A LIKELY STORY: An exploding star much like supernova 1987A, only much closer to home, may have initiated the collapse of the molecular cloud that would become the solar system, delivering newly synthesized material at the same time. Image: NASA, ESA, P. Challis and R. Kirshner (CfA)
One star dies, another is born. The remains of the old are gathered up, at least in some small measure, to become part of the new. That is the astronomical circle of life, the reason that stars have evolved through the eons, each generation incorporating new elements synthesized in the stars that came before. Unlike the earliest stars of hydrogen and helium, stars nowadays contain heavier elements passed down to them by their predecessors, such as carbon, iron and oxygen.
Aside from producing many of the elements that make up our planet and our bodies, the stellar cycle of birth and death appears to have spurred the formation of our solar system some 4.5 billion years ago. According to a new model outlined in a study in the July 1 issue of Astrophysical Journal Letters, a shock wave from an exploding massive star several light-years away probably triggered the collapse of the molecular cloud that would become our sun and planets.
Employing a bit of astrophysical forensics, researchers have located fingerprints of short-lived radioisotopes, long since decayed to more stable daughter elements, in primitive meteorites. For those radioisotopes to have been incorporated into primordial solar system bodies they must have been delivered, freshly synthesized, from some nearby cataclysm, whether a stellar explosion known as a supernova or an ailing star throwing off layers of material in less dramatic fashion.
Some researchers have hypothesized that the short-lived isotopes may have arrived in a shock wave strong enough to collapse the presolar cloud, thereby kick-starting the formation of the solar system and injecting newly synthesized material in one fell swoop.
But previous models had failed to deliver enough material to the nascent solar system to account for the observed prevalence of short-lived radioisotopes, says study co-author Alan Boss, a theoretical astrophysicist at the Carnegie Institution of Washington. Boss has been on the case for years, trying to solve the mystery of the solar system's formation. "Call it 'Crime Scene Solar System,'" he says. "It's a CSI show."
For years, Boss says, his models had relied on one kind of relatively thick shock front, based on the shell of material ejected in a planetary nebula. In the new study Boss and his Carnegie Institution colleague Sandra Keiser tweaked the modeled thickness and density of the shock wave. Varying the parameters of the shock front boosted the efficiency of the injection mechanism, to the point that the shock wave could indeed push enough material into the collapsing cloud to match observed radioisotopic levels. "Whammo, suddenly the injection efficiency went up quite a bit," Boss says, adding that the key was the incorporation of "more svelte, streamlined, slimmed-down shock fronts."
The svelte shock front in Boss's model better resembles a supernova spurred by the collapse of a massive star's core than it does an alternative proposed explanation, an expanding shell of material ejected from an aging type of star called an asymptotic giant branch (AGB) star. "There really is a good reason for thinking that a supernova did it," Boss says.
Based on encounter probabilities alone, the supernova mechanism appears more likely than a push from an AGB star, says Gary Huss, a cosmochemist at the University of Hawaii at Manoa. "A much wider variety of massive stars appear to be viable sources than AGB stars, most of which do not produce enough iron 60," one of the short-lived radioisotopes present in the early solar system, Huss says.
He notes that the new paper reinforces a number of previous studies pointing to a massive supernova as the source of short-lived elements in the early solar system. "I am comfortable with this conclusion, but the case is by no means closed," Huss says. "It will take many more studies like the one in this paper, many more observations of stars, star formation and stellar explosions, and many more models of stellar nucleosynthesis to close this case for good."
Of course, there is no reason that some mechanism could not have delivered the radioisotopes shortly after the presolar cloud began to collapse for a totally unrelated reason. "It's mostly economy of hypotheses" to link both actions to a common source, Boss says. "It's nice to do both things at once, and it does seem to work."
As with detectives working more conventional cold cases, Boss continues to apply better technology to the task. He is now moving from two- to three-dimensional modeling, a process that requires far more computing power but that provides better clues to solving the mystery of the solar system's formation once and for all. "Mother Nature did it," Boss says. "We know who the perp is, but we want to know how she did it."