Observations and three-dimensional ionization structure of the planetary nebula SuWt 2¹

Ashkbiz Danehkar,
Research Centre in Astronomy, Astrophysics & Astrophotonics, Macquarie University, Sydney, NSW 2109, Australia

Quentin A. Parker, and
Research Centre in Astronomy, Astrophysics & Astrophotonics, Macquarie University, Sydney, NSW 2109, Australia
Australian Astronomical Observatory, PO Box 296, Epping, NSW 1710, Australia

Barbara Ercolano
Universitäts-Sternwarte München, Ludwig-Maxmilians Universität München, Scheinerstr. 1, D-81679 München, Germany
Excellence Cluster Universe, Boltzmannstr. 2, D-85748 Garching, Germany

Date: Accepted 2013 June 17. Received 2013 June 16; in original form 2013 March 20

Abstract:

The planetary nebula SuWt 2 (PN G311.0+02.4), is an unusual object with a prominent, inclined central emission ellipse and faint bipolar extensions. It has two A-type stars in a proven binary system at the centre. However, the radiation from these two central stars is too soft to ionize the surrounding material leading to a so far fruitless search for the responsible ionizing source. Such a source is clearly required and has already been inferred to exist via an observed temporal variation of the centre-of-mass velocity of the A-type stars. Moreover, the ejected nebula is nitrogen-rich which raises question about the mass-loss process from a likely intermediate-mass progenitor. We use optical integral-field spectroscopy to study the emission lines of the inner nebula ring. This has enabled us to perform an empirical analysis of the optical collisionally excited lines, together with a fully three-dimensional photoionization modelling. Our empirical results are used to constrain the photoionization models, which determine the evolutionary stage of the responsible ionizing source and its likely progenitor. The time-scale for the evolutionary track of a hydrogen-rich model atmosphere is inconsistent with the dynamical age obtained for the ring. This suggests that the central star has undergone a very late thermal pulse. We conclude that the ionizing star could be hydrogen-deficient and compatible with what is known as a PG 1159-type star. The evolutionary tracks for the very late thermal pulse models imply a central star mass of $\sim 0.64 {\rm M}_{\bigodot}$, which originated from a $\sim 3 {\rm M}_{\bigodot}$ progenitor. The evolutionary time-scales suggest that the central star left the asymptotic giant branch about 25,000 years ago, which is consistent with the nebula's age.
 
Keywords: ISM: abundances - planetary nebulae: individual: PN SuWt 2
 
Journal Reference: A. Danehkar, Q. A. Parker, and B. Ercolano, Monthly Notices of the Royal Astronomical Society, 434:1513-1530, 2013. doi:10.1093/mnras/stt1116