Planetary nebulae, whose stars shed their layers over thousands of years, can turn into crazy whirligigs while puffing off shells and jets of hot gas. New images from the NASA/ESA Hubble Space Telescope have helped astronomers identify rapid changes in material blasting off stars at the centers of two nearby young planetary nebulae: NGC 6302 and NGC 7027.
This Hubble image shows NGC 7027, or the ‘Jewel Bug’ nebula. Image credit: NASA / ESA / J. Kastner, Rochester Institute of Technology.
NGC 6302, commonly known as the Butterfly Nebula, is located 2,417 light-years away in the constellation of Scorpius. Its butterfly shape stretches for more than two light-years, which is about half the distance from the Sun to Proxima Centauri.
NGC 7027 resides approximately 3,000 light-years away in the constellation of Cygnus. The object is unusually small, measuring only 0.2 by 0.1 light-years.
“The new multi-wavelength Hubble observations provide the most comprehensive view to date of both of these spectacular nebulae,” said Dr. Joel Kastner, an astronomers at the Rochester Institute of Technology.
“As I was downloading the resulting images, I felt like a kid in a candy store.”
The new Hubble images reveal in vivid detail how NGC 6302 and NGC 7027 are splitting themselves apart on extremely short timescales.
In particular, Hubble’s broad multi-wavelength views of each nebula are helping the researchers to trace the histories of shock waves in them.
Such shocks are typically generated when fresh, fast stellar winds slam into and sweep up more slowly expanding gas and dust ejected by the star in its recent past, generating bubble-like cavities with well-defined walls.
Astronomers suspect that at the heart of each nebula were two stars orbiting around each other.
Evidence for such a central ‘dynamic duo’ comes from the bizarre shapes of these nebulae. Each has a pinched, dusty waist and polar lobes or outflows, as well as other, more complex symmetrical patterns.
A leading theory for the generation of such structures in planetary nebulae is that the mass-losing star is one of two stars in a binary system.
The two stars orbit one another closely enough that they eventually interact, producing a gas disk around one or both stars. The disk then launches jets that inflate polar-directed lobes of outflowing gas.
Another, related, popular hypothesis is that the smaller star of the pair may merge with its bloated, more rapidly evolving stellar companion.
This very short-lived common envelope binary star configuration can also generate wobbling jets, forming the trademark bipolar outflows commonly seen in planetary nebulae. However, the suspect companion stars in these planetary nebulae have not been directly observed.
Scientists suggest this may be because these companions are next to, or have already been swallowed by, far larger and brighter red giant stars.
This Hubble image NGC 6302, commonly known as the Butterfly Nebula. Image credit: NASA / ESA / J. Kastner, Rochester Institute of Technology.
NGC 6302 exhibits a distinct S-shaped pattern seen in reddish-orange in the image. The ‘S’ only appears when captured by the Hubble camera filter that records near-infrared emission from singly ionized iron atoms.
“The S-shape in the iron emission from NGC 6302 is a real eye-opener,” Dr. Kastner said.
“The S-shape directly traces the most recent ejections from the central region, since the collisions within the nebula are particularly violent in these specific regions of NGC 6302.”
“This iron emission is a sensitive tracer of energetic collisions between slower winds and fast winds from the stars,” said Dr. Bruce Balick, an astronomer at the University of Washington.
“It’s commonly observed in supernova remnants and active galactic nuclei, and outflowing jets from newborn stars, but is very rarely seen in planetary nebulae.”
“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 accompanying 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.
“In some respects, the changes within this nebula are even more dramatic than those within NGC 6302,” Dr. Kastner said.
“Something recently went haywire at the very center, producing a new cloverleaf pattern, with bullets of material shooting out in specific directions.”
“We have a sneaking suspicion that this nebula is a great example of what happens when a red giant star abruptly swallows a companion,” said Dr. Rodolfo Montez Jr., an astronomer at the Harvard & Smithsonian Center for Astrophysics.
The team’s paper was published in the journal Galaxies.
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Joel H. Kastner et al. 2020. First Results from a Panchromatic HST/WFC3 Imaging Study of the Young, Rapidly Evolving Planetary Nebulae NGC 7027 and NGC 6302. Galaxies 8 (2): 49; doi: 10.3390/galaxies8020049
