3D photoionization structure and distances of Planetary Nebulae III. NGC 6781

Abstract

Continuing our series of papers on the three-dimensional (3D) structures and accurate distances to Planetary Nebulae (PNe), we present our study of the planetary nebula NGC6781. For this object we construct a 3D photoionization model and, using the constraints provided by observational data from the literature we determine the detailed 3D structure of the nebula, the physical parameters of the ionizing source and the first precise distance. The procedure consists in simultaneously fitting all the observed emission line morphologies, integrated intensities and the two-dimensional (2D) density map from the [SII] (sulfur II) line ratios to the parameters generated by the model, and in an iterative way obtain the best fit for the central star parameters and the distance to NGC6781, obtaining values of 950±143 pc (parsec – astronomic distance unit) and 385 L(solar luminosity) for the distance and luminosity of the central star respectively. Using theoretical evolutionary tracks of intermediate and low mass stars, we derive the mass of the central star of NGC6781 and its progenitor to be 0.60±0.03MQ (solar mass) and 1.5±0.5MQ respectively.