Multiwavelength imaging and long-slit spectroscopy of the planetary nebula NGC 6884: The discovery of a fast precessing, bipolar collimated outflow

We present H alpha, [N II] lambda 6583, [O III] lambda 5007 ground-based images, Hubble Space Telescope (HST) (V band) archival images, VLA-A 3.6 cm continuum data, and high-resolution long-slit spectra of the planetary nebula NGC 6884. The VLA and HST images show an extremely knotty and filamentary nebula with an elliptical central structure also recognizable in H alpha and [O III]. The [N II] image is different and is dominated by two bright arclike structures. The image ratio maps indicate a complex ionization structure within the nebula with low-excitation filaments surrounded by high-excitation regions. The spectra make it possible to distinguish three basic structural components in NGC 6884: (1) a compact (3.'18 x 2.'8, P.A. similar or equal to 32 degrees), high-excitation ([N II] lambda 6583/H alpha less than or equal to 0.15) ellipsoidal shell seen almost pole-on with polar and equatorial expansion velocities in the Ha line of similar or equal to 25 and similar or equal to 19 km s(-1), respectively; (2) two point-symmetric, low-excitation ([N II]/H alpha similar or equal to 0.6-1) spirals in which the distance to the central star and the radial velocity exhibit systematic variations; (3) two compact, apparently isolated, low-excitation bipolar knots. All data agree that the spirals represent precessing bipolar jetlike outflows. The observational properties of these precessing outflows can satisfactorily be explained with an approximately constant ejection velocity (similar or equal to 55 km s(-1)) and a large precession angle (2 phi similar or equal to 120 degrees). The deduced precession period (similar or equal to 500 x [D(kpc)/2] yr) is the shortest one measured so far in this kind of structure in planetary nebulae. The image ratio maps and the [N II]/H alpha intensity ratio suggest shock excitation in the precessing jets. A general discussion of NGC 6884 is presented within the context of planetary nebulae with jetlike components.