The Nature of the Stingray Nebula from Radio Observations

TL;DR

This study analyzes radio observations of the Stingray nebula over 25 years, revealing a decline in flux density, structural features, and potential signs of non-thermal emission, indicating evolving physical processes.

Contribution

First spatially resolved radio images of the Stingray nebula were produced, and the study provides insights into its changing emission properties and possible non-thermal components.

Findings

Radio flux density declined from 1991 to 2011, then stabilized.

A ring structure consistent with optical images was observed.

Hints of non-thermal emission emerging since 2013.

Abstract

We have analysed the full suite of Australia Telescope Compact Array data for the Stingray planetary nebula. Data were taken in the 4- to 23-GHz range of radio frequencies between 1991 and 2016. The radio flux density of the nebula generally declined during that period, but between 2013 and 2016 it shows signs of halting that decline. We produced the first spatially resolved radio images of the Stingray nebula from data taken in 2005. A ring structure, which appears to be associated with the ring seen in HST images, was visible. In addition, we found a narrow extension to the radio emission towards the eastern and western edges of the nebula. We derived the emission measure of the nebula - this decreased between 1992 and 2011, suggesting that the nebula is undergoing recombination. The radio spectral index is broadly consistent with a free-free emission mechanism, however a single data point hints that a steeper spectral index has possibly emerged since 2013, which could indicate the presence of synchrotron emission. If a non-thermal component component has emerged, such as one associated with a region that is launching a jet or outflow, we predict that it would intensify in the years to come.