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Session 53 - SNs, Planetaries and Circumstellar Disks.
Display session, Wednesday, June 10
We present new HST/WFPC2 imagery for the planetary nebula (PN) NGC 6818. Observations were made in line filters F437N, F487N, F502N, and F656N plus continuum filter F547M. The primary goal was to develop a high spatial resolution (\sim0.1'') map of the intrinsic line ratio [O III] 4363/5007 and thereby evaluate the electron temperature (T_e) and the mean-square T_e variation (t^2) across the PN. In this process we developed an extinction map from the F487N (H\beta) and F656N (H\alpha) images by comparing the observed line ratios in each pixel to the theoretical ratio and computing a c(H\beta) map which was used to correct the observed 4363/5007 ratios for reddening. We also adjust for the continuum contribution to the line filter data. We present color-coded pictures of the reddening (c(H\beta)) map, the [O III] T_e map, as well as our determinations of t^2. The T_e map shows a decline from \sim14000 K in the inner regions to \sim11000 K at the outer edge. Such a radial T_e gradient is expected for a high-excitation nebula with a prominent He^++ zone such as NGC 6818.
A composite of images taken in 3 filters (F656N, red; F487N, blue; and F502N, [O III] 5007, green) shows a roughly spherical outer envelope as well as a brighter vase-shaped interior ``bubble". There is a prominent orifice to the North and a smaller one to the South, along the major axis, possibly caused by a blow-out from a fast wind. This nebula has an appearance remarkably similar to that of the PN NGC 3918 previously imaged with HST by H.\ Bond.
We note from the continuum images (F547M) two stars in the nebular field that are fainter than the prominent central star; these are roughly 2--4'' N and NE of the central star. Further study is needed to establish whether or not there may be a physical association of either star with the central star.
Supported by AURA/STScI grant related to GO-6792.
Program listing for Wednesday