the stratified ionization structure associated with the bubble and the H~II region ionization front.
The bubble in NGC 7635 is the result of a fast stellar wind expanding into the interior of the larger H~II region. However, the central star BD +60 2522 is appreciably offset (by about 1') from the center of the bubble in the direction of the wall of the dense molecular cloud that defines this blister H~II region. This offset is the result of evolution of the wind bubble into the density and pressure gradient established by the photoevaporative flow away from the cavity wall. The physical conditions around the bubble vary according to the medium into which the bubble is expanding. Away from the cavity wall the bubble is expanding into the low density interior of the H~II region. Toward the wall, in the region of our images, the wind termination shock is very near the ionization front. The resulting physical structure, in which the photoevaporative flow away from the cloud wall is confined by the ram pressure of the wind, will be discussed.