Unveiling the Secrets of Protoplanetary Discs: A New Perspective with Polycyclic Aromatic Hydrocarbons
The formation of planets is a complex process, and scientists have long relied on models that make assumptions about the physical structure of protoplanetary discs. These discs, where planets are born, have layers of dust and gas that are crucial to understanding their evolution. One key aspect is the vertical structure, which determines how dust is distributed by the turbulent motion of gas.
In a recent study, researchers have used data from the NIRSpec instrument on the James Webb Space Telescope (JWST) to reveal a new perspective on the protoplanetary disc known as the Flying Saucer. This disc is unique because it is 'back-lit' by an ambient emission region containing Polycyclic Aromatic Hydrocarbons (PAHs). These PAHs, excited by ultraviolet photons, emit infrared photons that are absorbed by the disc at specific wavelengths, creating a silhouette effect.
The authors, led by E. Dartois from the Institut des Sciences Moléculaires d’Orsay, France, created a model using the RADMC3D radiative transfer model to simulate the path of light from the disc to the JWST. They included the ambient emitting region, light scattered by the disc, and interstellar dust extinction. By combining this model with the JWST's point-spread function, they compared it to observations, revealing the disc's vertical structure.
The results were fascinating. The disc's radius, when considering small grains, was found to be 235 au, which contradicts previous millimetre observations from the Atacama Large Millimeter Array (ALMA). The ALMA data traced larger grains, while JWST data focused on micron-sized dust, leading to the discrepancy. The cause of this difference remains unclear.
Furthermore, the study revealed that smaller grains are less settled than larger, millimetre-sized grains, aligning with theoretical expectations. The authors plan to conduct a follow-up study to fully analyze the disc's structure and properties.
One of the most intriguing findings was that the vertical extent of small, icy grains is larger than expected. The models suggest that a small fraction of ice-covered dust should be even higher up, defying the balance of forces due to gravity and gas pressure. However, the authors caution that model parameters could be degenerate, making it challenging to pinpoint the exact cause of these results.
The Flying Saucer disc offers a unique opportunity to study protoplanetary discs using PAH emission. The authors' follow-up study is highly anticipated, as it may provide further insights into the complex world of planet formation and the intricate structures within protoplanetary discs.
This research, accepted in Astronomy & Astrophysics (Open Access), showcases the power of advanced telescopes like JWST in unraveling the mysteries of the universe, one protoplanetary disc at a time.
