2008A&A...481..107V

We analyse the point-symmetric planetary nebula NGC6309 in terms of its three-dimensional structure and of internal variations of the physical conditions to deduce the physical processes involved in its formation. We used VLA-D λ3.6-cm continuum, ground-based, and HST-archive imaging as well as long slit high- and low-dispersion spectroscopy. The low-dispersion spectra indicate a high excitation nebula, with low to medium variations of its internal physical conditions (10600K≲T_e[OIII]≲10900K; 10100K≲T_e[NII]≲11800K; 1440cm^–3≲N_e[SII]≲4000cm^–3; 1700cm^–3≲N_e[ClIII]≲2600cm^–3; 1000cm^–3≲N_e[ArIV]≲1700cm^–3). The radio continuum emission indicates a mean electron density of ≃1900cm^–3, emission measure of 5.1x10⁵pc/cm6, and an ionised mass M(HII)≃0.07M_☉. In the optical images, the point-symmetric knots show a lack of [NII] emission as compared with similar features previously known in other PNe. A rich internal structure of the central region is seen in the HST images, resembling a deformed torus. Long slit high-dispersion spectra reveal a complex kinematics in the central region, with internal expansion velocities ranging from ≃20 to 30km/s. In addition, the spectral line profiles from the external regions of NGC6309 indicate expanding lobes (≃40km/s) as those generally found in bipolar nebulae. Finally, we have found evidence for the presence of a faint halo, possibly related to the envelope of the AGB-star progenitor. Our data indicate that NGC6309 is a quadrupolar nebula with two pairs of bipolar lobes whose axes are oriented PA=40° and PA=76°. Equatorial and polar velocities for these two pairs of lobes are 29 and 86 km/s for the bipolar system at PA=40° and 25 and 75km/s for the bipolar system at PA=76°. There is also a central torus that is expanding at 25 km/s. Kinematical age for all these structures is around 3700 to 4000yr. We conclude that NGC6309 was formed by a set of well-collimated bipolar outflows (jets), which were ejected in the initial stages of its formation as a planetary nebula. These jets carved the bipolar lobes in the previous AGB wind and their remnants are now observed as the point-symmetric knots tracing the edges of the lobes.