ALMA witnesses the birth of a massive binary star

A team of astronomers, among them Jonathan Tan (Chalmers) have made new observations with ALMA of a molecular cloud that is collapsing to form two massive protostars that will eventually become a binary star system.

While it is known that most massive stars possess orbiting stellar companions it has been unclear how this comes about. Are the stars born together from a common, spiraling gas disk at the center of a collapsing cloud, or do they pair up later by chance encounters in a crowded star cluster?

Understanding the dynamics of forming binaries has been difficult because the protostars in these systems are still enveloped in a thick cloud of gas and dust that prevents most light from escaping. Fortunately, it is possible to see them using radio waves, as long as they can be imaged with sufficiently high spatial resolution.

In the current research, published in Nature Astronomy, the researchers led by Yichen Zhang (RIKEN Cluster for Pioneering Research, Japan) and Jonathan Tan (Chalmers and University of Virginia), used the Atacama Large Millimeter/submillimeter Array (ALMA) telescope array in northern Chile to observe, at high spatial resolution, a star-forming region known as IRAS 07299-1651, which is located about 5,500 light-years (1.68 kiloparsecs) away in the constellation Puppis.

The observations showed that already at this early stage, the cloud contains two objects, a massive "primary" central star and another "secondary" forming star, also of high mass. For the first time, the research team were able to use these observations to deduce the dynamics of the system.

The observations showed that the two forming stars are quite far apart, separated by a distance of about 180 astronomical units (180 times the distance from the Earth to the Sun). They are currently orbiting each other with a period of at most 600 years, and have a total mass at least 18 times that of our Sun.

"This is an exciting finding because we have long been perplexed by the question of whether stars form into binaries during the initial collapse of the star-forming cloud or whether they are created during later stages. Our observations clearly show that the division into binary stars takes place early on, while they are still in their infancy," says Yichen Zhang.

Another finding of the study was that the binary stars are being nurtured from a common disk fed by the collapsing cloud and favoring a scenario in which the secondary star of the binary formed as a result of fragmentation of the disk originally around the primary. This allows the initially smaller secondary protostar to "steal" infalling matter from its sibling. Eventually they should emerge as quite similar "twins."

"This is an important result for understanding the birth of massive stars. Such stars are important throughout the universe, not least for producing, at the ends of their lives, the heavy elements that make up our Earth and are in our bodies," says Jonathan Tan.

"What is important now is to look at other examples to see whether this is a unique situation or something that is common for the birth of all massive stars," concludes Yichen Zhang.

Research Report: "Dynamics of a Massive Binary at Birth,"

Related Links
Onsala Space Observatory
Stellar Chemistry, The Universe And All Within It

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