An international team of astronomers has identified a rare laser phenomenon shining from the heart of the planetary nebula Menzel 3, otherwise known as the Ant Nebula. The discovery suggests the presence of an as yet unseen companion star, hiding at the core of the chaotic cosmic structure.
Menzel 3 is located roughly 8,000 light years from Earth in the direction of the constellation Norma. Discovered by Donald Menzel in the 1920s, it was given the moniker of the Ant Nebula, owing to its apparent similarity to the head and thorax of a garden ant.
The striking object belongs to a specific family of diffuse bodies known as planetary nebula. Despite their suggestive name, the formation of these beautiful structures has nothing to do with planets, and is instead rooted in the demise of middleweight stars similar to our Sun.
When such a star has depleted the majority of its hydrogen reserves, gravity causes the star to collapse, and shrink. This shrinking process compacts the material remaining in the star, causing it to heat up, and fuse the last of its hydrogen, which in turn makes it expand, and transform into a massive red giant.
At this point the star is capable of fusing the helium, which was created during the Sun-like phase of its existence from the fusion of hydrogen, into carbon. After about a billion years, the red giant would have expended its helium reserves. The core would then contract, releasing a burst of energy that would blow away the stellar material above. This cloud of debris would go on to form a planetary nebula, just like Menzel 3. The core meanwhile would remain at the heart of this cosmic cloud, as a white dwarf.
The Ant Nebula was recently observed by the European Space Agency's (ESA) Herschel Space Observatory as part of the Herschel Planetary Nebula Survey (HerPlaNS), which is seeking to explore the far-infared characteristics of these nebulae.
The powerful infrared capabilities of the Herschel telescope revealed the presence of a rare laser emission coming from the shrouded heart of Menzel 3, known as a hydrogen recombination line laser emission.
Only a handful of these infrared laser emissions have been discovered to date, and it is thought that they can only manifest under certain circumstances.
The chaotic clouds of ionized gas that make up the planetary nebula prevent astronomers from gazing directly at its core. However, the discovery of the laser emission beaming from the center of the structure allowed the scientists behind the new research to gain a better understanding of the conditions prevailing within.
In order for the hydrogen recombination line laser emissions to be created, there would have to be a dense cloud, roughly 10,000 times denser than the gas that makes up the visible head and thorax of the "ant," orbiting within 1.5 billion km of the star. This would be unusual, as this region of space should have been effectively cleared of stellar material when the star transitioned from a red giant into a white dwarf, and any residual gas should subsequently have been drawn in and devoured.
"The only way to keep gas close to the star is if it is orbiting around it in a disc," comments Albert Zijlstra, a co-author of the study detailing the new results, and Professor at the University of Manchester's School of Physics and Astronomy. "In this case, we have actually observed a dense disc in the very centre that is seen approximately edge-on. This orientation helps to amplify the laser signal. The disc suggests the white dwarf has a binary companion, because it is hard to get the ejected gas to go into orbit unless a companion star deflects it in the right direction."
Therefore, the discovery of the laser emissions indicates that there is a hidden second star lurking at the heart of the nebula. Material from this as of yet unseen companion star is possibly being ejected, and subsequently caught in the influence of the white dwarf.
The results of the study will help constrain the conditions under which these lasers emissions can occur, and will help inform current theories on stellar evolution.
A paper detailing the discovery has been accepted for publication in the Monthly Notices of the Royal Astronomical Society.
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