Meanwhile, astronomers have achieved a double breakthrough. Using state-of-the-art telescope technology, they were able to prove that magnetic fields do in fact play an important role in “feeding” stellar embryos. Thus, the German astrophysicist Max Kamensend’s theory, more than 30 years old, was confirmed.
This evidence is made possible by the European Southern Observatory’s (ESO) GRAVITY instrument in Chile. Commissioned in 2016, this interferometer combines signals from the four 8-meter telescopes of the Very Large Telescope (VLT) on Paranal. This creates images with resolution up to that of a 100m telescope.
Astronomers led by Jerome Bouvier of the University of Grénoble made the first breakthrough in early 2020 (Astronomy and astrophysics). They used the GRAVITY instrument to study the protostar DoAr 44. This is located about 476 light-years away from us in the star-forming region of Rho-Ophiuchus and is in the T Tauri phase. There is already a gap of about 30 astronomical units between it and the inner edge of its physical disk.
This protostar is exactly at the stage where it becomes difficult for it to obtain matter just by its own gravity. But GRAVITY data confirmed that DoAr 44 was still able to siphon gas from its surrounding disk. Using spectroscopy, astronomers discovered that strong ultraviolet radiation from the protostar initially ionized hydrogen gas at the inner edge of the accretion disk. The plasma of protons and electrons becomes conductive and can now be affected by the stellar magnetic field.
Bouvier and his team also observed a strikingly bright glowing region near the protostar’s surface. According to the spectrum, this radiation was produced by hot, fast-moving gas between the surface of the star and the disk — as predicted by the models. “Both the size of the ejected region and its slight offset relative to the central star indicate that this radiation originates in magnetic flux channels between the inner edge of the disk and the stellar surface,” according to the team’s report.
guided by the magnetic field
The second important observation was made shortly thereafter by Rebecca García-López of the Max Planck Institute for Astronomy and her colleagues (nature). They targeted the well-known T Tauri star TW Hydrae with the GRAVITY instrument. In this protostar, too, they detected telltale radiation of hot ionized hydrogen — and it didn’t come from the disk of material or the star’s surface, but from the gap in between.
“We can see how material from the surrounding disk is directed toward the star,” says Lopez. “This makes us the first researchers to elucidate the process by which new stars and eventually planets are born.” According to observations, ionized hydrogen gas flows in narrow columns from the disk into the star, following magnetic field lines. This confirms Cumminsend’s model of magnetic accumulation in the magnetosphere.
This answers one of the fundamental questions about the growth of protostars. However, many details are still open. This involves a more detailed reconstruction of the physical processes involved in magnetospheric accretion, particularly near the stellar surface. Details of the structure of the magnetic field of protostars are still not clear: “The magnetic fields can be more complex and have additional poles,” explains Thomas Henning of the Max Planck Institute for Astronomy.
Further investigations of T Tauri stars with the GRAVITY instrument should help clarify this. “This includes observations that track how the point of impact of gas on the star’s surface changes over time,” explains Henning’s colleague Wolfgang Brandner. “We hope to get clues about how far off the axis of rotation the star’s magnetic poles are.”
It can also reveal whether the initial magnetic fields change over time and how the magnetic field is related to the frequent flares of T Tauri stars. Even if some of the mysteries of the growth of young stars have already been solved, there is still much to be done for astronomers.
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