Physics
Scientific paper
Jul 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009hst..prop12157u&link_type=abstract
HST Proposal ID #12157. Cycle 18
Physics
Scientific paper
We will undertake differential proper-motion measurements of the enigmatic concentric arcs in the circumstellar shell of the Cygnus Egg Nebula using the archived two-epoch optical imaging-polarimetric data taken with WFPC2 and ACS. These circular concentric arcs are enigmatic because they co-exist and seemingly overlap with the object's well-known bipolar nebulosities, imposing a very perplexing mass-loss history to fathom. Previously we performed differential proper-motion measurements of the bipolar lobes of the object using two-epoch NICMOS images and determined the distance to the object to be 420 pc. Given this distance and the 7.25-yr time baseline of the optical wide-field imaging-polarimetric data, we can measure the expansion velocities of these concentric arcs directly and determine the mode of expansion of the arcs with respect to that of the bipolar lobes in order to assess the nature of these arc structures. Our proposed investigation is novel because we attempt to defy the confusion due to the projection effects by using imaging-polarimetric data. It is suspected that the observed concentric arcs are projections of the concentric spherical shells onto the plane of the sky, and hence, the apparent intervals between the neighboring arcs do not truly represent the physical spacing between the adjacent concentric spherical shells because of the confusion that arises as a result of the projection effects. Our proposed use of the polarized-flux-only maps permits us to recover the cross-section of the concentric spherical shells by exploiting the scattering-angle-dependent nature of the polarized flux. In such cross-sectional polarized surface brightness maps, the apparent spacing between two adjacent arcs truly represents the physical spacing between them. Therefore, we can measure relative motion of these concentric arcs/spheres without the confusion due to the projection effects and assess how the arcs expand in relation to the bipolar lobes, yielding new observational insights pertaining to kinematics of these enigmatic concentric arcs that can be tested against various existing theoretical models.;
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