Imaging of the Egg Nebula (CRL 2688) with WFPC2/HST: A history of age/post-AGB giant branch mass loss

Abstract

The proto-planetary nebula, CRL 2688, has been imaged through a wideband filter centered at 606 nm (F606W) with the Wide Field Planetary Camera 2 onboard the Hubble Space Telescope. CRL 2688 is the prototypical bipolar reflection nebula in which a star is surrounded by a dense, flattened cocoon of dust seen nearly edge-on and starlight escapes preferentially along the polar directions producing a pair of bright nebulosities, one above and one below the equatorial plane. We find a pair of radial "searchlight beams" emerging from within the dusty cocoon which intersect at the position expected for the central star when extrapolated inside the cocoon. The beams are crisscrossed by a large number of roughly round arcs with their center of curvature in the vicinity of the central star. The arcs are not systematically elongated along the polar axis of the nebula, as would be expected in the current model of CRL 2688 where the nebular density decreases with latitude. Our image directly shows the last ~ 13,000 yr history of mass ejection from the central star while it was on the tip of the asymptotic giant branch (AGE). We find that the average surface brightness varies as r^(-3.7), implying that the average mass-loss rate or the scattering opacity of the dust grains varies as r^(-0.7), i.e., one or both of these parameters have steadily increased with time. The temporal resolution of ≈25 yr in our images has provided direct evidence for episodic increases in the mass-loss rate by factors of ~2 or more, occurring every 150-450 yr, and lasting over periods of 75-200 yr. These irregularities in the mass-loss rate are roughly spherically symmetric. We have resolved the edges of the searchlight beams, as well as the peculiar structure of the inner region of the nebula (1".5 < radius < 6"). Our data require a new model for CRL 2688 in which the beams result from starlight escaping through a pair of nonuniform annular holes in the dust cocoon which are coaxial with the polar axis of the nebula. The holes have probably been generated by a young (less than 200 yr) high-velocity outflow which streams out through these holes and interacts with the surrounding dense AGE wind to produce the peculiar structure of the inner nebula. The cocoon contains dust grains of size about 0.6 µm, significantly larger than those in the extended nebula.

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