Modeling the Dynamics of the Orion Nebula

On January 22, 2015, Prof. Gary Ferland from the University of Kentucky visited NAOC and gave a talk about dynamics of molecular clouds, with a focus on the Orion Nebula. Prof. Ferland is a world-renown expert on this topic. He is famous in this field because he led the development of the widely-used computer modeling code “Cloudy” which was originally released in 1980 and has been updated approximately every two years since then. Prof. Ferland began his talk by outlining how he became interested in the topic of molecular clouds. He said that when he was a postdoctoral researcher, he was studying Active Galactic Nuclei (AGNs) and understood that the dusty torus around the central black hole in an AGN plays an important role in how radiation from the AGN is observed. This led him to start studying the complex dynamics of how radiation and magnetic fields interact with molecular clouds and how these processes can be interpreted in astronomical observations.

Prof. Ferland continued by introducing properties of the Orion Nebula, the nearest large molecular cloud to Earth. He showed a map of the region around the Orion Nebula that was taken with emissions from carbon monoxide, a good tracer of the dynamics in the cloud. It is clear from these emissions that the size of the Orion Nebula is much larger than what is apparent to a visual observer. He said that almost all the ionizing radiation that affects the dynamics of the Orion Nebula comes from a single bright star. This star heats and ionizes the surrounding gas, thus increasing pressure in the gas compared with other parts of the cloud. This increased pressure blows a hot bubble around the star and the heated gas eventually escapes the cloud via a “rocket funnel” in the same way that hot gas from a rocket engine escapes into the ambient air. He continued by showing how the energy generated by this hot star leads to dynamical interactions in the Orion Nebula, emphasizing that flows are dominated by the magnetic field in the cloud. He also illustrated how it is fortunate for observers on Earth that the stars in the Trapezium are on the near side of the Orion Nebula, behind a thin veil of gas. Because of this lucky arrangement, astronomers can easily observe interactions between these stars and the surrounding dust and gas, helping to advance knowledge in this complex field.

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