The Illumination and Growth of CRL 2688: An Analysis of New & Archival HST Observations

TL;DR

This study uses HST observations to analyze the structure, expansion, and dust properties of CRL 2688, revealing insights into its nebular evolution, ring ejection history, and geometric configuration.

Contribution

It provides new measurements of nebular expansion, ring ejection timing, and develops a detailed dust scattering model of CRL 2688, enhancing understanding of its formation and evolution.

Findings

The N-S lobes expanded uniformly by 2.5%.

Rings were ejected every 100 years for ~4 millennia.

Distance to CRL 2688 is estimated at 300-350 pc.

Abstract

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.