A Japanese-led international team of researchers has modelled a star’s birthplace and observed the chemical reactions taking place inside. Their work will shed light on how stars form.
Stars begin their lives in molecular clouds – cold, dark, dense regions of a galaxy where atoms and ions can bond to form molecules. These molecules experience temperatures of -263°C, where in theory any substance more complex than hydrogen or helium should be trapped in ice on the surface of dust grains.
In practice, the molecules have a little more leeway: observations show that some more complex substances can float freely around the cloud. Although scientists drawing on multiple different fields have tried to model the dynamics and chemistry inside a molecular cloud, we are still some way from fully understanding why these complex molecules aren’t locked inside ice.
We know of at least one way to free the molecules; ultraviolet radiation can strike the ice and release the chemicals inside in a process known as photodesorption. Researchers have observed this in some of the more massive gas clouds. However, in the densest, darkest regions where star formation begins, photodesorption isn’t efficient enough to explain the proportion of free-floating molecules. Something else must be at work.
When molecules land on a surface, they can react with other molecules on that surface, releasing energy and ejecting the products. This process, which scientists call chemical desorption, is thought to play a role in how molecular clouds evolve. However, looking for the telltale signs of chemical desorption in outer space is no easy task.
Instead, a team from Hokkaido University, Japan, and the University of Stuttgart in Germany, headed by Yasuhiro Oba and Naoki Watanabe, decided to search for chemical desorption by recreating the conditions inside a molecular cloud.
The experimental setup they used contained hydrogen sulphide (the chemical responsible for the smell of rotten eggs) and a special kind of ice known as amorphous solid water, all cooled to -263°C – the same temperature inside a molecular cloud. At the low temperatures and pressures found in interstellar space, ice no longer has the regular structure we see on Earth – it has no repeating structure at all and is called “amorphous”.
The team then exposed the hydrogen sulphide to molecular hydrogen. Because molecules vibrate constantly and absorb light at frequencies close to the ones at which they vibrate, the team could use infrared absorption spectroscopy to determine which molecules were present and thus what, if any, chemical reactions were taking place.
Oba’s team not only found that chemical desorption was taking place due to the hydrogen and hydrogen sulphide reacting with each other, but that chemical desorption is more efficient than previously thought. This raises implications for the evolution of molecular clouds and thus for star formation.
Measuring chemical desorption in space is difficult, but experiments here on Earth will give other researchers some picture of what might be happening in molecular clouds. Even if it turns out that our experiments on Earth don’t agree with observations, Oba’s study and others like it are paving the way for a better understanding of clouds and star formation.
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