Capturing the chaos in nearby nebulae

Astronomers have maxed out the Hubble Space Telescope’s imaging capability – using it across its full wavelength range – to create new images of “crazy fireworks” happening in two nearby young planetary nebulae.

As well as looking great, the observations in near-ultraviolet to near-infrared light reveal unprecedented levels of complexity and rapid changes in jets and gas bubbles blasting off stars from the centre of NGC 6303 (aka the Butterfly Nebula) and NGC 7027.

In particular, they show both nebulae are splitting themselves apart on extremely short timescales, which may be indirect evidence of one star merging with its companion star.

“The nebula NGC 7027 shows emission at an incredibly large number of different wavelengths, each of which highlights not only a specific chemical element in the nebula, but also the significant, ongoing changes in its structure,” says Joel Kastner of Rochester Institute of Technology, US, lead author of a paper in the journal Galaxies.

These are among the dustiest planetary nebulae known, the researchers say, and both contain unusually large masses of gas because they are so newly formed, which makes them interesting to study in parallel.

Based on their bizarre shapes, Kastner and colleagues suspect that at their hearts were two stars circling each other. Each has a pinched, dusty waist and polar lobes or outflows, as well as other, more complex symmetrical patterns.

One theory for the generation of such structures in planetary nebulae is that the two stars orbit one another closely enough that they eventually interact, producing a gas disc around one or both stars. The disc is the source of outflowing material directed in opposite directions from the central star.

Another hypothesis is that the smaller star merges with its bloated, more rapidly evolving stellar companion. This also can create outflowing jets of material that may wobble over time. This creates a symmetric pattern, perhaps like the one that gives NGC 6302 its “butterfly” nickname. Such outflows are commonly seen in planetary nebulas.

“The suspected companion stars in NGC 6302 and NGC 7027 haven’t been directly detected because they are next to, or perhaps have already been swallowed by, larger red giant stars, a type of star that is hundreds to thousands of times brighter than the Sun,” says co-author Bruce Balick, from the University of Washington in Seattle.

“The hypothesis of merging stars seems the best and simplest explanation for the features seen in the most active and symmetric planetary nebulas. It’s a powerful unifying concept, so far without rival.”

NGC 6302 has a distinct S-shaped pattern, seen in reddish-orange, but this only appears when captured by the Hubble filter that records near-infrared emission from singly ionised iron atoms. This is indicative of energetic collisions between both slow and fast winds, which is most commonly observed in active galactic nuclei and supernova remnants.

“This is very rarely seen in planetary nebulae,” says Balick. “Importantly, the iron emission image shows that fast, off-axis winds penetrate far into the nebula like tsunamis, obliterating former clumps in their paths and leaving only long tails of debris.”

The new image of NGC 7027, which resembles a jewel bug, indicates that it had been slowly puffing away its mass in quiet, spherically symmetric or perhaps spiral patterns for centuries — until relatively recently.

“Something recently went haywire at the very centre, producing a new cloverleaf pattern, with bullets of material shooting out in specific directions,” says Kastner.