The new observations confirm a critical step in star formation

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Artist’s impression of the geometry of the accretion disk surrounding the young star and the outflowing rotating disk wind. Parts of the disk wind moving toward us turn blue, so the image is blue; Areas moving away from us are redshifted (in red). Credit: D. Mueller, R. Lanhart (MPIA)

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Artist’s impression of the geometry of the accretion disk surrounding the young star and the outflowing rotating disk wind. Parts of the disk wind moving toward us turn blue, so the image is blue; Areas moving away from us are redshifted (in red). Credit: D. Mueller, R. Lanhart (MPIA)

The new observations have confirmed a key step in the star formation process: the swirling „cosmic wind” made up of molecules, which is critical for gas clouds to contract sufficiently to form a hot, dense young star.

The result obtained from radio observations, combined with a sophisticated analysis, allowed astronomers to study the flow of material around a young star in the dark cloud, CB26, in greater detail than ever before. Work Published in the magazine Astronomy & Astrophysics.

Observations by Ralf Lahnhardt, group leader at the Max Planck Institute for Astronomy, and colleagues have captured a key part of the standard conditions for the formation of new stars: the mechanism by which gas clouds can collapse and give birth to a new star. , without being torn by their own cycle in the process.

Gas in the cosmic hydrogen cloud collapses under its own gravity, forming new stars as its temperature rises. Beyond a certain range of density and temperature, nuclear fusion occurs, where hydrogen nuclei combine to form helium nuclei. The energy released by this process makes the stars shine. But there is a problem. No cloud of gas in the universe is completely stationary—all clouds rotate slightly. As the gas contracts, the vortex accelerates. Physicists call this „conservation of angular momentum”.

Known outside of astronomy, from figure skating: when a figure skater wants to spin very fast, they extend both arms and one leg and start spinning slowly. Then, they pull their joints closer to their axis of rotation, and the speed of rotation increases significantly.

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A problem and its (possible) solution

For star formation, this spells a potential problem. The fast spin includes centrifugal forces, flinging material away from the axis of rotation. For a swing ride or swing carousel, that’s part of the fun: As the carousel spins, riders’ chain-supported chairs are thrown outward. For a protostar, on the other hand, centrifugal forces are dangerous: if enough material is ejected as the cloud collapses and speeds up its rotation, there won’t be enough to form a protostar in the first place.

This is called the „angular momentum problem” of star formation. A theoretical solution to much of the problem was found in the 1980s. As additional material falls onto the new central protostar, it forms a so-called accretion disk: a flat rotating disk of gas and dust whose material eventually falls onto the central protostar.

The physics behind accretion disks is involved: some of the gas in the disk becomes plasma, and each of the hydrogen atoms splits into an electron and a proton. As the plasma rotates in the disc, it creates a magnetic field. This field affects the plasma flow: a small amount of plasma moves along the magnetic field lines.

Each time, drifting plasma particles collide with (electrically neutral) molecules; As a result some molecular gas is also carried away. These molecules form a „disc wind” that takes considerable angular momentum away from the disc. The loss of angular momentum slows the rotation, reduces centrifugal forces, and can solve the protostar’s angular momentum problem.

From hypothesis to observation

At first, this scenario is no more than a plausible hypothesis. To an observer on Earth, even an accretion disk-like structure around a nearby nascent star is too small. That’s why it took more than 20 years for astronomers to find temporal evidence of rotation in this kind of mass flow: In 2009, Ralf Lahnhardt and colleagues at the Max Planck Institute for Astronomy were able to observe an outflow around a young star. A small hydrogen cloud with the name CB26. At less than 460 light-years from Earth, CB26 is one of the closest disk systems around a protostar.

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The observations in question are made with radio telescopes operating at millimeter wavelengths, in this case an array of antennas called a Platte de Pure interferometer. In fact, such antennas are connected in a clever way so that they act like a single, very large radio dish. These types of radio telescopes can detect the characteristic radiation of different types of molecules—in this case, carbon monoxide (CO). As the molecules move toward or away from the observer, that characteristic radiation is shifted to slightly longer or shorter wavelengths („Doppler shift”), allowing astronomers to track gas motion along the line of sight.

Observations in 2009 showed that the outflow of gas from the young star was indeed in motion, and in the right way one would expect from a rotating disk wind that dissipates angular momentum. But they don’t provide enough fine-grained detail to allow any judgment about the distance from the star where the wind is ejected from the disk—a key property that determines how much angular momentum the gas stream carries (think „love”).

Observing rotating disc winds

New results just published confirm the case. For this work, Lanhart and colleagues were able to make observations with higher angular resolution. They used the configuration of the Plateau de Pure Observatory, in which the radio antennas were placed much further away than their first observations. They also fielded a sophisticated physico-chemical model of the disc, which allowed them to distinguish between contributions from the disc and contributions from the disc wind.

All this allowed astronomers to determine the dimensions of the cone-shaped outflow: near the disk, the lower end of the cone has a radius roughly 1.5 times the Earth-Neptune distance—enough for the disk wind to carry. Lots of angular momentum. This is the first time that those dimensions have been determined directly from (reconstructed) images.

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With these measurements, the argument wins: disc winds solve most of the angular momentum problem for protostars. Lanhart and colleagues were able to compare their measurement with indirect reconstructions of the dimensions of the disc wind in nine other young star-disc systems published in a 2009 paper.

This comparison shows a clear trend in the mean radius of the disk, in which the disk wind is growing with time: initially, in the first tens of thousands of years, there are more concentrated disk winds. For a million years the disc wind is very widespread.

Next steps

Astronomers are already planning the next observations of CB26. Meanwhile, the Plateau de Bure Interferometer has been improved. The new observatory, called NOEMA, has 12 antennas instead of the previous 6, and offers structures that can tease out twice as much detail as its predecessors.

But while those refinements hold considerable promise, the current paper takes a key step: confirming that disc winds are indeed a key factor in allowing protostars to form in the first place and solving the angular momentum problem.

More information:
R. A resolved rotating disc wind from a young T Tauri star in Boke globule CB 26, Lanhart et al. Astronomy & Astrophysics (2023) DOI: 10.1051/0004-6361/202347483

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