Before They Were (Binary) Stars

Astronomers have taken a behind-the-scenes look at a set of dense gas clumps, catching a quadruple star system in the fleeting act of formation.

Alpha, Beta, and Proxima Centauri

Alpha and Beta Centauri feature in this image uploaded by Skatebiker on Wikipedia. Faint Proxima Centauri is circled in red.

Take a closer look at the stars tonight and you’ll find that many aren’t solo performers. The Alpha Centauri system, Algol and its companions, and the Dog Star and the Pup of Sirius are just a few well-known multiples that come to mind. In fact, half of all Sun-like stars belong to a multiple system, a fraction that increases with stellar mass, and those companions affect everything from forming planets to the rate of Type Ia supernova explosions.

But there's not much observational evidence to back various multiple-formation ideas. And that’s not surprising, considering that stars form lightning-fast by astronomical standards — gas goes from cold clumps to hot fusion in “just” a hundred thousand years.

Barnard 5 star-forming filaments

Very Large Array observations revealed filaments and condensations within the star-forming core Barnard 5 in the Perseus star-forming region.

Now, Jaime Pineda (ETH Zurich, Switzerland) and colleagues report in the February 12th Nature their observations of radio and submillimeter emissions from a multiple star system forming 815 light-years away. The images put some much-needed evidence into the theoretical playing field.

It’s not hard to make multiples in theory. One idea is that a star-forming clump might split into several, like identical twins in the womb. Another proposes that the disk of material that feeds a forming star might be gravitationally unstable, fragmenting and collapsing into another star that orbits the first. Some theorists have also suggested complex three-body encounters that can lead to stellar capture or a modification of existing partnerships.

But Pineda says these theories have a hard time explaining the formation of wide binaries, stellar siblings separated by thousands of astronomical units (a.u., the distance between Earth and the Sun). Instead, his observations point to a fourth option: fragmenting filaments.

Pineda’s team used the Very Large Array in New Mexico to image radio waves emitted from ammonia molecules in the Perseus star-forming region. That radiation traces the presence of dense gas and reveals long filaments that have crumbled into four distinct clumps in a star-forming core called Barnard 5. One of these clumps contains a well-known protostar, a star that hasn’t yet ignited its core fusion. The other three clumps surround the protostar at distances as close as 3,300 a.u. and as far as 11,400 a.u.

The protostar and the three surrounding clumps each contain between a tenth and a third of the Sun’s mass, based off the submillimeter-wavelength brightness as seen with the James Clerk Maxwell Telescope on Mauna Kea. But predicting the mass of the stars-to-be is more difficult.

Artist's conception of the Barnard 5 star-forming core

Artist's conception of the B5 complex as seen today (left), and as it will appear as a multiple-star system in about 40,000 years (right).

The authors estimate that the clumps’ gravitational collapse will take roughly another 40,000 years, so it all comes down to how much gas they can collect in that time. Gas flowing along the filaments may continue to feed the growing clumps, or the individual masses could fragment further even as they continue to collect gas from their surroundings.

Making some reasonable assumptions, Pineda and colleagues determine the clumps will indeed form a bound system of four low-mass stars.

A closer look at the clumps themselves tells the story of how this multiple system was able to form. The clumps are smaller than what you might predict if you simply pit gravity’s inward pull against the thermal motion of gas molecules. Instead, it looks like random flows of turbulence have broken up the condensations within this filament.

Barnard 5's neighborhood

The Barnard 5 gas complex seen within its neighborhood.

“The idea of turbulent fragmentation in filamentary structures has been around for a decade,” says Kaitlin Kratter (University of Arizona), who wrote Nature’s perspective piece on the research. But even though simulations using that idea have been able to create wide binaries, she adds that observational confirmation has only recently entered the realm of possibility.

Kratter admits she was surprised that these results came from the VLA and JCMT rather than the Atacama Large Millimeter/submillimeter Array (ALMA). The Very Large Array was upgraded in 2011, providing the high spatial resolution and high sensitivity required to image the four faint, tightly packed clumps.

“Given that this was possible even without ALMA,” Kratter adds, “I imagine we will have more systems like this in the not-so-distant future.”

Want to chase down some multiples of your own? Expand your astronomical library with Double Stars For Small Telescopes, an annotated catalog compiled by one of today's most experienced double-star observers.

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