Unusual laser emission in the Ant Nebula suggests hidden double star system

Unusual laser emission in the Ant Nebula suggests hidden double star system
Credit: University of Manchester

An international team of astronomers have discovered an
unusual laser emission that suggests the presence of a double
star system hidden at the heart of the “spectacular” Ant
Nebula.


The extremely rare phenomenon is connected to the death of a
star and was discovered in observations made by European Space
Agency’s (ESA) Herschel observatory.

When low- to middleweight stars like our Sun approach the end
of their lives they eventually become dense, white dwarf
. In the process, they cast off their outer
layers of gas and dust into space, creating a kaleidoscope of
intricate patterns known as a planetary nebula. Our Sun is
expected to one day form such a planetary nebula.

A nebula is an interstellar cloud of dust, hydrogen, helium and
other ionized gases. The Ant Nebula earns its nickname from the
twin lobes that resemble the head and body of an ant.

The recent Herschel observations have shown that the dramatic
demise of the central star in the core of the Ant Nebula is
even more theatrical than implied by its colourful appearance
in visible images – such as those taken by the NASA/ESA Hubble
Space Telescope.

The new data shows that the Ant Nebula also beams intense
from its core. Lasers are
well-known down on earth in everyday life, from special visual
effects in music concerts to health care and communications. In
space, is detected at very
different wavelengths and only under certain conditions. Only a
few of these infrared space lasers are known.

By coincidence, astronomer Donald Menzel who first observed and
classified this particular planetary nebula in the 1920s (it is
officially known as Menzel 3 after him) 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.

Dr. Isabel Aleman, lead author of a paper describing the new
results, said “We detected a very rare type of emission called
hydrogen recombination laser emission, which is only produced
in a narrow range of physical conditions.

“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 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 observations with models found that the
density of the gas emitting the lasers is around ten thousand
times denser than the gas seen in typical planetary nebulae and
in the lobes of the Ant Nebula 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.

Co-author Prof Albert Zijlstra, from the Jodrell Bank Centre
for Astrophysics in the School of Physics & Astronomy, added:
“The only way to keep such dense gas close to the star is if it
is orbiting around it in a disc.In this nebula, 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 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 disc around the dying star, deep inside
the .”

Astronomers have not yet seen the expected second star, hidden
in the heart of the Ant .

Göran Pilbratt, ESA’s Herschel project scientist, added: “It is
a nice conclusion that it took the Herschel mission to connect
together Menzel’s two discoveries from almost a century ago.”

Explore further:
What will happen
when our sun dies?

More information: Isabel Aleman et al, Herschel
Planetary Nebula Survey (HerPlaNS)★: Hydrogen Recombination
Laser Lines in Mz 3, Monthly Notices of the Royal
Astronomical Society
(2018). DOI: 10.1093/mnras/sty966 ,
dx.doi.org/10.1093/mnras/sty966

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