Cosmic chemistry: closing the gap in the origin of the elements.

For the past 25 years, astronomers have puzzled over an elemental mystery. Researchers widely accept the notion that all of the hydrogen and helium in the universe, as well as trace amounts of lithium, were produced in the Big Bang. Heavier nuclei, beginning with carbon, were forged in the furnacelike interiors of massive stars and then dumped into space when these stars exploded as supernovas. That still leaves no explanation for two lightweight elements-beryllium and boron-and the bulk of the lithium.

"The elements in between those produced in the Big Bang and those produced by supernovas are not very common, but it hasn't been entirely clear where they came from," notes Douglas K. Duncan, an astronomer at the University of Chicago and the Adler Planetarium & Astronomy Museum in Chicago.

In the early 1970s, astronomers came up with a theory that seemed to explain the formation of these three elements. These researchers proposed that the trio represented the debris left over when cosmic-ray protons-protons accelerated to high speeds in the galaxy-smashed into and shattered stationary carbon, oxygen, and nitrogen nuclei in the interstellar medium.

"That's what I was taught in graduate school, and that's what was generally believed," says Duncan.

Calculations showed, however, that this process alone can't generate the three elements in the abundances observed today in and near the solar system. Collisions of carbon, oxygen, and nitrogen nuclei with high-speed protons would produce twice the measured ratio of boron to beryllium and only half the ratio of the isotope boron-11 to boron-10, a sibling with one fewer neutron.

Observations with the Hubble Space Telescope have only made matters worse. Using Hubble's Goddard high-resolution spectrograph, astronomers have for the first time measured the abundance of boron in eight stars that rank among the oldest in our galaxy. The stars date from the formation of the Milky Way, some 10 billion years ago, and provide a record of boron abundance from that long-ago era.

To the surprise of many astronomers, the Hubble studies show that the abundance of boron way back when wasn't much lower than it is in the interstellar medium today. That finding is at odds with the notion that boron arose from the collision of high-speed protons with heavier elements. Ten billion years ago, "there wasn't very much carbon, oxygen, and nitrogen in the galaxy, so there weren't very many targets for the cosmic-ray protons to hit," notes Duncan. Boron, as well as lithium and beryllium, should therefore have been much scarcer in the distant past.

To explain how the early universe could have contained so much of the three light elements, Duncan and his collaborators have reversed the roles of the key players in the old theory. Rather than having high-speed protons slam into low-speed carbon, nitrogen, and oxygen, the researchers propose that the heavier nuclei, accelerated to high speed, shattered when they ran into low-speed protons in the interstellar medium.

"We're reversing which is the target and which is the thing hitting it," says Duncan.

That role reversal might seem to make little difference, but the new model accounts more fully for the elemental abundances seen both today and in the early universe.

In areas of the cosmos where supernova explosions were common, heavy nuclei-including carbon, nitrogen, and oxygen-would have been accelerated into space in significant numbers while remaining relatively rare in the interstellar medium at large. Protons, which are nothing more than the nuclei of hydrogen atoms, were already abundant in the interstellar medium in early times, thanks to their production in the Big Bang.

Duncan's team bases its work on an analysis of recent Hubble data as well as on previous observations reported in 1992 by Duncan, David L. Lambert of the University of Texas at Austin, and Michael Lemke of the University of Erlangen-Nuremberg in Bamberg, Germany.

Lambert says that Duncan "has [analyzed] more stars, he has more data points, but basically he gets the same result that we got the first time."

Duncan and his colleagues presented their work in September at a meeting on results from the Goddard spectrograph at NASA's Goddard Space Flight Center in Greenbelt, Md. At the same meeting, Reuven Ramaty of Goddard and his collaborators presented calculations showing that supernovas could have supplied the requisite carbon, nitrogen, and oxygen early in the history of the Milky Way.

Two separate lines of evidence support the new model. In 1994, Hans Bloemen of the Space Research Organization in Utrecht, the Netherlands, and his colleagues used a telescope aboard the Compton Gamma Ray Observatory (GRO) to analyze a series of broad emission lines from the Orion molecular cloud complex, the nearest stellar nursery to Earth (SN: 2/4/95, p. 70). The region is chock-full of massive stars and supernovas, and Bloemen says the broadness of the gamma-ray emissions and their energies indicate that they come from carbon and oxygen nuclei moving at high speeds, presumably because they've been hurled into space by supernovas.

Bloemen's interpretation may be open to question: A shorter survey of Orion by another GRO telescope has failed to detect the emissions. But if he's correct, supernovas would indeed provide a plentiful source of the speedy heavy nuclei that Duncan's team requires to produce lithium, beryllium, and boron.

In another study, Steven R. Federman of the University of Toledo in Ohio and his colleagues, including Lambert, used the Goddard spectrograph to measure the relative abundances of boron-10 and boron-11 along the line of sight to three stars in the nearby interstellar medium.

Federman's team has completed an analysis of data along the line of sight of two of the stars, one in the constellation Scorpio, the other in Orion. These stars reside in opposite parts of the sky, each some 300 light-years from Earth. The researchers conclude that the interstellar medium in Earth's neighborhood contains four times as much boron-11 as boron-10. That value, Federman adds, is similar to the ratio previously found in meteorites, which date from the formation of the solar system 4.5 billion years ago.

He notes that the new recipe, based on collisions of high-speed carbon and oxygen nuclei with low-speed protons, is required to explain the observed ratio of boron-11 to boron-10. Federman adds that collisions which follow the rules of the older model, in which high-speed protons collide with heavier, stationary atoms, make a significant, but much smaller, contribution to the abundance of the boron isotopes.

Says Federman, "The nice thing about the measurements of [his team and Duncan's] is that they're complementary. Duncan measures the elemental abundance of boron, while we measure the isotopic ratios of boron." The two studies, he says, lead to the same conclusion. "People are fascinated in a general sense about the origin of elements," says Duncan. "'You mean that you know where the elements in my body came from?' they ask. To finally complete that picture is very appealing." n