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Session 44 - Circumstellar Material.
Display session, Tuesday, January 16
North Banquet Hall, Convention Center
As part of a multi-wavelength study of the mass-loss processes which red giant stars undergo on the Asymptotic Giant Branch (AGB) and beyond, we have obtained high spatial-resolution images of two post-AGB objects using the WFPC2 aboard the HST. The first is CRL2688 (the Egg Nebula), belonging to a class of bipolar reflection nebulae surrounding post-AGB stars in transition to becoming planetary nebulae. Our F606W (wide-band 606 nm filter) image shows a dark edge-on flaring disk obscuring the central star, a pair of radial "searchlight-beam" like features, criss-crossed by a large number (at least 25) of roughly circular arcs around the center. The arcs probably represent local peaks in a quasi-periodic mass-ejection process. Intriguingly, some of the arcs clearly intersect and/or bifurcate. Very faint radial striations can be seen within the "searchlight-beam" structures, indicating that these are jets of matter. We have also used the WFPC2 POLQ polarisation-quad filter to image the polarisation characteristics of the Egg Nebula. The second object is the young planetary nebula, MyCn-18, selected because of its striking resemblance to the triple-ring nebula around SN1987A (Burrows et al. 1995, ApJ, in press). We have imaged MyCn-18 in several narrow emission-line filters. Our H\alpha and [NII] images show an hourglass-shaped nebula, with the outer rings representing the hourglass rims. A pair of intersecting elliptical rings in the central region may be the rims of a smaller hourglass. A series of arcs appear to be engraved on the hourglass walls. Within the inner hourglass, one can see a barrel-shaped structure whose symmetry axis is apparently orthogonal to the axis of the hourglasses. We will characterise the detailed structure of the circumstellar material in CRL2688 and MyCn-18 which has been revealed unambiguously for the first time in our WFPC2 images, and discuss possible mechanisms for producing such structures, with reference to current theories for the formation of planetary nebulae.
