Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) may have solved the mystery of the coldest known object in the cosmos the Boomerang Nebula, a pre-planetary nebula produced by a dying red giant. The research is published in the Astrophysical Journal.

Composite image of the Boomerang Nebula: ALMA observations (orange) showing the hourglass-shaped outflow, which is embedded inside a roughly round ultra-cold outflow; the hourglass outflow stretches more than 1.9 trillion miles from end to end, and is the result of a jet that is being fired by the central star, sweeping up the inner regions of the ultra-cold outflow like a snow-plow; the ultra-cold outflow is about 10 times bigger. The ALMA data are shown on top of an image from the NASA/ESA Hubble Space Telescope (blue). Image credit: ALMA / ESO / NAOJ / NRAO / NASA / ESA / Hubble / AUI / NSF.

An ancient red giant has produced the Boomerang Nebula, the coldest object found in the Universe so far. How this star was able to create an environment strikingly colder than the natural background temperature of deep space has been a mystery for more than two decades.

According to astronomers using ALMA, the answer may be that a small companion star has plunged into the heart of the red giant, ejecting most the matter of the larger star as an ultra-cold outflow of gas and dust.

This outflow is expanding so rapidly that its temperature has fallen to less than half a degree Kelvin (minus 458.5 degrees Fahrenheit, or minus 272.5 degrees Celsius).

The ALMA observations enabled the astronomers to unravel this mystery by providing the first precise calculations of the Boomerang Nebulas extent, age, mass, and kinetic energy.

These new data show us that most of the stellar envelope from the massive red giant star has been blasted out into space at speeds far beyond the capabilities of a single, red giant star, said lead author Dr. Raghvendra Sahai, an astronomer at NASAs Jet Propulsion Laboratory.

The only way to eject so much mass and at such extreme speeds is from the gravitational energy of two interacting stars, which would explain the puzzling properties of the ultra-cold outflow.

Such close companions may be responsible for the early and violent demise of most stars in the Universe.

The extreme properties of the Boomerang Nebula challenge the conventional ideas about such interactions and provide us with one of the best opportunities to test the physics of binary systems that contain a giant star, said co-author Prof. Wouter Vlemmings, an astronomer at Chalmers University of Technology in Sweden.

Also known as the Bow Tie Nebula and the Centaurus Bipolar Nebula, the Boomerang Nebula is located about 5,000 light-years from Earth in the constellation Centaurus.

When the nebula was first observed in 1995, astronomers noted that it was absorbing the light of the Cosmic Microwave Background, the fossil light resulting from a time when the Universe was hot and dense, only 380,000 years after the Big Bang.

This radiation provides the natural background temperature of space only 2.725 degrees above absolute zero.

For the Boomerang Nebula to absorb that radiation, it had to be even colder than this lingering, dim energy that has been continually cooling for more than 13 billion years.

The new ALMA observations also produced an evocative image of the nebula, showing an hourglass-shaped outflow inside a roughly round ultra-cold outflow.

The hourglass outflow stretches more than 1.9 trillion miles (3 trillion km, or about 21,000 times the distance from the Sun to the Earth) from end to end, and is the result of a jet that is being fired by the central star, sweeping up the inner regions of the ultra-cold outflow like a snowplow.

The ultra-cold outflow is more than 10 times bigger. Traveling more than 335,540 mph (150 km/s), it took material at its outer edges approximately 3,500 years to reach these extreme distances after it was first ejected from the dying star.

These conditions, however, will not last long. Even now, the Boomerang Nebula is slowly warming.

We see this remarkable object at a very special, very short-lived period of its life, said co-author Dr. Lars-ke Nyman, an astronomer at the Joint ALMA Observatory in Santiago, Chile.

Its possible these super cosmic freezers are quite common in the Universe, but they can only maintain such extreme temperatures for a relatively short time.

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R. Sahai et al. 2017. The Coldest Place in the Universe: Probing the Ultra-cold Outflow and Dusty Disk in the Boomerang Nebula. ApJ 841, 110; doi: 10.3847/1538-4357/aa6d86

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ALMA Probes Boomerang Nebula, Universe's Coldest Known Object - Sci-News.com