Planetary Nebulae Observed Releasing Water-Building Molecules
June 17, 2014

Planetary Nebulae Observed Releasing Water-Building Molecules

Brett Smith for - Your Universe Online

The region in space around a planetary nebula is filled with harsh radiation. Yet despite this hostile environment – the area is seeded with a molecule essential to the formation of water.

According to two new studies -- "Herschel Planetary Nebula Survey" and "Herschel spectral mapping of the Helix nebula" -- based on observations from the European Space Agency’s Herschel Observatory, harsh radiation around a planetary nebula does not restrict the formation of OH+. In fact, the radiation may even fuel its formation.

When stars the size of our Sun or a bit smaller approach the end of their existence, they become dense, white dwarf stars. In the process, they shed their surface layers of dust and gas into space, generating a kaleidoscopic planetary nebula.

Called ‘planetary nebula’ in the late 18th century by astronomer William Herschel because they resembled planets as seen through his telescope, these cosmic phenomena are currently being studied by the space observatory named after the famed astronomer.

After shedding its layers, a white dwarf begins to emit radiation previously thought to destroy the molecules ejected by the star. In one of the new studies, published in the journal Astronomy & Astrophysics, researchers scoured data from the Herschel observatory to find 3 of 11 planetary nebulae they surveyed were home to the water-forming OH+ molecule. The study team noted all three of these nebulae were among the hottest they observed.

“We think that a critical clue is in the presence of the dense clumps of gas and dust, which are illuminated by UV and X-ray radiation emitted by the hot central star,” said study author Isabel Aleman of the University of Leiden, the Netherlands. “This high-energy radiation interacts with the clumps to trigger chemical reactions that leads to the formation of the molecules.”

The other study, published in the same journal, focused exclusively on one of the hotter known nebulae – the Helix Nebula, located just 700 light years from Earth. An analysis of Herschel data revealed OH+ to be most plentiful in places where carbon monoxide molecules, ejected by the star, would be later destroyed by strong UV radiation.

Once oxygen atoms have been freed from the carbon monoxide, they can form OH+, evidence that the UV radiation may be fostering their creation.

Taken together, the two studies are the first to distinguish in planetary nebulae this significant molecule necessary for the formation of water. It remains to be seen if the circumstances would actually allow water formation to move forward.

“The proximity of the Helix Nebula means we have a natural laboratory on our cosmic doorstep to study in more detail the chemistry of these objects and their role in recycling molecules through the interstellar medium,” said Mireya Etxaluze, an author on the Helix study from the Instituto de Ciencia de los Materiales de Madrid, Spain.

“Herschel has traced water across the Universe, from star-forming clouds to the asteroid belt in our own Solar System,” said Göran Pilbratt, ESA’s Herschel project scientist who was not an author of either study. “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.”