The Boomerang Nebula, called the “coldest place in the Universe,”
reveals its true shape with ALMA. The background blue structure, as seen
in visible light with the Hubble Space Telescope, shows a classic
double-lobe shape with a very narrow central region. ALMA’s resolution
and ability to see the cold molecular gas reveals the nebula’s more
elongated shape, as seen in red. Credit: Bill Saxton; NRAO/AUI/NSF;
NASA/Hubble; Raghvendra Saha. Low-resolution JPEG image - High-resolution TIFF image
At a cosmologically crisp one degree Kelvin (minus 458 degrees
Fahrenheit), the Boomerang Nebula is the coldest known object in the
Universe – colder, in fact, than the faint afterglow of the Big Bang,
which is the natural background temperature of space.
Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA)
telescope have taken a new look at this intriguing object to learn more
about its frigid properties and to determine its true shape, which has
an eerily ghost-like appearance.
As originally observed with
ground-based telescopes, this nebula appeared lopsided, which is how it
got its name. Later observations with the Hubble Space Telescope
revealed a bow-tie-like structure. The new ALMA data, however, reveal
that the Hubble image tells only part of the story, and the twin lobes
seen in that image may actually be a trick of the light as seen at
visible wavelengths.
“This ultra-cold object is extremely
intriguing and we’re learning much more about its true nature with
ALMA,” said Raghvendra Sahai, a researcher and principal scientist at
NASA’s Jet Propulsion Laboratory in Pasadena, California, and lead
author of a paper published in the Astrophysical Journal. “What
seemed like a double lobe, or ‘boomerang’ shape, from Earth-based
optical telescopes, is actually a much broader structure that is
expanding rapidly into space.”
The Boomerang Nebula, located
about 5,000 light-years away in the constellation Centaurus, is a
relatively young example of an object known as a planetary nebula.
Planetary nebulae, contrary to their name, are actually the end-of-life
phases of stars like our Sun that have sloughed off their outer layers.
What remains at their centers are white dwarf stars, which emit intense
ultraviolet radiation that causes the gas in the nebulae to glow and
emit light in brilliant colors.
The Boomerang is a pre-planetary
nebula, representing the stage in a star's life immediately preceding
the planetary nebula phase, when the central star is not yet hot enough
to emit enough ultraviolet radiation to produce the characteristic glow.
At this stage, the nebula is seen by starlight reflecting off its dust
grains.
The outflow of gas from this particular star is
expanding rapidly and cooling itself in the process. This is similar in
principle to the way refrigerators use expanding gas to produce cold
temperatures. The researchers were able to take the temperature of the
gas in the nebula by seeing how it absorbed the cosmic microwave
background radiation, which has a very uniform temperature of 2.8
degrees Kelvin (minus 455 degrees Fahrenheit).
“When astronomers
looked at this object in 2003 with Hubble, they saw a very classic
‘hourglass’ shape,” commented Sahai. “Many planetary nebulae have this
same double-lobe appearance, which is the result of streams of
high-speed gas being jettisoned from the star. The jets then excavate
holes in a surrounding cloud of gas that was ejected by the star even
earlier in its lifetime as a red giant.”
Observations with
single-dish millimeter wavelength telescopes, however, did not detect
the narrow waist seen by Hubble. Instead, they found a more uniform and
nearly spherical outflow of material.
ALMA’s unprecedented
resolution allowed the researchers to reconcile this discrepancy. By
observing the distribution of carbon monoxide molecules, which glow
brightly at millimeter wavelengths, the astronomers were able to detect
the double-lobe structure that is seen in the Hubble image, but only in
the inner regions of the nebula. Further out, they actually observed a
more elongated cloud of cold gas that is roughly round.
The
researchers also discovered a dense lane of millimeter-sized dust grains
surrounding the star, which explains why this outer cloud has an
hourglass shape in visible light. The dust grains have created a mask
that shades a portion of the central star and allows its light to leak
out only in narrow but opposite directions into the cloud, giving it an
hourglass appearance.
“This is important for the understanding
of how stars die and become planetary nebulae,” said Sahai. “Using ALMA,
we were quite literally and figuratively able to shed new light on the
death throes of a Sun-like star.”
The new research also
indicated that the outer fringes of the nebula are beginning to warm,
even though they are still slightly colder than the cosmic microwave
background. This warming may be due to the photoelectric effect -- an
effect first proposed by Einstein in which light is absorbed by solid
material, which then re-emits electrons.
Additional authors on
this paper include Wouter Vlemmings, Chalmers University of Technology,
Onsala, Sweden; Patrick Huggins, New York University, New York; Lars-Ake
Nyman, Joint ALMA Observatory, Santiago de Chile; and Yiannis
Gonidakis, CSIRO, Australia Telescope National Facility.
ALMA,
an international astronomy facility, is a partnership of Europe, North
America and East Asia in cooperation with the Republic of Chile. ALMA
construction and operations are led on behalf of Europe by ESO, on
behalf of North America by the National Radio Astronomy Observatory
(NRAO), and on behalf of East Asia by the National Astronomical
Observatory of Japan (NAOJ). The Joint ALMA Observatory (JAO) provides
the unified leadership and management of the construction, commissioning
and operation of ALMA.
The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.
Contact:
Charles Blue
(434) 296-0314
(434) 296-0314
Email: cblue@nrao.edu
