Herschel reveals role of starlight in forming building blocks of life

Herschel reveals role of starlight in forming building blocks of lifeScientists using data from the European Space Agency’s Herschel Space Observatory spacecraft have discovered that ultraviolet light from stars plays a vital role in forming molecules that are necessary for building other chemicals that are essential to life. These fundamental substances, composed of atoms of carbon connected to hydrogen, oxygen, and other elements, were previously thought to be formed by “shock” events that create turbulence.

The researchers studied the ingredients of carbon chemistry in the Orion nebula, the closest star-forming region to Earth that forms massive stars. They measured the amount, temperature, and motions of the carbon-hydrogen molecule (CH, or “methylidyne”), the carbon-hydrogen positive ion (CH+), and the carbon ion (C+). An ion is an atom or molecule with an imbalance of protons and electrons that result in a net charge.

“On Earth, the Sun is the driving source of almost all the life on Earth. Now, we have learned that starlight drives the formation of chemicals that are precursors to chemicals that we need to make life,” said Patrick Morris, first author of the paper and researcher at the Infrared Processing and Analysis Center at Caltech in Pasadena.

CH and CH+ were two of the first three molecules discovered in space, back in the early 1940s. When scientists studied the Orion Nebula with Herschel, they were surprised to find that the CH+ is emitting rather than absorbing light, indicating that it is warmer than the background gas. The CH+ molecule requires a lot of energy to form and is extremely reactive, so it gets destroyed if it interacts with the hydrogen in the cloud. This makes its warm temperature and high abundance quite mysterious.

Many previous studies have tried to answer the question of why CH+ is so abundant in molecular clouds like the Orion Nebula, but their observations were limited because few background stars were available to study. Herschel can study an area of the electromagnetic spectrum – the far infrared, which is associated with cold objects – that no other space telescope has reached before, so it can study the entire Orion Nebula instead of just the individual stars within. The instrument the researchers used to capture their data, the Heterodyne Instrument for the Far-Infrared (HIFI), is also highly sensitive to the motions of the gas clouds.

One theory about the origins of basic hydrocarbons was that they were formed in “shocks” – events, such as exploding supernovae or young stars spitting out material, which create turbulence that results in shock waves. Like a large wave hitting a boat, shock waves cause vibrations in any material that they encounter. These vibrations can knock electrons off atoms, turning them into ions, which are more likely to combine into molecules. The recent study did not find a correlation between these shocks and the abundance of CH+ in the Orion Nebula.

Data collected by Herschel indicates that these CH+ molecules were more likely created by the ultraviolet emissions of very young stars in the Orion Nebula. When a molecule absorbs a photon of light, it becomes “excited” and has more energy to react with other particles,

The Orion Nebula has long been known to contain a lot of hydrogen gas. When ultraviolet light from large stars heats up the surrounding hydrogen molecules, this creates ideal conditions forming hydrocarbons. As the hydrogen gets warmer, carbon atoms formed in stars begin to react with the molecular hydrogen, creating CH+. The CH+ eventually captures an electron to form the neutral CH molecule.

The study’s findings also have implications for the formation of basic hydrocarbons in other galaxies. While it is well known that other galaxies have shocks, dense regions in which ultraviolet light dominates heating and chemistry may also play a key role in forming fundamental hydrocarbon molecules.

“It’s still a mystery how certain molecules get excited in the cores of galaxies,” Pearson said. “Our study is a clue that ultraviolet light from massive stars could be driving the excitation of molecules there, too.”
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