Studies of NGC 6720 with Calibrated HST WFC3 Emission-Line Filter Images--III:Tangential Motions using AstroDrizzle Images

TL;DR

This study measures the tangential motions within the Ring Nebula using Hubble images taken over nearly 13 years, confirming homologous expansion and estimating the nebula's distance and age.

Contribution

It provides precise tangential velocity measurements and confirms the homologous expansion model for NGC 6720, also estimating its distance and age.

Findings

Expansion is faster along the major axis.

Dark knots expand more slowly than gas.

Distance to the nebula is approximately 720 parsecs.

Abstract

We have been able to compare with astrometric precision AstroDrizzle processed images of NGC 6720 (the Ring Nebula) made using two cameras on the Hubble Space Telescope. The time difference of the observations was 12.925 yrs. This large time-base allowed determination of tangential velocities of features within this classic planetary nebula. Individual features were measured in [N II] images as were the dark knots seen in silhouette against background nebular [O III] emission. An image magnification and matching technique was also used to test the accuracy of the usual assumption of homologous expansion. We found that homologous expansion does apply, but the rate of expansion is greater along the major axis of the nebula, which is intrinsically larger than the minor axis. We find that the dark knots expand more slowly that the nebular gas, that the distance to the nebula is 720 pc +/-30%, and the dynamic age of the Ring Nebula is about 4000 yrs. The dynamic age is in agreement with the position of the central star on theoretical curves for stars collapsing from the peak of the Asymptotic Giant Branch to being white dwarfs.