An international team of researchers using the Karl G. Jansky Very Large Array (VLA) have discovered previously-unseen companions in the protostellar systems L1165-SMM1 and CB230 IRS1. The discovery supports one of the competing theories for how double-star systems form.
This is an artist’s impression of the binary system L1165-SMM1. Image credit: Bill Saxton / NRAO / AUI / NSF.
Astronomers know that about half of all Sun-like stars are members of double or multiple-star systems, but have debated over how such systems are formed.
The discovery, described in a paper published in the Astrophysical Journal (arXiv.org), supports the idea that double-star systems form when a disk of gas and dust whirling around one young star fragments, forming another new star in orbit with the first.
Young stars that still are gathering matter from their surroundings form such disks, along with jet-like outflows rapidly propelling material in narrow beams perpendicular to the disk.
When the astronomers studied gas-enshrouded young stars roughly 1,000 light-years from Earth, they found that two, L1165-SMM1 and CB230 IRS1, had previously-unseen companions in the plane where their disks would be expected, perpendicular to the direction of the outflows from the systems.
One of the systems, L1165-SMM1, also clearly had a disk surrounding both young stars.
“This fits the theoretical model of companions forming from fragmentation in the disk. This configuration would not be required by alternative explanations,” said study lead author Dr John Tobin of the National Radio Astronomy Observatory.
The new observations add to a growing body of evidence supporting the disk-fragmentation idea.
Binary star formation through disk fragmentation starts, left, with a young star surrounded by a rotating disk of gas and dust. The disk fragments under its own gravity, with a second star forming within the disk, center, surrounded by its own disk. At right, the two stars form an orbiting pair. Image credit: Bill Saxton / NRAO / AUI / NSF.
In 2006, a different VLA observing team found an orbiting pair of young stars, each of which was surrounded by a disk of material. The two disks, they found, were aligned with each other in the same plane.
In 2012, Dr Tobin with co-authors found a large circumstellar disk forming around a protostar in the initial phases of star formation. This showed that disks are present early in the star formation process, a necessity for binary pairs to form through disk fragmentation.
“Our new findings, combined with the earlier data, make disk fragmentation the strongest explanation for how close multiple star systems are formed,” said co-author Dr Leslie Looney from the National Radio Astronomy Observatory and the University of Illinois.
“The increased sensitivity of the VLA, produced by a decade-long upgrade project completed in 2012, made the new discovery possible,” added co-author Dr Claire Chandler, also from the National Radio Astronomy Observatory.
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Tobin JJ et al. 2013. VLA and CARMA Observations of Protostars in the Cepheus Clouds: Sub-arcsecond Proto-binaries Formed via Disk Fragmentation. ApJ 779, 93; doi: 10.1088/0004-637X/779/2/93
