The universe is filled with double and, more rarely, triple or quadruple star systems. We are only just starting to understand how they form. So the discovery of a disk of gas and dust in the act of turning into three stars provides an important piece to our understanding of the star formation process.
Astronomers have proposed two origins for multiple star systems. One involves a break up of the loose cloud of material that goes on to become stars. The other involves circumstellar disks, well on the way to becoming stars, fragmenting and creating dense pockets that go on to become separate stars.
A team, led by the University of Oklahoma's Dr John Tobin, witnessed this late-stage break up in action using the Atacama Large Millimeter/submillimeter Array (ALMA) and the Karl G. Jansky Very Large Array (VLA). Their observations of a triple star system called L1448 IRS3B are reported in Nature.
L1448 IRS3B is about 750 light-years away in the constellation Perseus. Two protostars with a combined mass similar to that of the Sun are 61 AU apart, double the distance between the Sun and Neptune. A much less massive star in formation lies 183 AU away. Material in the disk can be seen out to 400 AU from the center. Despite its lower mass, the third star is much brighter at the wavelengths ALMA collects.
"This whole system probably is less than 150,000 years old." said Dr Kaitlin Kratter of the University of Arizona in a statement. Kratter suspects the largest star broke away from the other two 10,000 to 20,000 years ago, a minuscule moment in stellar terms.
It is rare for stars to be 500 to 1,000 astronomical units (the distance from the Earth to the Sun) apart, yet separations that are either wider than 1,000 AU or narrower than 500 AU are common. This observation inspired the search for two distinct processes to form multi-star systems.
For widely spread star systems, it’s likely that vast gas clouds – from which stars originate – have dense patches that gradually form into stars thousands of astronomical units apart. We have recently seen this process in action. For closely spaced star systems, the result is likely from the fragmentation of circumstellar disks
Once a disk has formed, gravity acts to keep it together, eventually coalescing into a single star. However, gravitational instability can disrupt this process. The authors created a model of the L1448 IRS3B system based on the density, temperature, and motion of material revealed by the two giant radio telescope arrays. They found such instability was to be expected at 150 to 320 AU from the disk's center of mass.
"We now expect to find other examples of this process and hope to learn just how much it contributes to the population of multiple stars," Tobin said.
L1448 IRS3B is part of a larger system with three other protostars, so any planets that form will have six Suns engaged in a complex gravitational dance. Nightfall, anyone?
Artist's conception of the development of a triple system like L1448 IRS3B. Left as it is today, and right as it will eventually become. Credit: Bill Saxton, NRAO/AUI/NSF.
