It has been known that many hot white dwarfs atmospheres, essentially of pure hydrogen or pure helium, are contaminated by other elements – like carbon, silicon and iron. What was not known, however, was the origins of these elements, known in astronomical terms as metals.
"The precise origin of the metals has remained a mystery and extreme differences in their abundance between stars could not be explained," said Professor Barstow, a Pro-Vice-Chancellor at the University of Leicester whose research was assisted by his daughter Jo, a co-author of the paper, during a summer work placement in Leicester. She has now gone on to be an astronomer working in Oxford - on extra-solar planets.
"It was believed that this material was "levitated" by the intense radiation from deeper layers in the star," said Professor Barstow. Now the researchers have discovered that many of the stars show signs of contamination by rocky material, the left overs from a planetary system.
The researchers surveyed 89 white dwarfs, using the Far Ultraviolet Spectroscopic Explorer to obtain their spectra (dispersing the light by color) in which the "fingerprints" of carbon, silicon, phosphorous and sulphur can be seen, when these elements are present in the atmosphere. They found that in stars with polluted atmospheres the ratio of silicon to carbon matched that seen in rocky material, much higher than found in stars or interstellar gas.
White Dwarf Type Ia supernovae are brighter than whole galaxies and visible billions of light-years away (see image at top of page).
The new work indicates that at around a one-third of all hot white dwarfs are contaminated in this way, with the debris most likely in the form of rocky minor planet analogues. This implies that a similar proportion of stars like our Sun, as well as stars that are a little more massive like Vega and Fomalhaut, build systems containing terrestrial planets. This work is a form of celestial archaeology where we are studying the 'ruins' of rocky planets and/or their building blocks, following the demise of the main star.
The mystery of the composition of these stars is a problem we have been trying to solve for more than 20 years. It is exciting to realise that they are swallowing up the left overs from planetary systems, perhaps like our own, with the prospect that more detailed follow-up work will be able to tell us about the composition of rocky planets orbiting other stars", said Professor Barstow.
On Early Earth, a little less than 2 billion years ago, metals that were originally seeded by billions of Supernova explosions, including copper, molybdenum and zinc became available to primitive cells, at the same time that the cells began to become much more complex. Scientists indicate that they have identified the event that introduced these metals, which made it possible for those primitive cells to develop, evolve, and spread. The secret, according to researchers, was granite.
Life on Earth originated more than 3.5 billion years ago, as basic prokaryotic cells, which did not have a nucleus. Advanced cells -- called eukaryotes -- added a nucleus. They evolved around 2 billion years ago. Then, sometime between 2 billion and 1 billion years ago, these cells proliferated, sexual reproduction evolved, and the first multicellular organisms developed.
John Parnell, a geologist from the University of Aberdeen, in the U.K., said that he brought together two areas of research to form this conclusion. The first was from scientists who identified that critical advances in life lined up with increased access to metals about 1.5 to 2 billion years ago. A second group had shown that granites bearing many of these same metals formed at about the same time."Nobody had thought about it in this way before," said Parnell.
The research suggests that the large amount of heat within the Earth at this time caused metal-laden magmas to rise from great depths, which cooled into granites near the surface. The scientists conclude that this event caused the substantial change in the Earth's surface and ocean chemistry that began about 2 billion years ago. This hypothesis challenges the prevailing consensus that changes in ocean chemistry were responsible for enabling life to undertake this transition.
"There's no doubt that probably a lot of metal was locked up in the oceans," said Parnell"[We're] suggesting that it's the terrestrial environment where the metal was really being made newly available."
Parnell brought together several pieces of evidence to show that granites formed, came to the surface of continents, then weathered. Weathering freed up metals, he said, which traveled with runoff to fill lakes and shallow seas, places where primitive life could incorporate the metals and become more complex.
"We propose that metals were delivered due to a critical combination of continental growth, near-surface metal concentration, and erosion into the surface environment during the Mesoproterozoic [a time period from roughly 1.6-1.0 billion years ago]," Parnell and his colleagues wrote in their paper, published this in Geology.
"It's really interesting and intriguing," said Ariel Anbar, a geologist at Arizona State University in Tempe. "A lot of us in the community have noted loosely that there's this correlation and we've wondered if there's something to it." Anbar also called the article "thought-provoking." He suggested that the increased availability of oxygen in the environment may have been more responsible for life's increasing complexity.
"I think we should all be cautious about saying, 'Oh, this one factor was the key,'" said Anbar. "There are probably multiple factors."
Regardless of the root cause that made new elements available to ancient life, metals such copper, molybdenum, and zinc remain important.
"Many people will know that we have plenty of iron in our body, and you can see that in the red color in our blood," said Parnell. "We have just as much zinc in our bodies as we have iron."
More information: Research paper: http://geology.gsa … 6.1.abstract
The Daily Galaxy via University of Leicester, Geology.com, and Inside Science News Service