Herschel Discovers Water Building Molecules Around Dying Stars

NASA, ESA, and C. Robert O'Dell (Vanderbilt University). Ring nebula.

Using the European Space Agency’s Herschel space observatory, two independent teams of researchers have detected a molecule critical for the formation of water in the cloudy remnants of dying stars known as planetary nebulas.

When a star comes to the end of its life, a series of events ensue that are dependent on the star’s mass. Stars that are at least several times more massive than the Sun explode dramatically as supernovas which will then go on to form either a neutron star or a black hole. Stars that have masses similar to our Sun go through a slightly different sequence when they die, first swelling dramatically into a red giant. The star then starts to shed its outer layers of gas and dust into space in clumps, leaving a striking remnant known as a planetary nebula.

Both supernovas and planetary nebulas enrich their surroundings with various elements which will eventually be used to forge new stars. Supernovas are able to produce heavy elements, whereas planetary nebulas contain lighter elements such as carbon, nitrogen and oxygen.

When a planetary nebula enters the next phase of stellar evolution, forming a white dwarf, a large amount of UV radiation is released into the environment. It was believed that this harsh radiation would likely obliterate the vast majority of molecules expelled previously by the star and also hinder the creation of new molecules.

Intriguingly, however, two independent studies using Herschel data have found evidence for a water-building molecule in this harsh environment. The molecule, OH+, is a positively charged combination of one oxygen and one hydrogen atom.

In one of the studies, a team of researchers analyzed 11 planetary nebulas and found evidence for this molecule in three of them. Interestingly, it transpires that these three were also the hottest nebulas. According to lead author of the study, Dr Isabel Aleman, the high energy UV and X-ray radiation emitted by the central star interacts with the encircling clumps of gas and dust, triggering chemical reactions that lead to OH+ formation.

The second study honed in on a nearby planetary nebula known as the Helix Nebula, located a mere 700 light years away. While the central star is half as massive as the Sun, it is far hotter with temperatures of around 120,000oC. The researchers discovered that OH+ was predominantly located in regions where previously released carbon monoxide molecules were likely being destroyed by the radiation. The radiation would split apart the CO molecules, freeing the oxygen so that it can combine with hydrogen and thus form OH+.

Although OH+ could go on to form water molecules in principle, it remains unknown whether the conditions around these stars would actually permit water formation.

“Herschel has traced water across the Universe, from star-forming clouds to the asteroid belt in our own solar system,” said Herschel project scientist Gӧran Pilbratt in a news-release. “Now we have even found that stars like our Sun could contribute to the formation of water in the Universe, even as they are in their death throes.”

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