SIMBAD references

We analyze optical images and high-resolution, long-slit spectra of three planetary nebulae that possess collimated, low-ionization features. NGC 3918 is composed of an inner, spindle-shaped shell mildly inclined with respect to the plane of the sky. Departing from the polar regions of this shell, we find a two-sided jet expanding with velocities that increase linearly with distance from 50 to 100 km.s^–1. The jet is probably coeval with the inner shell (age ~1000D yr, where D is the distance in kpc), suggesting that its formation should be ascribed to the same dynamical processes that also shaped the main nebula, and not to a more recent mass-loss episode. We discuss the formation of the aspherical shell and jet in the light of current hydrodynamical and magnetohydrodynamical theories. K1-2 is a planetary nebula with a close binary nucleus that shows a collimated string of knots embedded in a diffuse, elliptical shell. The knots expand with a velocity similar to that of the elliptical nebula (∼25 km.s^–1), except for an extended tail located out of the main nebula, which linearly accelerates up to ∼45 km.s^–1. We estimate an inclination on the line of the sight of ∼40° for the string of knots; once the orientation of the orbit is also determined, this information will allow us to test the prediction of current theories of the occurrence of polar jets from close binary systems. Wray 17-1 has a complex morphology, showing two pairs of low-ionization structures located in almost perpendicular directions from the central star, and embedded in a large, diffuse nebula. The two pairs show notable similarities and differences, and their origin is very puzzling.