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Session 15 - Planetary Nebulae and White Dwarfs.
Display session, Wednesday, January 07
Exhibit Hall,
We present high spatial resolution ground-- and space--based (HST) images of ionized gas and dust in the young, low--excitation Planetary Nebula M 4--18. Images of M 4--18 have been obtained in the optical (with WFPC2/HST), the near--IR, and at mid--IR wavelengths. The optical--H\alpha image shows a complex morphology: a central cavity around the central star is surrounded by a toroidal shell. The emission peaks in two clumps which are disposed symmetrically (N-S) about the central star, and lie outside the cavity. These clumps appear to delineate the limb--brightened walls of the torus. The mid-IR images of thermal emission from warm (T\approx200K) dust also show a two--lobed morphology, but the position angle of the lobes lies orthogonal to the equatorial axis, as defined by the H\alpha toroid. The difference between the dust emission and the ionized gas suggests that, the mid-IR peaks trace regions of hotter dust grains, rather than define the higher density equatorial plane of the nebula. Alternatively, it is possible that gas and dust are spatially separated, i.e. regions of highest gas density do not coincide with regions of highest dust density. We have created a code that constructs 3-D spatial and kinematical models of PNe, and applied it to M 4--18 and MyCn 18 (the Hourglass Nebula). Our results illustrate that optically thin, axially symmetric models are able to reproduce the overall structure and kinematics of the two sources.
