Previous abstract Next abstract
Session 7 - Planetary & Other Nebulae.
Display session, Monday, June 09
South Main Hall,
HST/WFPC2 observations of NGC 6888 made in June 1995 are presented. The 1100 second exposures in [OIII] (F502N), H\alpha (F656N), and [SII] (F673N) were centered on a portion of the bright northeastern limb.
NGC 6888 is a wind-blown nebula associated with the Wolf-Rayet (WN6) star WR 136 (HD 192163). The morphology and dynamics of such nebulae serve as laboratories into stellar mass loss and wind interactions, as well as the effect of such systems on the interstellar medium. Fabry-Perot imaging of the nebula has shown it to be a prolate ellipsoid 7.6 pc x 5.0 pc in size, inclined 45 degrees out of the plane of the sky (Lozhinskaya 1970). The wind from the central star (10^-6 - 10^-5 M_ødotyr^-1, V_\infty \sim 2000 km-s^-1) has swept up matter lost in earlier phases of mass loss into a shell. The shell has been fragmented via interaction with the stellar wind; the breakout of this stellar wind can be seen in the [OIII] emission external to the filament system, confirming previous observations (Dufour 1989). Gasdynamical simulations of the wind-wind interactions qualitatively support this picture (Garcìa-Segura and Mac Low 1995). Previous studies have shown that the filaments are overabundant in nitrogen and helium (Parker 1978), reflecting the nucleosynthetic processes of the central star.
The high spatial resolution of the images (1 WF pixel = 2.2 \times 10^10 km with an assumed distance of 1450 pc) affords a view of the ionization structure of individual filaments in the fragmented shell. This stratification is the result of the combination of radiative shocks and ionization fronts responsible for the observed emission. Preliminary analysis of the region will be presented.
Program listing for Monday