A
composite RGB image of the Ring Nebula (also known as Messier 57 and NGC 6720)
constructed from four WEAVE/LIFU emission-line images. The bright outer ring is
made up of light emitted by three different ions of oxygen, while the
"bar" across the middle is due to light emitted by a plasma of
four-times-ionised iron atoms. North is up and East is to the left in the
image. Credit: University College London
A
mysterious bar-shaped cloud of iron has been discovered inside the iconic Ring
Nebula by a European team led by astronomers at University College London (UCL)
and Cardiff University.
The cloud of iron atoms, described for
the first time in Monthly Notices of the
Royal Astronomical Society, is in the shape of a bar or strip: it just fits
inside the inner layer of the elliptically shaped nebula, familiar from many
images including those obtained by the James Webb Space Telescope at infrared
wavelengths.
The bar's length is roughly 500 times
that of Pluto's orbit around the sun and, according to the team, its mass of
iron atoms is comparable to the mass of Mars.
The Ring Nebula, first spotted in 1779
in the northern constellation of Lyra by the French astronomer Charles Messier,
is a colorful shell of gas thrown off by a star as it ends the nuclear
fuel-burning phase of its life. Our own sun will expel its outer layers in a
similar way in a few billion years' time.
An
illustrative set of 8 individual WEAVE LIFU emission-line images of the Ring
Nebula. The colour in each panel tracks the brightness of emission, with
brown-red being the most intense, shading through yellow and green to blue for
the faintest emission. North is up and east, left. Credit: University College
London
Discovery and analysis using WEAVE
The iron cloud was discovered in
observations obtained using the Large Integral Field
Unit (LIFU)
mode of a new instrument, the WHT Enhanced Area Velocity Explorer (WEAVE),
installed on the Isaac Newton Group's 4.2-meter William Herschel Telescope.
The LIFU is a bundle of hundreds of
optical fibers. It has enabled the team of astronomers to obtain spectra (where light is separated into its constituent
wavelengths) at every point across the entire face of the Ring Nebula, and at
all optical wavelengths, for the first time.
Lead author Dr. Roger Wesson, based
jointly at UCL and Cardiff University, said, "Even though the Ring Nebula
has been studied using many different telescopes and instruments, WEAVE has
allowed us to observe it in a new way, providing so much more detail than
before.
"By obtaining a spectrum
continuously across the whole nebula, we can create images of the nebula at any
wavelength and determine its chemical composition at any position.
"When we processed the data
and scrolled through the images, one thing popped out as clear as anything—this
previously unknown 'bar' of ionized iron atoms, in the middle of the familiar
and iconic ring."
Unanswered questions and future research
How the iron bar formed is
currently a mystery, the authors say. They will need further, more detailed
observations to unravel what is going on. There are two potential scenarios:
the iron bar may reveal something new about how the ejection of the nebula by
the parent star progressed, or (more intriguingly) the iron might be an arc of
plasma resulting from the vaporization of a rocky planet caught up in the
star's earlier expansion.
Co-author Professor Janet Drew,
also based at UCL, said, "We definitely need to know more—particularly
whether any other chemical elements co-exist with the newly-detected iron, as
this would probably tell us the right class of model to pursue. Right now, we
are missing this important information."
The team are working on a follow-up
study, and plan to obtain data using WEAVE's LIFU at higher spectral resolution
to better understand how the bar might have formed.
WEAVE is carrying out eight surveys
over the next five years, targeting everything from nearby white dwarfs to very
distant galaxies. The Stellar, Circumstellar and Interstellar Physics strand of
the WEAVE survey, led by Professor Drew, is observing many more ionized nebulae
across the northern Milky Way.
"It would be very surprising
if the iron bar in the Ring is unique," explains Dr. Wesson. "So
hopefully, as we observe and analyze more nebulae created in the same way, we
will discover more examples of this phenomenon, which will help us to
understand where the iron comes from."
Professor Scott Trager, WEAVE
Project Scientist based at the University of Groningen, added, "The
discovery of this fascinating, previously unknown structure in a night-sky
jewel, beloved by sky watchers across the Northern Hemisphere, demonstrates the
amazing capabilities of WEAVE.
"We look forward to many more discoveries from this new instrument."
Provided by Royal Astronomical Society
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