VLT Observations of the Asymmetric Etched Hourglass Nebula, MyCn 18

The Etched Hourglass Nebula is a typical tight-waisted, asymmetric, bipolar Planetary Nebula (PN). Here we consider near infra-red imagery, infra-red and long-slit narrowband spectroscopy and morpho-kinematic modelling with the view of understanding the structure of the nebula in terms of its ionisation along different directions, the origins of the extreme bipolar shaping and the effect of the off-centre central star. A series of infra-red images of MyCn 18, obtained using NACO (Naos-Conica) on ESO’s VLT (UT4) telescope, shows the full extent of the main hourglass shell and the finely detailed features of the nebula. Two-micron ISAAC spectra from a slit position intersecting the narrow waist of the nebula demonstrate that the ionised gas resides closer to the centre of the nebula than the molecular emission. The nebula is clearly ionisation-bounded in this central direction, in contrast to the open ends of the hourglass structure. The shape and structure of MyCn 18 was reconstructed using a morpho-kinematic code to better describe the nebula’s morphology. The putative central star, the much smaller ‘inner hourglass’, and the main nebulosity are somewhat offset from the geometric centre of the nebula, indicating the possibility of a binary companion. The role of exoplanets is also considered in the breaking of spherical symmetry of MyCn 18.