Unlocking The Secrets Of Mysterious Interstellar Molecules

NASA Goddard Space Flight Center, 'Young Stars at Home in Ancient Cluster,' via Flickr. CC BY 2.0.

Back in 1919, student Mary Lea Heger discovered something strange whilst observing the spectrum of light from stars in a binary system- fuzzy, dark lines on the rainbow-like range of light wavelengths emitted by the objects. These missing colors were absorption features, meaning that something was absorbing certain wavelengths of light before it reached Earth.

While this was a normal feature, what was interesting was that the bands weren’t actually coming from the stars, but rather the matter between star systems in a galaxy- the interstellar medium. For the last century this phenomena has puzzled astronomers and despite a significant amount of research, no one has been able to figure out precisely what is causing these “diffuse interstellar bands” (DIBs).

It is known that this matter contains an abundance of small molecules, called interstellar molecules, which account for much of the carbon, hydrogen and silicon in the universe. Scientists have long suspected that these molecules might be responsible for DIBs since they naturally absorb light from luminous bodies, but given that there exists a lot of uncertainty about the precise chemical composition and arrangement of these molecules, this has been tricky to prove.

Now, a team of researchers from the Harvard-Smithsonian Center for Astrophysics have gathered data that suggests these molecules could be silicon-capped hydrocarbons. The work has been published in The Journal of Chemical Physics.

As mentioned, DIBs are caused by the absorption of photons of certain wavelengths of light as they travel through space before reaching us on Earth. We know that matter absorbs light, and that the wavelengths absorbed are specific to the type of matter. That means we can identify the presence of particular elements in stars or interstellar medium, but it’s much more difficult to do this for diffuse, unfamiliar molecules.

Scientists have been attempting to use spectroscopy to identify these enigmatic interstellar molecules for some time by demonstrating which molecules in the lab leave the same absorptive patterns, but this had been a rather fruitless endeavor.

“Not a single one has been definitively assigned to a specific molecule,” study co-author Neil Reilly said in a news-release. However, Reilly and colleagues now believe they may have identified a source of the DIBs- silicon terminated carbon chains.

The team started off by creating 3 of these chains in the lab with different numbers of carbon atoms (SiC3H, SiC4H, SiC5H) using a jet-cooled discharge of two molecules- a silicon containing compound (silane) and a hydrocarbon (acetylene). They then analyzed the optical spectra of these chains and carried out theoretical calculations to predict that longer chains in this family could be responsible for at least some of the DIBs.

While the research is promising, the team acknowledges that more needs to be done before they can definitively prove that these longer silicon capped hydrocarbon chains are a source of DIBs. Specifically, they need to define the types of transitions that these molecules undergo and demonstrate that these are representative of astronomical observations. Still, it appears that researchers could be tantalizingly close to finally identifying one of the mysterious sources of DIBs, which is exciting. 

[Header image "Young Stars at Home in Ancient Cluster," from NASA Goddard Space Flight Center, via Flickr. Used in accordance with CC BY 2.0]

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