The hypersonic, bipolar, knotty outflow from the engraved hourglass planetary nebula MyCn 18

The remarkable velocity structure of the different components of the young planetary nebula MyCn 18 have been revealed by obtaining imagery and spatia lly resolved spectrometry of the H alpha and [N II] lambda lambda 6548, 658 4 lines with the Manchester echelle spectrometer combined with the 3.9 m An glo-Australian telescope. The bright, bipolar, nebular core is shown to be composed of two extended hemispherical cavities whose axes are tilted at 52 degrees to the plane of the sky. Ionized flows, at less than or equal to 9 0 km s(-1) and parallel to the walls of these cavities, are occurring. The full extent of the elongated bipolar assembly of highspeed knots which appa rently lie along the same axis is now revealed in a continuum-subtracted im age in the light of the H alpha and [N II] lambda lambda 6548, 6584 nebular emission lines. Complete spatial coverage of line profiles from these knot s is also presented for the first time. In their most likely configuration, these knots are shown to have a range of outflowing speeds of less than or equal to 630 km s(-1) that are proportional to their distance from the cen tral star. There is some degree of point/velocity symmetry, indicating that some pairs of knots have been ejected in opposing directions at the same s peed. Curiously, the line profiles from the knots are very narrow, i.e., fr om 15 to 30 km s(-1). Among several possible explanations of the origin of these hypersonic knots is a recurrent nova-like ejection from a central bin ary star.