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Nested morphology of gas streams confirms a mechanism that helps infant stars to grow by ingesting disk material.

Planet-forming disks, maelstroms of gas and dust swirling around young stars, are nurseries that give rise to planetary systems, including our solar system. Astronomers have discovered new details of gas flows that sculpt and shape those disks over time. The observed nested structure of those flows confirms a long-theorized mechanism that allows the star to grow by tapping disk material.

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Every second, more than 3,000 stars are born in the visible universe. Many are surrounded by what astronomers call a protoplanetary disk – a swirling “pancake” of hot gas and dust that feeds the central star’s growth and provides the building blocks of new planets. However, the exact processes that give rise to stars and planetary systems are still poorly understood.

JWST takes a detailed look at disk winds

A team of astronomers led by University of Arizona researchers supported by scientists from the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany, used the James Webb Space Telescope (JWST) to obtain some of the most detailed insights into the forces that shape protoplanetary disks. The observations offer glimpses into what our solar system may have looked like 4.6 billion years ago.

Specifically, the team was able to trace so-called disk winds in unprecedented detail. These winds are streams of gas blowing from the planet-forming disk out into space. Primarily powered by magnetic fields, these winds can travel dozens of kilometres in just one second. The researchers’ findings, published in Nature Astronomy, help astronomers better understand how young planetary systems form and evolve.

According to the paper’s lead author, Ilaria Pascucci, a professor at the University of Arizona’s Lunar and Planetary Laboratory, one of the most important processes at work in a protoplanetary disk is the star eating matter from its surrounding disk, which astronomers call accretion.

“How a star accretes mass has a big influence on how the surrounding disk evolves over time, including the way planets form later on,” Pascucci said. “The specific ways in which this happens have not been understood, but we think that winds driven by magnetic fields across most of the disk surface could play a very important role.”

Magnetized disk winds help with stellar growth

Young stars grow by pulling in gas from the disk swirling around them, but for that to happen, the gas must first shed some of its inertia. Otherwise, the gas would consistently orbit the star and never fall onto it. Astrophysicists call this process “losing angular momentum,” but how exactly that happens has proved elusive.

To better understand how angular momentum works in a protoplanetary disk, it helps to picture a figure skater on the ice: Tucking her arms alongside her body will make her spin faster while stretching them out will slow down her rotation. Because her mass does not change, the angular momentum remains the same.

For accretion to occur, gas across the disk has to lose angular momentum. Still, astrophysicists have a hard time agreeing on how exactly this happens. In recent years, magnetically driven disk winds have emerged as essential players funnelling away some gas from the disk surface – with it, angular momentum – allowing the leftover gas to move inward and ultimately fall onto the star.

How to distinguish between wind mechanisms

Because other processes at work also shape protoplanetary disks, it is critical to be able to distinguish between the different phenomena, according to the paper’s second author, Tracy Beck at NASA’s Space Telescope Science Institute.

While the star’s magnetic field pushes out material at the inner edge of the disk in what astronomers call an X-wind, the outer parts of the disk are eroded by intense starlight, resulting in so-called thermal winds, which blow at much slower velocities. JWST’s high sensitivity and resolution were ideally suited to distinguish between the magnetic field-driven wind, the thermal wind and the X-wind.

A crucial property distinguishing the magnetically driven from the X-wind is that they are located farther out and extend across broader regions, including the inner, rocky planets of our solar system – roughly between Earth and Mars. These winds also extend farther above the disk than thermal winds, reaching hundreds of times the distance between Earth and the sun.

“We had already found observational indications for such a wind based on interferometric observations at radio wavelengths,” MPIA astronomer Dmitry Semenov points out. He is also a co-author of the underlying study. However, those observations could not probe the entire disk wind morphology, let alone image them in detail. In particular, the nested structure of the various wind components, a hallmark of those disk winds, was beyond the observations’ capabilities. In contrast, the new JWST observations revealed that structure without any doubt. The observed morphology matches the expectations for a magnetically driven disk wind.

“Our observations strongly suggest that we have obtained the first detailed images of the winds that can remove angular momentum and solve the longstanding problem of how stars and planetary systems form,” Pascucci said.

For their study, the researchers selected four protoplanetary disk systems, all appearing edge-on when viewed from Earth. Their orientation allowed the dust and gas in the disk to act as a mask, blocking some of the bright central star’s light, which otherwise would have overwhelmed the winds.

JWST’s NIRSpec resolves nested wind morphology

The team could trace various wind layers by tuning JWST’s NIRSpec detector to distinct atoms and molecules in certain states of transition. NIRSpec is JWST’s high-resolution near-infrared spectrograph. The astronomers obtained spatially resolved spectral information across the entire field of view by employing the spectrograph’s Integral Field Unit (IFU), essentially a grid looking at distinct positions in the sky. This way, the scientists synthesized images at various diagnostic wavelengths, each being comparably coarse but still good enough to resolve the morphology.

The observations revealed an intricate, three-dimensional structure of a central jet nested inside a cone-shaped envelope of winds originating at progressively larger disk distances, similar to the layered structure of an onion. According to the researchers, an important new finding was the consistent detection of a pronounced central hole inside the cones, formed by molecular winds in each of the four disks.

Next, Pascucci’s team hopes to expand these observations to more protoplanetary disks to understand better how common the observed disk wind structures are in the universe and how they evolve.

“We believe they could be common, but with four objects, it’s a bit difficult to say,” Pascucci said. “We want to get a larger sample with JWST and then also see if we can detect changes in these winds as stars assemble and planets form.”

Background information

The MPIA scientists involved in this study are Dmitry Semenov and Kamber Schwarz.

Other researchers include Ilaria Pascucci (Lunar and Planetary Laboratory, University of Arizona, Tucson, USA [UofA], study lead), Tracy L. Beck (Space Telescope Science Institute, Baltimore, USA), Sylvie Cabrit (Observatoire de Paris, LERMA, CNRS, Paris, France), and Naman S. Bajaj (UofA).

NIRSpec is part of the European Space Agency’s (ESA) contribution to the Webb mission, built by a consortium of European companies led by Airbus Defence and Space (ADS). NASA’s Goddard Space Flight Centre provided two sub-systems (detectors and micro-shutters). MPIA was responsible for procuring electrical components of the NIRSpec grating wheels.

JWST is the world’s premier space science observatory. It is an international program led by NASA jointly with its partners, ESA (European Space Agency) and CSA (Canadian Space Agency).

Funding for this work was provided by NASA and the European Research Council.

This text is largely based on a press release published by the University of Arizona, written by Daniel Stolte.

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