Exploding star missing from formation of solar system
Researchers didn’t find high abundances of a radioactive isotope that would provide evidence for the presence of a supernova.
December 18, 2012
A new study published by University of Chicago researchers challenges the notion that the force of an exploding star caused the formation of the solar system.
Scientists in the University of Chicago’s Origins Laboratory have published the latest in a series of papers about the origin of the solar system. Infant stars glow reddish-pink in this infrared image of the Serpens star-forming region, captured by NASA’s Spitzer Space Telescope. Four-and-a half billion years ago, the Sun may have looked much like one of the baby stars deeply embedded in the cosmic cloud of gas and dust that collapsed to create it. // Courtesy of NASA/JPL-Caltech/L. Cieza (University of Texas at Austin)
In this study, authors Haolan Tang and Nicolas Dauphas found the radioactive isotope iron-60 — the telltale sign of an exploding star — low in abundance and well mixed in solar system material. As cosmochemists, they look for remnants of stellar explosions in meteorites to help determine the conditions under which the solar system formed.
Some remnants are radioactive isotopes — unstable, energetic atoms that decay over time. Scientists in the past decade have found high amounts of the radioactive isotope iron-60 in early solar system materials. “If you have iron-60 in high abundance in the solar system, that’s a ‘smoking gun’ — evidence for the presence of a supernova,” said Dauphas, professor in geophysical sciences.
Iron-60 can only originate from a supernova, so scientists have tried to explain this apparent abundance by suggesting that a supernova occurred nearby, spreading the isotope through the explosion.
But Tang and Dauphas’ results were different from previous work: They discovered that levels of iron-60 were uniform and low in early solar system material. They arrived at these conclusions by testing meteorite samples. To measure iron-60’s abundance, they looked at the same materials that previous researchers had worked on, but they used a different, more precise approach that yielded evidence of low iron-60.
Previous methods kept the meteorite samples intact and did not remove impurities completely, which may have led to greater errors in measurement. Tang and Dauphas’ approach, however, required that they “digest” their meteorite samples into solution before measurement, which allowed them to thoroughly remove the impurities.
This process ultimately produced results with much smaller errors. “[Tang] has dedicated five years of very hard work to reach these conclusions, so we did not make those claims lightly. We’ve been extremely careful to reach a point where we’re ready to go public on those measurements,” Dauphas said.
To address whether iron-60 was widely distributed, Tang and Dauphas looked at another isotope of iron, iron-58. Supernovae produce both isotopes by the same processes, so they were able to trace the distribution of iron-60 by measuring the distribution of iron-58.
“The two isotopes act like inseparable twins: Once we knew where iron-58 was, we knew iron-60 couldn’t be very far away,” Dauphas said.
They found little variation of iron-58 in their measurements of various meteorite samples, which confirmed their conclusion that iron-60 was uniformly distributed. To account for their unprecedented findings, Tang and Dauphas suggest that the low levels of iron-60 probably came from the long-term accumulation of iron- 60 in the interstellar medium from the ashes of countless stars past, instead of a nearby cataclysmic event like a supernova.
If this is true, Dauphas said, there is then “no need to invoke any nearby star to make iron-60.” However, it is more difficult to account for the high abundance of aluminum-26, which implies the presence of a nearby star.
Instead of explaining this abundance by supernova, Tang and Dauphas propose that a massive star (perhaps more than 20 times the mass of the Sun) shed its gaseous outer layers through winds, spreading aluminum-26 and contaminating the material that would eventually form the solar system, while iron-60 remained locked inside the massive star’s interior. If the solar system formed from this material, this alternate scenario would account for the abundances of both isotopes.
“In the future, this study must be considered when people build their story about solar system origin and formation,” Tang said.