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

¹ Instituto de Astronomía, Universidad Nacional Autónoma de México, Apdo. Postal 877, 22800 Ensenada, B. C., Mexico e-mail: [vazquez;mcontreras;fguillen]@astrosen.unam.mx
² Instituto de Astrofísica de Andalucía, CSIC, PO Box 3004, 18080 Granada, Spain e-mail: lfm@iaa.es
³ Instituto de Astronomía, Universidad Nacional Autónoma de México, Apdo. Postal 70-264, 04510 México, D. F., Mexico e-mail: lorenzo@astrosen.unam.mx
⁴ Centro de Radioastronomía y Astrofísica, Universidad Nacional Autónoma de México, Apdo. Postal 3-72 (Xangari), 58089 Morelia, Mich., Mexico e-mail: s.ayala@astrosmo.unam.mx
⁵ Instituto de Ciencias del Espacio (CSIC) – IEEC, Facultat de Física, Universitat de Barcelona, Av. Diagonal 647, 08028 Barcelona, Spain e-mail: torrelles@ieec.fcr.es

Received: 21 August 2007
Accepted: 18 December 2007

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 -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 ([] ; [] ; [] ; [] ; [] ). 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() . In the optical images, the point-symmetric knots show a lack of [] 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 = and PA = . Equatorial and polar velocities for these two pairs of lobes are 29 and 86 km s^-1 for the bipolar system at PA = and 25 and 75 km s^-1 for the bipolar system at PA = . 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.