Physics
Scientific paper
Jul 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007hst..prop11232o&link_type=abstract
HST Proposal ID #11232. Cycle 16
Physics
Scientific paper
The Ring Nebula NGC 6720 represents an important stage in the evolution of planetary nebulae, being large enough that it has entered the post fast-wind stage yet has not reached the late ballistic phase of objects like the Helix Nebula. Understanding this nebula well presents the opportunity to determine how nebulae transition from their creation phase into the form they have as their material enters the interstellar medium. A recent study based on ground-based spectroscopy has derived a new and accurate model for the Ring Nebula. A well defined characteristic of this model is that it predicts a tangential velocity of 20 km/s whereas the application of its quite uncertain trigonometric parallax distance of 700 +450/-200 pc with the angular expansion velocity determined from HST observations with a 2 year time base indicates a tangential velocity of 69 +45/-20 km/s. This disagreement indicates that either the distance is even more uncertain than thought or that the earlier angular velocity is incorrect. We propose to make a new set of observations of the Ring Nebula in the diagnostic emission line filters F469N HeII, F502N [OIII], and F658N [NII] that will produce much more accurate angular velocities than the previous study by having a time base of 8.8 years and imaging the nebula entirely within a single CCD of the WFPC2. The primary result from this study will be an accurate distance to this important nebula and from this to be able to use this object to refine our picture of how planetary nebulae evolve during middle-age.;
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