Science & Technology

Organic molecules observed around young star thanks to violent icy outbursts

The outer part of the disk is cold and dust particles are covered with ice. ALMA detected various complex organic molecules around the snow line of water in the disk (National Astronomical Observatory of Japan)
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Astronomers have used the ALMA telescope to observe complex organic molecules around a young star – V883 Ori – thanks to a sudden and violent, icy-outburst, employing these observations to reconstruct the evolution of the compounds we see around us.

Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have detected various complex organic molecules around the young star V883 Ori — located 1300 light-years from Earth. A sudden outburst from this star is releasing molecules from the icy compounds in the planet-forming disk. The outbursts occurring – FU Orionis type outbursts – involve a sudden increase of luminosity due to a bursting torrent of material flowing from the disk to the star.

The distribution of dust is shown in orange and the distribution of methanol, an organic molecule, is shown in blue (ALMA/ LEE)

The distribution of dust is shown in orange and the distribution of methanol, an organic molecule, is shown in blue (ALMA/ LEE)

These outbursts last only on the order of 100 years, therefore the chance to observe a burst is rather rare. However, since young stars with a wide range of ages experience FU Ori bursts, astronomers expect to be able to trace the chemical composition of ice throughout the evolution of young stars.

The chemical composition of this disk is similar to that of comets in the modern Solar System which have remained virtually unchanged since its formation. These observations enable astronomers to reconstruct the evolution of organic molecules from the birth of the Solar System to the objects we see today.

The research team led by Jeong-Eun Lee, from Kyung Hee University, Korea,  detected complex organic molecules including methanol (CH3OH), acetone (CH3COCH3), acetaldehyde (CH3CHO), methyl formate (CH3OCHO), and acetonitrile (CH3CN). This is the first time that acetone was unambiguously detected in a planet-forming region or protoplanetary disk.

Various molecules are frozen in ice around micrometre-sized dust particles in protoplanetary disks. V883 Ori’s sudden flare-up is heating the disk and sublimating the ice, which releases the molecules into gas. The region in a disk where the temperature reaches the sublimation temperature of the molecules is called the “snow line.” The radii of snow lines are about a few astronomical units (AU) around normal young stars, however, they are enlarged almost 10 times around bursting stars.

The outer part of the disk is cold and dust particles are covered with ice. ALMA detected various complex organic molecules around the snow line of water in the disk (National Astronomical Observatory of Japan)

The outer part of the disk is cold and dust particles are covered with ice. ALMA detected various complex organic molecules around the snow line of water in the disk (National Astronomical Observatory of Japan)

 

Lee says: “It is difficult to image a disk on the scale of a few AU with current telescopes.

“However, around an outburst star, ice melts in a wider area of the disk and it is easier to see the distribution of molecules. We are interested in the distribution of complex organic molecules as the building blocks of life.”

Ice, including frozen organic molecules, could be closely related to the origin of life on planets. In our Solar System, comets are the focus of attention because of their rich icy compounds. For example, the European Space Agency’s (ESA)  legendary comet explorer Rosetta found rich organic chemistry around the comet Churyumov-Gerasimenko. Comets are thought to have been formed in the outer colder region of the proto-Solar System, where the molecules were contained in ice.

Thus, probing the chemical composition of ice in protoplanetary disks is directly related to probing the origin of organic molecules in comets, and the origin of the building blocks of life.

Thanks to ALMA’s sharp vision and the enlarged snow line due to the flare-up of the star, the astronomers obtained the spatial distribution of methanol and acetaldehyde. The distribution of these molecules has a ring-like structure with a radius of 60 AU, which is twice the size of Neptune’s orbit. The researchers assume that inside of this ring the molecules are invisible because they are obscured by thick dusty material, and are invisible outside of this radius because they are frozen in ice.

Yuri Aikawa at the University of Tokyo, a member of the research team, says: “Since rocky and icy planets are made from solid material, the chemical composition of solids in disks is of special importance. An outburst is a unique chance to investigate fresh sublimates, and thus the composition of solids.”

This isn’t the first time that organic molecules have been detected around V883 ORI. In 2018, methane was believed to have been observed around the young star. The results, which were published in the journal The Astrophysical Journal letters, noted that the observations were unfortunately too faint to resolve structures within the water-snow line.

Research available at: https://www.nature.com/articles/s41550-018-0680-0

 

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