Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992aas...181.6701r&link_type=abstract
American Astronomical Society, 181st AAS Meeting, #67.01; Bulletin of the American Astronomical Society, Vol. 24, p.1226
Astronomy and Astrophysics
Astronomy
Scientific paper
NGC 6888 is an oblong, clumpy shell of emission surrounding a WN6 star. It is ~ 15 ' long at its major axis, lies at a distance of about 1.5 kpc, and is expanding with a velocity of 85 km s(-1) . The shell is especially prominent in Hα and {[N II]}, but also has an {[O III]} `skin' which is tied to the shell in the NE and SW, but which extends well beyond it in the NW and SE where the shell appears to have been breached. We have used the Wide Field version of the imaging Fabry-Perot system developed for the 60" Palomar telescope to obtain narrow band {[O III]} and Hα images of NGC 6888. The instrument has a 16' X 16' field of view, a total system speed of f/1.65, and a velocity resolution of 20 km s(-1) . The spatial resolution was {0.96 "/pixel} with integration times of 15 minutes for {[O III]} and 5 minutes for Hα at each etalon setting. The {[O III]} skin around NGC 6888 appears to consist of wind-driven radiative shocks. Material behind the shocks has presumably cooled to the point of photoionization equilibrium with the radiation field of the star. The extremely high ratio of {[O III]}/Hβ in these back-illuminated shocks ( > 20) shows that they are optically thin to ionizing radiation. Further, the absence of a bright Hα halo indicates that the ambient medium has a lower density than was previously claimed, which in turn supports the idea of a significant contribution to the shell from stellar mass loss. Dynamically, shocks to the NW fountain from the breach rather than radiating from the star, suggesting a shock driven by internal pressure , and not a simple wind-driven shell. The patterns of the local shock velocities around the breaches in the shell allow us to estimate the time at which Rayleigh-Taylor instabilities became important.
Hester Jeff J.
Reed Josh E.
van Buren Dale
Vogel Stuart N.
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