Bibcode
DOI
Miranda, Luis F.; Guerrero, Martín A.; Torrelles, José M.
Bibliographical reference
The Astronomical Journal, Volume 117, Issue 3, pp. 1421-1432.
Advertised on:
3
1999
Citations
36
Refereed citations
25
Description
We present Hα, [N II] lambda6583, [O III] lambda5007 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 Hα 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 (3.8"x2.8", P.A.~=32^deg),
high-excitation ([N II] lambda6583/Hα<=0.15) ellipsoidal shell
seen almost pole-on with polar and equatorial expansion velocities in
the Hα line of ~=25 and ~=19 km s^-1, respectively; (2) two
point-symmetric, low-excitation ([N II]/Hα~=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 (~=55 km s^-1) and a large
precession angle (2phi~=120^deg). The deduced precession period
(~=500x[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]/Hα 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.