Hubble provides holistic view of stars gone haywire

Date:
June 18, 2020
Source:
NASA/Goddard Space Flight Center
Summary:
As nuclear fusion engines, most stars live placid lives for hundreds
of millions to billions of years. But near the end of their lives
they can turn into crazy whirligigs, puffing off shells and jets
of hot gas.

Hubble has dissected such crazy fireworks in two nearby young
planetary nebulas.



FULL STORY ==========================================================================
As nuclear fusion engines, most stars live placid lives for hundreds
of millions to billions of years. But near the end of their lives
they can turn into crazy whirligigs, puffing off shells and jets of hot
gas. Astronomers have employed Hubble's full range of imaging capabilities
to dissect such crazy fireworks happening in two nearby young planetary nebulas. NGC 6303 is dubbed the Butterfly Nebula because of its wing-like appearance. In addition, NGC 7027 resembles a jewel bug, an insect with
a brilliantly colorful metallic shell.


==========================================================================
The researchers have found unprecedented levels of complexity and rapid
changes in jets and gas bubbles blasting off of the stars at the centers
of both nebulas. Hubble is allowing the researchers to converge on an understanding of the mechanisms underlying the chaos.

"When I looked in the Hubble archive and realized no one had observed
these nebulas with Hubble's Wide Field Camera 3 across its full wavelength range, I was floored," said Joel Kastner of Rochester Institute of
Technology, Rochester, New York, leader of the new study. "These new multi-wavelength Hubble observations provide the most comprehensive view
to date of both of these spectacular nebulas. As I was downloading the resulting images, I felt like a kid in a candy store." By examining
this pair of nebulas with Hubble's full, panchromatic capabilities --
making observations in near-ultraviolet to near-infrared light -- the
team has had several "aha" moments. In particular, the new Hubble images
reveal in vivid detail how both nebulas are splitting themselves apart
on extremely short timescales -- allowing astronomers to see changes
over the past couple decades.

Some of this rapid change 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," said Kastner. The research team also observed the
Butterfly Nebula, which is a counterpart to the "jewel bug" nebula: Both
are among the dustiest planetary nebulas known and both also contain
unusually large masses of gas because they are so newly formed. This
makes them a very interesting pair to study in parallel, say researchers.

Hubble's broad multi-wavelength views of each nebula are helping the researchers to trace the nebulas' histories of shock waves. Such shocks typically are 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.



========================================================================== Researchers suspect that at the hearts of both nebulas are -- or
were -- two stars circling around each other, like a pair of figure
skaters. Evidence for such a central "dynamic duo" comes from the bizarre shapes of these nebulas.

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
nebulas 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 is the source of outflowing material directed in opposite directions
from the central star.

Similarly, the smaller star of the pair may merge 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," said team member Bruce
Balick of 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." The Butterfly Nebula Imagine
a lawn sprinkler spinning wildly, tossing out two S-shaped streams. At
first it appears chaotic, but if you stare for a while, you can trace its patterns. The same S-shape is present in the Butterfly Nebula, except in
this case it is not water in the air, but gas blown out at high speed
by a star. And 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 the Butterfly Nebula is a real
eye- opener," 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," Balick explained. "It's commonly
observed in supernova remnants and active galactic nuclei, and outflowing
jets from newborn stars, but is very rarely seen in planetary nebulas."
"The fact that the iron emission is only showing up along these opposing,
off- center directions implies that the source of the fast flows is
wobbling over time, like a spinning top that's about to fall," added
Kastner. "That's another tell-tale sign of the presence of a disk, which directs the flow, and also a binary companion." The 'Jewel Bug' Nebula
The planetary nebula NGC 7027 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 the Butterfly," Kastner
said. "Something recently went haywire at the very center, producing
a new cloverleaf pattern, with bullets of material shooting out in
specific directions." The research team's new images of NGC 7027 show
emission from singly ionized iron that closely resembles observations
made by NASA's Chandra X-ray Observatory in 2000 and 2014 as part
of earlier research by Kastner, team member Rodolfo Montez Jr. of the
Center for Astrophysics | Harvard & Smithsonian, and collaborators. The
iron emission traces the southeast-to- northwest-oriented outflows that
also produce the X-ray-emitting shocks imaged by Chandra. "We have a
sneaking suspicion that this nebula is a great example of what happens
when a red giant star abruptly swallows a companion," Montez Jr. said.


========================================================================== Story Source: Materials provided by
NASA/Goddard_Space_Flight_Center. Note: Content may be edited for style
and length.


========================================================================== Journal Reference:
1. Joel H. Kastner, Jesse Bublitz, Bruce Balick, Rodolfo Montez,
Adam Frank,
Eric Blackman. First Results from a Panchromatic HST/WFC3 Imaging
Study of the Young, Rapidly Evolving Planetary Nebulae NGC 7027
and NGC 6302.

Galaxies, 2020; 8 (2): 49 DOI: 10.3390/galaxies8020049 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200618150315.htm

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