A mysterious bar-shaped cloud of iron has been discovered inside the iconic Ring Nebula by a European team led by astronomers at 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 Messier1, 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.2
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)3, installed on the Isaac Newton Group's 4.2-metre William Herschel Telescope4.
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 UCL's Department of Physics & Astronomy 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 ionised iron atoms, in the middle of the familiar and iconic ring."
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 nebula ejection by the central star progressed, or (more intriguingly) the iron might be a highly stretched-out arc of plasma resulting from the vaporisation of a rocky planet caught up in the central star's prior 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."
