Bruce Balick, Thomas Gomez, Dejan Vinković, Javier Alcolea, Romano L. M. Corradi, Adam Frank
We present four-color images of CRL 2688 obtained in 2009 using the
Wide-Field Camera 3 on HST. The F606W image is compared with archival images in
very similar filters to monitor the proper motions of nebular structure. We
find that the bright N-S lobes have expanded uniformly by 2.5% and that the
ensemble of rings has translated radially by 0.07 in 6.65 y. The rings were
ejected every 100y for ~4 millennia until the lobes formed 250y ago. Starlight
scattered from the edges of the dark E-W dust lane is coincident with extant H2
images and leading tips of eight pairs of CO outflows. We interpret this as
evidence that fingers lie within geometrically opposite cones of opening angles
{\approx} 30{\circ} like those in CRL618. By combining our results of the rings
with 12CO absorption from the extended AGB wind we ascertain that the rings
were ejected at ~18 km s-1 with very little variation and that the distance to
CRL2688, v_{exp}$ / ${\dot\theta}_exp$, is 300 - 350 pc. Our 2009 imaging
program included filters that span 0.6 to 1.6{\mu}m. We constructed a
two-dimensional dust scattering model of stellar radiation through CRL2688 that
successfully reproduces the details of the nebular geometry, its integrated
spectral energy distribution, and nearly all of its color variations. The model
implies that the optical opacity of the lobes >~ 1, the dust particle density
in the rings decreases as radius^{-3} and that the mass and momentum of the AGB
winds and their rings have increased over time.
View original:
http://arxiv.org/abs/1110.5678
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