A bright iron bar is haunting the Ring Nebula – and no one knows why

We’ve known about the iconic Ring Nebula for nearly 250 years, but only now have astronomers discovered a huge mystery at its core.

There, extending into the heart of the cosmic cloud of dust and gas, is a vast, strangely linear, bar-shaped cloud of glowing, ionized iron atoms. A structure of this nature has never been found in a nebula before, and it has a whole host of unusual properties that make it challenging to interpret.

It is the hope of a team of astronomers led by Roger Wesson of Cardiff University in the UK that further observations of other nebulae will reveal where many of these strange iron clouds came from.

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The Ring Nebula is a planetary nebula 2,570 light-years away in the constellation Lyra. found out In 1779 by the French astronomer Charles Messier. These bright blobs in the sky have nothing to do with planets, but the beautiful guts of stars like the dying Sun.

At the end of their lives, these stars slowly shed their outer layers, while the star’s core collapses into a white dwarf.

Because the process is much quieter than the violent supernova deaths of massive stars, the ejected material can often form beautiful, clean, circular structures in the sky.

There are thousands of known and possible planetary nebulae in the Milky Way, so astronomers know exactly what to expect. In addition, the Ring Nebula is one of the most famous and best studied, so it was not expected to cause any strange surprises.

However, here we are.

The observations were made using the Large Integral Field Unit (LIFU) mode of the new WHT Enhanced Area Velocity Explorer (WEAVE) instrument on the 4.2-meter William Herschel Telescope. This mode allows WEAVE to capture a large area in a single shot, providing a comprehensive spectroscopic view of the entire object.

“While the Ring Nebula has been studied using many different telescopes and instruments, WEAVE has allowed us to observe it in a new way, providing more detail than ever before,” says astronomer Roger Wesson of Cardiff University in the UK.

“As we processed the data and scrolled through the images, one thing popped out as clearly as anything—this previously unknown ‘bar’ of ionized iron atoms in the middle of this familiar and iconic ring.”

Previous spectroscopic observations of the ring nebula were made only using slit spectroscopy, which is what it looks like: looking at a single, thin slice of the nebula. This explains why the iron bar was neglected for so long; Slit observations could only find this if the slits were precisely aligned in the orientation of the bar.

Ignorance of this is not the only strange thing about iron bars. At first glance, it looks like a jet of material blasted from a star – but it’s not. A closer examination revealed that the white dwarf responsible for the ring nebula is offset from the center of the bar, so it is unlikely to be the source of the iron atoms.

Bar speed is also wrong for a jet. Emission lines from the length of the bar suggest that the entire structure is moving away from us; One end does not come close as the other descends, as you would expect from a star with two jets pouring in opposite directions.

The mystery deepens with the composition of the bar; About 14 percent of Earth’s mass is completely bare, glowing iron atoms (more than the mass of Mars) hanging in the middle of a nebula, with very few clues about how it got there.

Iron is usually present in nebulae Stuck in the dustNot hanging around naked and ionized. And there is no other emission in the iron bar-shaped nebula.

One possibility is that large amounts of dust were somehow destroyed, leaving the iron behind. That lines up with JWST observations, which reveal dust on either side of the iron bar, but not overlapping it.

However, there is no evidence of the conditions necessary to release iron from dust into the nebula. To ionize iron, you need either a very powerful shock or a very high temperature. The quiet center of the Ring Nebula shows no evidence at all.

The press release on the strip offers a torn-up planet as an explanation … but the debris of a torn-up planet does not form a clean, straight strip, and it shows a clear velocity pattern (orbital or expansion) that does not match the observations. Also, it contains other elements such as magnesium and silicon, which are shown in the observations.

We also need to consider that we cannot see the full 3D shape of the iron cloud; It can extend far beyond our line of sight, like a plank of wood seen over an edge.

The whole thing is a big, squishy question mark with no easy answer. Which means looking for more examples and hoping they turn up some clues.

“If the iron bar in the ring is unique it will be very surprising,” says Wesson. “So hopefully, as we observe and analyze more nebulae created in that way, we’ll find more examples of this phenomenon, which will help us understand where the iron comes from.”

Research has been published in Monthly Notices of the Royal Astronomical Society.

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