The physical structure of the point-symmetric and quadrupolar planetary nebula NGC 6309

Abstract

Aims:We analyse the point-symmetric planetary nebula NGC 6309 in terms of its three-dimensional structure and of internal variations of the physical conditions to deduce the physical processes involved in its formation.
Methods: We used VLA-D λ3.6-cm continuum, ground-based, and HST-archive imaging as well as long slit high- and low-dispersion spectroscopy.
Results: The low-dispersion spectra indicate a high excitation nebula, with low to medium variations of its internal physical conditions (10 600~K ⪉ T_e[O III] ⪉ 10 900~K; 10 100~K ⪉ T_e[N II] ⪉ 11 800~K; 1440~cm^-3 ⪉ N_e[S II] ⪉ 4000~cm^-3; 1700~cm^-3 ⪉ N_e[Cl III] ⪉ 2600~cm^-3; 1000~cm^-3 ⪉ N_e[Ar IV] ⪉ 1700~cm^-3). The radio continuum emission indicates a mean electron density of ≃1900 cm^-3, emission measure of 5.1 × 10⁵ pc cm^-6, and an ionised mass M(H II) ≃ 0.07~M_⊙. In the optical images, the point-symmetric knots show a lack of [N II] 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 30 km s^-1. In addition, the spectral line profiles from the external regions of NGC 6309 indicate expanding lobes (≃40 km s^-1) 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.
Conclusions: Our data indicate that NGC 6309 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^-1 for the bipolar system at PA = 40° and 25 and 75 km s^-1 for the bipolar system at PA = 76°. There is also a central torus that is expanding at 25 km s^-1. Kinematical age for all these structures is around 3700 to 4000 yr. We conclude that NGC 6309 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.