High-velocity collimated outflows in planetary nebulae: NGC 6337, He 2-186, and K4-47

We have obtained narrowband images and high-resolution spectra of the plane tary nebulae NGC 6337, He 2-186, and K4-47 with the aim of investigating th e relation between their main morphological components and several low-ioni zation features present in these nebulae. The data suggest that NGC 6337 is a bipolar PN seen almost pole-on, with polar velocities of greater than or equal to 200 km s(-1). The bright inner ring of the nebula is interpreted to be the "equatorial" density enhancement. It contains a number of low-ion ization knots and outward tails that we ascribe to dynamical instabilities leading to fragmentation of the ring or transient density enhancements due to the interaction of the ionization front with previous density fluctuatio ns in the interstellar medium. The lobes show a pronounced point-symmetric morphology and two peculiar low-ionization filaments, the nature of which r emains unclear. The most notable characteristic of He 2-186 is the presence of two high-velocity (greater than or equal to 135 km s(-1)) knots from wh ich an S-shaped lane of emission departs toward the central star. K4-47 is composed of a compact core and two high-velocity, low-ionization blobs. We interpret the substantial broadening of line emission from the blobs as a s ignature of bow shocks, and using the modeling of Hartigan, Raymond, & Hart man, we derive a shock velocity of similar to 150 km s(-1) and a mild incli nation of the outflow on the plane of the sky. We discuss possible scenario s for the formation of these nebulae and their low-ionization features. In particular, the morphology of K4-47 hardly fits into any of the usually ado pted mass-loss geometries for single asymptotic giant branch stars. Finally , we discuss the possibility that point-symmetric morphologies in the lobes of NGC 6337 and the knots of He 2-186 are the result of precessing outflow s from the central stars.