More than 3,000 new stars are formed in the universe every second as clouds of dust and gas collapse due to gravity. The remaining dust and gas then settle into a swirling disk that promotes the star's growth and eventually aggregates into planets – also known as a protoplanetary disk. While this model, known as the nebula hypothesis, is the most widely accepted theory, the exact processes that lead to the formation of stars and planetary systems are not yet fully understood. Elucidating these processes is one of the many goals of the James Webb Space Telescope (JWST).
In a recent study, an international team of astronomers led by researchers at the University of Arizona and supported by scientists at the Max Planck Institute for Astronomy (MPIA) used JWST's advanced infrared optics to study protoplanetary disks around new stars. These observations provided the most detailed insights into the gas flows that shape and shape protoplanetary disks over time. They also confirm what scientists have long suspected, providing clues about what our solar system looked like about 4.6 billion years ago.
The research was led by Ilaria Pascucci, a professor of astrophysics and planetary science at the University of Arizona's Lunar and Planetary Laboratory (LPL). She was joined by researchers from the Space Telescope Science Institute (STScI), the Observatoire de Paris, the National Optical-Infrared Astronomy Research Laboratory (NOIRLab), the Carl Sagan Center at the SETI Institute, the Max Planck Institute for Astronomy, and several Universities. The article describing their results recently appeared in Nature Astronomy.
Artist's impression of a young star surrounded by a protoplanetary disk of gas and dust. Photo credit: LMU/Thomas Zankl, media with downcast eyes
In order for young stars to grow, they must suck in gas from the protoplanetary disk surrounding them. To do this, the gas must lose its angular momentum (inertia); otherwise it would constantly orbit the star and never grow on it. However, the mechanism that makes this possible remains controversial among astronomers. Magnetic-driven disc winches have emerged as a possible mechanism in recent years. Driven primarily by magnetic fields, these “winds” carry streams of gas from the planet-forming disk into space at tens of kilometers per second.
This causes it to lose angular momentum and the remaining gas falls inward towards the star. For their study, the researchers selected four protoplanetary disk systems that can be seen edge-on from Earth. Using Webb's Near Infrared Spectrograph (NIRSpec), the team was able to track different layers of wind by tuning the instrument to detect different atoms and molecules in specific transition states. Using the spectrograph's Integral Field Unit (IFU), the team also obtained spatially resolved spectral information across the entire field of view.
This allowed the team to track the disk winds in unprecedented detail and discover a complicated, three-dimensional layered structure: a central jet embedded in a cone-shaped shell of winds at increasing distances. The team also noticed a distinct central hole in the cones of all four protoplanetary disks. According to Pascucci, one of the most important processes is the way the star accumulates matter from its surrounding disk:
“How a star increases in mass has a major impact on how the surrounding disk evolves over time, including how planets later form. The exact mechanisms by which this occurs are not yet clear, but we believe that winds driven by magnetic fields across most of the disk surface may play a very important role.”
But other processes are also responsible for the formation of protoplanetary disks. This includes “X-wind,” in which the star’s magnetic field pushes material on the inner edge of the disk outward. There are also “thermal winds” that blow at much slower speeds and are caused by intense starlight eroding its outer edge. The high sensitivity and resolution of the JWST were ideal for distinguishing between the magnetic field-driven wind, the X-wind and the thermal wind. These observations revealed a never-before-seen nested structure of the various wind components.
Observed gas jet and wind structure of the protostar HH 30, with offsets in astronomical units (au), the mean distance between the Sun and Earth. Photo credit and ©: I. Pascucci et al./MPIA
A key difference between the magnetically driven winds and the X-winds is that they are located further out and cover larger regions. These winds cover regions corresponding to the inner rocky planets of our solar system, such as between Earth and Mars. They also extend further across the disk than thermal winds, reaching hundreds of times the distance between Earth and the Sun. While astronomers previously found observational evidence of these winds based on interferometric observations at radio wavelengths, they were unable to image the entire disk in detail to determine the morphology of the winds.
In contrast, the new JWST observations revealed a nested structure and morphology that matched astronomers' expectations for a magnetically driven disc wind. Looking forward, Pascucci and her team hope to extend these observations to additional protoplanetary disks to see how common the observed disk wind structures are and how they evolve.
“Our observations strongly suggest that we have obtained the first detailed images of the winds that can remove angular momentum and solve the long-standing problem of how stars and planetary systems form,” she said. “We think they could be common, but with four objects it's a little difficult to say. We want to use JWST to get a larger sample and then also see if we can detect changes in these winds as stars assemble and planets form.”
Further reading: MPIA, natural astronomy
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