Herschel Detects Unusual Laser Emission from Ant Nebula
Astronomers using ESA’s Herschel Space Observatory have discovered an unusual laser emission from the planetary nebula Menzel 3 (often referred to as the Ant Nebula), which suggests the presence of a binary stellar system in its center. The discovery is described in the Monthly Notices of the Royal Astronomical Society.
This Hubble image shows the bipolar planetary nebula Menzel 3. Image credit: NASA / ESA/ Hubble Heritage Team / STScI / AURA / R. Sahai, Jet Propulsion Lab / B. Balick, University of Washington.
Menzel 3 is a young bipolar planetary nebula located in the constellation Norma, about 8,000 light-years from Earth.
This object was discovered by the American astronomer Donald Howard Menzel in 1922.
Professor Menzel was also one of the first to suggest that in certain conditions natural ‘light amplification by stimulated emission of radiation’ — from which the acronym ‘laser’ derives — could occur in nebulae in space. This was well before the discovery of lasers in laboratories.
The new Herschel data show that Menzel 3 beams intense laser emission from its core.
“We detected a very rare type of emission called hydrogen recombination laser emission, which is only produced in a narrow range of physical conditions,” said study lead author Dr. Isabel Aleman, an astrophysicist with the Leiden Observatory and the University of São Paulo.
“Such emission has only been identified in a handful of objects before and it is a happy coincidence that we detected the kind of emission that Professor Menzel suggested, in one of the planetary nebulae that he discovered.”
“This kind of laser emission needs very dense gas close to the star.”
Comparison of the Herschel observations with models found that the density of the gas emitting the lasers is around 10,000 times denser than the gas seen in typical planetary nebulae and in the lobes of Menzel 3 itself.
Normally, the region close to the dead star — close in this case being about the distance of Saturn from the Sun — is quite empty, because its material is ejected outwards. Any lingering gas would soon fall back onto it.
“The only way to keep such dense gas close to the star is if it is orbiting around it in a disk,” said Professor Albert Zijlstra, from the Jodrell Bank Centre for Astrophysics at University of Manchester, UK.
“In this nebula, we have actually observed a dense disk in the very center that is seen approximately edge-on. This orientation helps to amplify the laser signal.”
“The disk suggests there is 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. The laser gives us a unique way to probe the disk around the dying star, deep inside the planetary nebula.”
Astronomers have not yet seen the expected second star, hidden in the heart of Menzel 3, but they think that the mass from the dying companion star is being ejected and then captured by the compact central star of the original planetary nebula, producing the disk where the laser emission is produced.
“We used Herschel to characterise various components of gas and dust in nebula around old stars, but we were not necessarily looking for a laser phenomenon,” said Dr. Toshiya Ueta, a researcher at the University of Denver and principal investigator of the Herschel Planetary Nebula Survey project.
“Such emission has only been identified in a handful of objects before; this was a remarkable discovery that we did not anticipate. There is certainly more to stellar nebulae than meets the eye!”
“This study suggests that the distinctive Menzel 3 nebula as we see it today was created by the complex nature of a binary star system, which influences the shape, chemical properties, and evolution in these final stages of a star’s life,” said Herschel project scientist Dr. Göran Pilbratt, who was not involved in the study.
“Herschel offered the perfect observing capabilities to detect this extraordinary laser in Menzel 3. The findings will help constrain the conditions under which this phenomenon occurs, and help us to refine our models of stellar evolution.”
“It is a nice conclusion that it took the Herschel mission to connect together Professor Menzel’s two discoveries from almost a century ago.”
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Isabel Aleman et al. 2018. Herschel Planetary Nebula Survey (HerPlaNS): hydrogen recombination laser lines in Mz 3. MNRAS 477 (4): 4499-4510; doi: 10.1093/mnras/sty966