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 spatially resolved spectrometry of the Hα and [N ΙΙ] λλ6548, 6584 lines with the Manchester echelle spectrometer combined with the 3.9 m Anglo-Australian telescope. The bright, bipolar, nebular core is shown to be composed of two extended hemispherical cavities whose axes are tilted at 52° to the plane of the sky. Ionized flows, at ≤90 km s^-1 and parallel to the walls of these cavities, are occurring. The full extent of the elongated bipolar assembly of high-speed knots which apparently lie along the same axis is now revealed in a continuum-subtracted image in the light of the Hα and [N ΙΙ] λλ6548, 6584 nebular emission lines. Complete spatial coverage of line profiles from these knots is also presented for the first time. In their most likely configuration, these knots are shown to have a range of outflowing speeds of ≤630 km s^-1 that are proportional to their distance from the central star. There is some degree of point/velocity symmetry, indicating that some pairs of knots have been ejected in opposing directions at the same speed. Curiously, the line profiles from the knots are very narrow, i.e., from 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 binary star.