Astronomers at the University of Arizona have discovered excephosphorous, a critical ingredient for life as we know it, in an unexpected location: the outskirts of the Milky Way galaxy. According to conventional wisdom, the element is produced by fusion processes inside very massive stars, which are not believed to exist in the outer reaches of the Milky Way.
Phosphorous found on outskirts of Milky Way
Other, less heavy elements necessary for life, such as carbon, oxygen and nitrogen, can form in lower-mass stars, which are much more abundant. When those stars reach the end of their life, they release those elements relatively calmly into the interstellar medium.
"But to make phosphorus, you need some kind of violent event," explained Lucy Ziurys, Regents Professor of chemistry and biochemistry and astronomy and astronomer at Steward Observatory. "It is thought that phosphorus is created in supernova explosions, and for that, you need a star that has at least 20 times the mass of the sun."
When a star goes supernova, it spills its innards into the surrounding space, including the elements that formed the building blocks for life in our solar system.
"In other words, if you're going to have life, you better be near a supernova, if that's indeed the only source where phosphorous is created," Ziurys said.
The study, published in Nature, calls into question the conventional wisdom that nature's only way of producing phosphorous is through supernova explosions.
"The phosphorus we detected is at the edge of the galaxy, where it shouldn't be," said Lilia Koelemay, a doctoral student studying chemistry and the first author of the paper. "And so that means that there has to be some other way phosphorus is created."
One such mechanism that had been proposed in the past invokes "galactic fountains" that lift phosphorous from the inner reaches of the Milky Way far above the galaxy's plane and shower it back onto the disk farther out. However, evidence of such fountains remains scarce, and even if they exist, Koelemay says it would be unlikely they reach farther than a kiloparsec or so, or about 3,260 light-years.
"Even then it would take so long for the material to fall back into the galaxy that it probably wouldn't have formed the molecules that we were seeing," she added.
So, if supernovae can't account for phosphorous at the edges of the galaxy, how did it get there?
According to one theory, Ziurys said, low and intermediate mass stars may generate excess neutrons by stripping them off of carbon atoms during the end of their life cycle and accumulating them in pockets between their hydrogen-burning and their helium-burning shells. Adding those neutrons onto silicon atoms would result in phosphorous.
"This has been postulated in theory, and so it could possibly explain another source of phosphorus in addition to supernovae, and I think we have good evidence supporting this now," Ziurys said.
Koelemay and her co-author, chemistry doctoral student Katherine Gold, used the 12-meter radio telescope of the Arizona Radio Observatory on Kitt Peak and a 30-meter radio telescope near Granada, Spain, operated by the Institute for Radio Astronomy in the Millimeter Range, or IRAM.
Their observations detected the telltale signatures of phosphorous - specifically phosphorus monoxide and phosphorous nitride - in a molecular cloud named WB89-621. Located nearly 74,000 light-years from the center of the Milky Way, the discovery extends the presence of phosphorous almost twice as far out as where it was known to exist. Because matter becomes sparser the farther one ventures out from the galaxy's center, the outer reaches simply don't have enough mass to support the formation of stars big enough to end their lives as supernovae.
The project began with a homework assignment: Koelemay and Gold took a course in astrochemistry that Ziurys teaches, and she encouraged them to scan the cloud for phosphorous-containing molecules, admitting that it might be a l...
