Young star trio born from clumping disc of gas and dust

A stellar set of triplets has been spotted in their early years, swaddled in a vast spiral of gas and dust that gave clues to their birth.

Using the Atacama Large Millimetre/sub-millimetre Array (ALMA) in Chile, John Tobin from the University of Oklahoma and colleagues from the US, Germany and Sweden peered at the triple star system L1448 IRS3B – which, at 150,000 years old, is a cosmic baby.

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In a paper published in Nature, they concluded the disc of the gas and dust surrounding the stars fragmented, and from this at least one of the stars arose.

In stellar nurseries, stars like our sun arise in massive clumps called molecular clouds. And around half of sun-like stars have a stellar companion or two (or a few). These buddies are thought to crop up in a couple of ways.

Turbulence kicked up by one star’s birth can trigger gravitational instabilities in the surrounding material, prompting more star formation.

But small-scale gravitational instabilities can occur too – particularly when material in a disc surrounding a young star, also called a protostar, cools and clumps. (A similar phenomenon is thought to be at play in the formation of planets in young solar systems.)

The problem with checking out these theories is observing stars in the process of formation with companions is tricky. Not only do we have to be lucky enough to catch them in their early years, they’re also small and faint – they don’t start burning hydrogen and glowing brightly for another few million years.

And telescopes must be super-sensitive to pick up the stars’ dim appearance with high enough resolution to distinguish separate stars in a binary (or triplet) system.

The ALMA’s ability to pick up wavelengths of light on the order of millimetres is able to do this.

It picked up light from the three protostars in L1448 IRS3B – called IRS3B-a, IRS3B-b and IRS3B-c – and their surrounds with a sensitivity 10 times higher and resolution twice that of previous studies.

This increased sensitivity exposed the disc’s “dominant one- or two-armed spiral that links IRS3B-a and IRS3B-b with the more widely separated IRS3B-c, which is embedded in the outermost arm”, the researchers write.

This spiral is a telltale sign that L1448 IRS3B recently underwent small-scale gravitational instability, they conclude, giving rise to at least one of the three stars – most likely IRS3B-c, but also perhaps IRS3B-b too.

In a News and Views article, Adele Plunkett from the European Space Observatory in Chile writes that the “strength of this study is the attention to detail”.

For instance, Tobin and his crew calculated the radius at which the disc would fragment – between 150 and 300 astronomical units (where one astronomical unit is the distance between the Earth and sun).

Indeed, this is in line with the ALMA-observed 183 astronomical units between the two central protostars IRS2B-a and IRS3B-b, and the third star IRS3B-c out in the spiral arm.

Systems such as L1448 IRS3B, Plunkett concludes, “are probably not rare – rather, they are waiting to be studied in more detail using the powerful (sub-) millimetre-wavelength telescopes that are now available”.