Chemical Process First Proposed 50 years Ago Helps Understand How Stars Form

The Taurus molecular cloud where such chemical processes might be taking place. ESO

Fifty years after it was first proposed, researchers have reproduced in the lab a chemical process that can explain how certain gases escape from frozen particles at almost absolute zero. This process is thought to be crucial in star-forming nebulae.

It might seem counterintuitive but the gas that eventually collapses into a star needs to be cold. If the gas is too warm, it would move too fast to condense beyond a certain critical point. The temperatures of these clouds are just around a few degrees above absolute zero, and according to theory, everything but hydrogen and helium should be frozen (with not much chemistry going on).

“Interstellar chemistry is of great importance to understanding the formation of stars, as well as water, methanol and possibly to more complex molecular species,” co-author Naoki Watanabe, from Hokkaido University, said in a statement. The study is published in Nature Astronomy.

Observations show that there are molecules flying around, and the researchers explain how simple molecules could be escaping from these frozen dust particles. The process is called chemical desorption, and it implies that molecules can free themselves by “stealing” a bit of energy from a chemical reaction happening nearby.

A similar type of desorption (which is the opposite of absorption) is well documented in molecular clouds and is a consequence of intense radiation. Photodesorption can liberate chemicals from frozen dust, but in the denser region of the nebulae where stars are on the verge of birth, it is too dark for this to be an efficient process.  

That’s why 50 years ago, researchers proposed chemical desorption as the key mechanism. But there was no proof that this could happen in outer space. Until now. A Japanese-German collaboration with researchers from the Hokkaido University and the University of Stuggart have created an experimental system to reproduce the extreme conditions of a giant molecular cloud.

They placed amorphous solid water (which lacked the crystalline structure of ice) at 10 Kelvin (-263.15°C/-441.67°F) with hydrogen sulfide and monitored the reaction. They discovered that the desorption was caused by the chemical reaction and it was a lot more efficient than previously estimated. In particular, it is more efficient than desorption produced by light, suggesting this has a key role in the formation of stars.

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