Jets, knots, and tails in planetary nebulae: NGC 3918, K1-2, and Wray 17-1

We analyze optical images and high-resolution, long-slit spectra of three p lanetary 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, w e find a two-sided jet expanding with velocities that increase linearly wit h distance from 50 to 100 km s(-1) The jet is probably coeval with the inne r shell (age approximate to 10000 yr, where D is the distance in kpc), sugg esting that its formation should be ascribed to the same dynamical processe s that also shaped the main nebula, and not to a more recent mass-loss epis ode. We discuss the formation of the aspherical shell and jet in the light of current hydrodynamical and magnetohydrodynamical theories. K1-2 is a pla netary nebula with a close binary nucleus that shows a collimated string of knots embedded in a diffuse, elliptical shell. The knots expand with a vel ocity similar to that of the elliptical nebula (similar to 25 km s(-1)), ex cept for an extended tail located out of the main nebula, which linearly ac celerates up to similar to 45 km s(-1). We estimate an inclination on the l ine of the sight of similar to 40 degrees for the string of knots; once the orientation of the orbit is also determined, this information will allow u s to test the prediction of current theories of the occurrence of polar jet s from close binary systems. Wray 17-1 has a complex morphology, showing tw o pairs of low-ionization structures located in almost perpendicular direct ions from the central star, and embedded in a large, diffuse nebula. The tw o pairs show notable similarities and differences, and their origin is very puzzling.