Studies of NGC 6720 with Calibrated HST WFC3 Emission-Line Filter Images--I: Structure and Evolution

TL;DR

This study provides a detailed 3-D structural model of NGC 6720 using HST WFC3 images and spectral data, revealing the nebula's complex geometry and ionization history.

Contribution

It introduces a new 3-D model of NGC 6720 based on calibrated HST WFC3 images and spectral analysis, enhancing understanding of its structure and evolution.

Findings

Main Ring is an ionization-bounded irregular disk with a central cavity.

Outer halos are fossil radiation from earlier ionization stages.

Calibrated WFC3 filters accurately isolate emission lines despite broadening.

Abstract

We have performed a detailed analysis of the Ring Nebula (NGC 6720) using HST WFC3 images and derived a new 3-D model. Existing high spectral resolution spectra played an important supplementary role in our modeling. It is shown that the Main Ring of the nebula is an ionization-bounded irregular non-symmetric disk with a central cavity and perpendicular extended lobes pointed almost towards the observer. The faint outer halos are determined to be fossil radiation, i.e. radiation from gas ionized in an earlier stage of the nebula when it was not ionization bounded. The narrow-band WFC3 filters that isolate some of the emission-lines are affected by broadening on their short wavelength side and all the filters were calibrated using ground-based spectra. The filter calibration results are presented in an appendix.