Mysterious iron ‘bar’ discovered in famous nebula

A mysterious bar-shaped cloud of iron has been discovered inside the iconic Ring Nebula by a European team led by astronomers at Cardiff University and University College London (UCL).

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 colourful 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.

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-metre William Herschel Telescope.

The LIFU is a bundle of hundreds of optical fibres.  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 Cardiff University School of Physics and Astronomy and UCL’s Department of Physics and Astronomy, 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.”

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 vaporisation of a rocky planet caught up in the star’s earlier expansion.

Co-author Professor Janet Drew, also based at UCL, advises caution: “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 is 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.

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.”