New 3D model reveals how shadows reshape chemistry in stellar nurseries

How do young stars reshape the clouds from which they are born? A new high-resolution 3D model developed by an international team led by Dr Brandt Gaches from the University of Duisburg-Essen shows how the uneven structure of stellar nurseries creates shadows that alter their chemistry and influence the infrared emission observed by the James Webb Space Telescope. The model has now been published in Astronomy & Astrophysics.

Stars are born inside vast clouds of cold gas and dust. As massive young stars begin to shine, their intense ultraviolet light heats the gas, breaks apart molecules and triggers a cascade of chemical reactions in the clouds’ outer layers. These boundary zones are known as photodissociation, or photon-dominated, regions. Their light provides astronomers with important clues about how stars form and how gas is recycled to create future generations of stars and planets both within our own galaxy and in investigations of star formation across cosmological history.

“The Orion Bar is an ideal natural laboratory for studying these processes,” says Dr. Brandt Gaches. Located about 1,350 light-years from Earth, it can be observed in exceptional detail. “We see it nearly edge-on, so its different chemical layers are particularly easy to distinguish. That makes it a benchmark for testing models of photon-dominated regions,” adds Gaches, who leads an Emmy Noether Research Group on molecular astrophysics at the University of Duisburg-Essen (UDE).

Modelling the Orion Bar in 3D

Until now, photodissociation region models of the Orion Bar have generally represented it as a series of one-dimensional layers. In reality, however, interstellar clouds are clumpy and irregular. The new model captures this complex three-dimensional structure at high resolution and follows in detail how molecular hydrogen—the most abundant molecule in the universe—behaves under intense stellar radiation.

Calculating how radiation travels through the complex 3D structure of the Orion Bar is computationally demanding. “We developed a new highly efficient algorithm to overcome the computational bottleneck. This allowed us to model the Orion Bar in 3D at unprecedented spatial resolution,” says Dr. Zhengping Zhu from Zhejiang Lab in China, who developed the ray-tracing algorithm used in these models.

Shadows leave signatures in the light

The simulations reveal where important chemical transitions take place, including the change from atomic to molecular hydrogen and the recombination of ionized carbon. They also show how the cloud’s uneven structure can produce the arcs and filaments seen in observations by the James Webb Space Telescope (JWST). Dense regions block part of the incoming radiation, creating shadows and shielded pockets in which the chemistry develops differently.

“The three-dimensional structure of the gas makes a crucial difference,” says Gaches. “It creates shadows and shields parts of the cloud from the intense radiation. That changes the chemistry and leaves a clear signature in the infrared to radio wavelengths of light we observe.”

The model also allows researchers to create realistic images of how such a region should appear to JWST and compare them with what the telescope actually sees. This makes it possible to study how the three-dimensional structure of stellar nurseries shapes their chemistry in ways that simpler one-dimensional models cannot capture.

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