Seeking Protostellar Companions in Orion

photograph of the constellation orion

Visible-light image of the constellation Orion and the Orion nebula — located below the three stars of the “belt” — which is part of the Orion molecular cloud complex. [Wikipedia]

Many mature stars have stellar companions, living out their lives in close-knit clusters or swinging through space in binary pairs. This suggests that most stars form in small groups, but the onset of star formation has long been difficult to study. Now, astronomers have turned arrays of sensitive radio telescopes toward one of the most active star-forming regions in the Milky Way to investigate star formation at its earliest stages.

Star Formation in Our Backyard

When it comes to studying young stars, there’s no better place than the Orion molecular cloud complex: a network of active star-forming regions located just over a thousand light-years away. The Orion molecular clouds contain hundreds of protostars still siphoning gas from their nascent nebulae, making it a perfect arena for studying star formation.

The two leading theories for how stars form are disk fragmentation and turbulent fragmentation. The disk fragmentation theory suggests that a rotating disk of star-forming material can splinter into multiple stars. The turbulent fragmentation theory posits that small fluctuations within a dense clump of gas can ripple outward and induce a gas cloud to collapse. Key to distinguishing between these hypotheses are their length scales; disk fragmentation is thought to produce stars separated by roughly 100 au, while turbulent fragmentation likely generates more widely separated companions.

A First Look at New Stars

Studying star formation at small spatial scales is challenging, because short-wavelength light emitted by young stars is heavily obscured by gas and dust, and long-wavelength observations are inherently lower in resolution. Luckily, the advent of radio telescope arrays has increased the achievable resolution of radio images and allowed astronomers to probe smaller scales than ever before. A team led by John Tobin (National Radio Astronomy Observatory) has used the Atacama Large Millimeter/submillimeter Array (ALMA) and the Very Large Array (VLA) to investigate stellar companionship in the Orion molecular clouds at the earliest stages of star formation — and to explore what these findings mean for how stars form.

plots of radio flux from protostars

Example of observations from ALMA (left) and the VLA (right) of a young protostellar system. [Adapted from Tobin et al. 2022]

Tobin and collaborators surveyed 328 protostars in the Orion molecular clouds, including 94 in the earliest phase of protostar evolution. The team used an iterative algorithm to search for protostars with companions within 20–10,000 au. (For context, if the Sun had a companion at 10,000 au, it would be located in our solar system’s Oort cloud.) The authors found that roughly 30% of all systems surveyed contained multiple stars, with binary systems being more common than triple- or quadruple-star systems.

Competing Creation Scenarios

plot of fraction of companion protostars observed in orion as a function of separation in astronomical units

Separation distribution for multiple-star systems containing the youngest (Class 0) protostars. The dotted curve approximates the distribution for Sun-like stars in the field. [Adapted from Tobin et al. 2022]

The authors noted that the companion separation distribution for the youngest protostars has two peaks — one around 100 au and another around 3,000 au. This suggests that multiple formation mechanisms are at play in these systems; stars that form at large (>500 au) separations via turbulent fragmentation can migrate inward over time, but comparisons with simulations suggest that there are more close-in protostellar companions than can be accounted for by this migration.

The team concluded that more than half of all companions within 500 au likely formed through disk fragmentation, while those at larger separations likely only formed due to turbulent fragmentation. Hopefully, future studies of this rich protostar data set will reveal even more insights about star formation!


“The VLA/ALMA Nascent Disk And Multiplicity (VANDAM) Survey of Orion Protostars. V. A Characterization of Protostellar Multiplicity,” John J. Tobin et al 2022 ApJ 925 39. doi:10.3847/1538-4357/ac36d2