Rogue star gatecrashes cosmic baby-making party

A twisted magnetic field reveals a hidden baby star.

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Astronomers recently came across a star hiding a secret. A protostar in a distant stellar nursery called the Aquila Rift had a twisted magnetic field, and researchers weren’t sure why. Behind a veil of dust, astronomers found the culprit: a baby star locked in a gravitational embrace with the protostar, hidden in a cosmic nest.

The discovery will help astronomers find more star pairs like this by searching for peculiarly-shaped magnetic fields, which can act as a beacon toward baby stars. The discovery also provides hints on how binary systems come to exist. That’s a big part of understanding the universe, because half of all Sun-like stars in space come in twos.

The protobinary L483. The dark region near the center is a cloud of star-making material, called an envelope.


Astronomer Erin Guilfoil Cox at Northwestern University led the team that peered into L483, the cloud at the heart of the study. Cox and colleagues turned to several observatories including the airplane-based Stratospheric Observatory for Infrared Astronomy (SOFIA), the Atacama Large Millimeter Array (ALMA) Radio Telescope in Chile, and the Pico dos Dias Observatory in Brazil.

A paper published Tuesday in The Astrophysical Journal details the findings.

Here’s the background — Binary stars are “notoriously difficult to observe at their youngest stages in life,” Guilfoil Cox tells Inverse. “That’s because they’re enshrouded in this gas and dust, and so they are pretty invisible to most telescopes. We don’t know much about their very earliest stages of life.”

Stellar deaths are grand, bursting as supernovas in an instant and producing nebulae many times the diameter of the forlorn star.

But baby stars are harder to follow. The cosmic cloud of dust and gas, which coalesces to become a new protostar, also forms a haze obscuring a youngling from astronomers. Baby stars grow slowly, too, with only minor changes occurring across a human lifetime.

Thankfully astronomers might have found a new roundabout way to learn about these early chapters. Guilfoil Cox’s team found that a previously-unknown baby star’s journey towards another protostar may have twisted a magnetic field.

Now astronomers want to find more instances like this, and if they do, they’ll have a brand new way to learn about how baby stars morph into their grown-up selves.

Messier 42, also known as the Orion Nebula.

NASA, ESA, M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team

What’s new — A magnetic field is a force field. We’re probably most familiar with the way it directs a compass towards the North and South Poles, where the magnetic field lines enter our planet. The cores of both Earth and the Sun generate magnetic fields, but the Big Bang may have done that too, with lingering effects. Curiously, mysterious magnetic field lines are threaded throughout all parts of our galaxy, including the stellar nursery Guilfoil Cox was observing.

It turns out that the processes that created and carried the newly-discovered baby star — initially hidden out of sight within the nursery of another known protostar — had twisted a magnetic field line.

Why it matters — Learning how stars form will help us learn more about the Solar System, and help us comprehend our origins.

“Stars are the basic unit of the universe,” says Guilfoil Cox. The forces that produce stars affect how their planets form, and collectively, stars and planets make up galaxies. Therefore, understanding stellar life cycles is crucial for the field of astrophysics.

An illustration of a baby star’s development. A massive young star produces jets of charged particles that shoot out from its poles, because its magnetic field is overwhelmed by the amount of material it has pulled from the envelope, the dark-orange region of star material seen here on the bottom right.


What they did — Astronomers peered at L843, an “isolated star-forming region” — that just means this cosmic nursery isn’t crowded with stellar fledglings.

They took an extensive view of this area, from about 10,000 Earth-Sun distances (or astronomical units), and then zoomed down in size, first beginning with what’s known as a molecular cloud, down to an envelope of material, down to a disk, then down to a protostar, ending at about 1,000 astronomical units. These structures become progressively denser on smaller scales.

They noticed a magnetic field running parallel to an outflow of discarded star-making material, producing jets. When astronomers zoomed into this scene, they saw the twist.

“It doesn’t seem likely that it should happen,” says Guilfoil Cox. The field should appear more disordered the more they zoomed into the scene, when observing smaller scales. But they saw that this field wasn’t scattered.

“So we zoomed in even closer using the ALMA telescope down in Chile, and we see that the system that we looked at is actually forming two stars. And they are pretty close together.”

“We thought, ‘oh, this is interesting,’” she says. “We thought this was a single star when we first proposed to observe it.”

The yellow region at the top of this illustration is a cloud of material that can create multiple stars. The middle of the image shows three pockets of dust and gas, called protostellar clouds, that will collapse to form three stars. At the bottom we zoom into one of these would-be stars to see a disk condensing into the protostar seen at the disk’s center.


What they discovered – A possible explanation is that the two baby stars, which are now roughly 30 astronomical units apart — the distance between the Sun and Pluto — were once much farther apart when they were forming.

Guilfoil Cox says computer simulations show that binary stars can form far apart — more than 500 astronomical units from each other — and then gradually come closer together.

“If there’s a change in the dynamics of the system really close together, at the scales of the protostars, it can shift the direction of the magnetic field.”

What’s next — “We think we might be able to use the magnetic field signature as a way to determine the formation history of the binary,” says Guilfoil Cox.

To prove that a magnetic-field signature can be a new astronomy tool, researchers need to collect a statistical sample that shows other similar instances to Guilfoil Cox’s work. The sample will have to be at the scale of envelopes, those tight yet wafting regions of stellar yolk that surround protostars and make them grow.

Guilfoil Cox will survey a bunch of young stars to see if there are any binary stars there while also looking at the morphology of their magnetic fields using the TolTEC instrument on the Large Millimeter Telescope in Mexico.

The team looks forward to the “really exciting time” when they can use newer telescopes with higher sensitivity to peer into these stellar nurseries like never before.

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