The planets in our solar system and the thousands of known exoplanets formed in circular disks around their host star. The past decade of observations and simulations has revolutionized how astronomers understand planet formation and disk physics, however measuring the regions where Earth-like planets form is still very challenging. These terrestrial planet-forming regions are so close to the central star that they are nearly impossible to resolve from larger regions in the disk. In an article in Nature Astronomy published on June 19, a team of international astronomers, led by Purple Mountain Observatory (PMO) and Kavli Institute of Astronomy and Astrophysics at Peking University (PKU/KIAA), reported the highest resolution images ever obtained of the wind from one of these disks. The physical scale of 3.5 AU is small enough to measure the physics in regions where terrestrial planets form. Interpreting the images with state-of-the-art simulations led to surprising results that alter our view of the physics that determines how disks evolve.
Disks evolve with time as mass moves through the disk, ending up either in the star, a planet, or ejected from the system in winds. The ejection occurs through a combination of magnetic winds, launched near the star, and photoevaporative winds, launched at larger distances. Both types of winds can be measured using emission from neutral oxygen. New observations of oxygen with the MUSE instrument on the Very Large Telescope, owned and operated by European Southern Observatory, revealed that most of the oxygen emission is produced in the terrestrial-forming regions of the disk. "These observations demonstrate that the oxygen emission is almost entirely produced in the disk and magnetic disk wind very near the star," said Dr. Min Fang of PMO and lead of the study. "While we see the photoevaporative wind, the emission is much fainter than expected from models, a puzzle that we still need to solve."
The observations were interpreted with advanced simulations of winds that include magnetic fields, irradiation of the disk, and many other processes. "When we compared the observations to predictions from our generic simulations, without any tweaks, the magnetic winds perfectly matched the MUSE images and also the kinematics of oxygen emission," said Lile Wang, a professor at PKU/KIAA and second author of the study. "Our models also reproduced neon emission, which has been the primary observational evidence for photoevaporation."
The study originated with discussions by team members about the unique capabilities of the MUSE instrument to study winds from disks with a multi-conjugate adaptive optics system. The data were originally obtained to search for young planets in a team that included co-author Sebastiaan Haffert, a researcher at University of Arizona. "While we recognized the power of MUSE, the analysis really became powerful when we were able to combine it with simulations," says co-author Gregory Herczeg, a professor also at PKU/KIAA. "We are also deeply appreciative of the observers. It is unfortunate that the primary goal of searching for planets did not lead to a discovery, however, the open availability of data through powerful archives allows teams with different expertise to assess data for other purposes."
This work paves the way for improved tests of disk physics with even better observations in the future, including with the Chinese Space Station Telescope (CSST). "One of the instruments planned for CSST, the integral field unit spectrograph, is ideal for expanding from this case study," said Dr. Fang. "The exquisite point spread function of CSST will allow us to use this instrument to test photoevaporation with unsurpassed images of disks.
Figure 1: A painting by Dawei Li (Xiamen Univeersity) that visualizes the swirling wind expected from protoplanetary disks. The wind is launched away from the star but continues to rotate, following the rotation of the disk around the star. The launch region is often inferred from the velocity profile in spectral lines, such as oxygen, but has not been previously measured with such high spatial resolution.
Figure 2: The left panel shows the oxygen emission extracted from the VLT/MUSE observation of TW Hya, with a physical resolution of about 3.7 au. The right panel shows that the radial profile of the oxygen emission, convolved with the point spread function of MUSE is consistent with expectations for a magnetic wind but much weaker than expected for a photoevaporative wind.
Published Paper: https://www.nature.com/articles/s41550-023-02004-x