A triple protostar system surrounded by a disk with a spiral structure has been observed by astronomers using the Atacama Large Millimeter/submillimeter Array.
Combined ALMA and VLA image of the L1448 IRS3B system, with two young stars at the center and a third distant from them; spiral structure in the dusty disk surrounding them indicates instability in the disk. Image credit: Bill Saxton / ALMA / ESO / NAOJ / NRAO / AUI / NSF.
The stellar system in question, called L1448 IRS3B, is a member of the Perseus molecular cloud at a distance of 750 light-years.
The three young stars in IRS3B — denoted a, b, and c — have a hierarchical configuration: the most central of the young stars, IRS3B-a, is separated from the other two by 61 and 183 astronomical units (AU), respectively.
All three are surrounded by a disk of material that the Atacama Large Millimeter/submillimeter Array (ALMA) revealed to have spiral structure, a feature that indicates instability in the disk.
Until ALMA, no one had observed a triple protostar system forming in a disk.
“What is important is that we discovered that companion stars can form in disk material surrounding a dominant star,” said lead author Prof. John Tobin, from the University of Oklahoma and Leiden Observatory in the Netherlands.
“We had observed IRS3B in the past with ALMA’s predecessors, but this is the first time we have been able to clearly analyze the disk and the newborn stars within it.”
“ALMA revealed the spiral arms and disk that led to the formation of the tri-star system.”
“Triple systems like this one are rare, and this is the only one with a configuration like this, but we are actively searching for more.”
Artist’s conception of how the triple-star system develops. Left: disk of material fragments into separate protostars. Right: the resulting stellar system. Image credit: Bill Saxton / NRAO / AUI / NSF.
How multiple stellar systems form has been a mystery for some time, and there are different theories about how they form.
“This new work directly supports the conclusion that there are two mechanisms that produce multiple star systems — fragmentation of circumstellar disks, such as we see here, and fragmentation of the larger cloud of gas and dust from which young stars are formed,” Prof. Tobin said.
The findings were published in the Oct. 27 issue of the journal Nature.
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John J. Tobin et al. 2016. A triple protostar system formed via fragmentation of a gravitationally unstable disk. Nature 538, 483-486; doi: 10.1038/nature20094
