NEW RESEARCH EXPLAINS STELLAR ORIGINS OF UNUSUAL ISOTOPES
COLUMBUS, Ohio -- Researchers at Ohio State University may have helped solve the mysterious origins of four isotopes of the elements molybdenum and ruthenium that challenge current theories in nuclear astrophysics.
A better understanding of where these elements came from will help astronomers decipher the processes that make elements inside stars.
According to current theory, stars consume light chemical elements like hydrogen as fuel and create heavier elements as by-products. When a star dies, as is the case with a supernova, the explosion sprays the heavier elements out in all directions. These elements, in turn, form the seeds for new stars and planets.
This means that all the heavy elements in the universe, like carbon and iron, came from stars that exploded long ago. And, since astronomers cant directly examine what goes on inside stars, they get clues about stellar processes by measuring the
abundances of these elements in rocks and meteorites on earth.
We think we understand many of these processes very well, said Richard Boyd, professor of physics and astronomy at Ohio State, and associate dean of the college of mathematical and physical sciences. What were trying to do now is see if there are things we might have missed.
Four molybdenum and ruthenium isotopes may represent one such case: the isotopes contain the fewest neutrons for the set of nuclei with that number of protons, and so must have originated in a very unique chemical reaction. Normal stellar processes have great difficulty making the large amounts of these isotopes that scientists find on earth.
Most of the very neutron-poor isotopes are rare, Boyd said. This is not the case for these molybdenum and ruthenium isotopes, and this is what makes them so special.
Scientists believe that stars could create these heavy elements with few neutrons and many protons through the rapid-hydrogen burning (rp) process, when light elements churning at high-speed inside a stellar furnace collide with protons and stick together. The protons occasionally remain in the nucleus, creating a new element, while the stars radiative energy knocks out the neutrons.
But nuclei can only capture so many protons before they decay by ejecting the last proton. If one of these proton-rich nuclei lives longer than the rp-process lasts, then it blocks the progression of the rp-process. Because such a bottleneck exists at 68Se -- a nucleus with 34 protons and 34 neutrons and a half-life of 35.5 seconds --and since the rp-process is usually thought to last only a few seconds, the molybdenum and ruthenium isotopes that Boyd is studying cannot be made from the very abundant nuclei around mass 56 by the simple rp-process.
Boyd and his colleagues at Ohio State and Clemson University think they have found a way around this problem with a new process they call repetitive rp. Boyd presented this research February 14 in Seattle at the annual meeting of the American Association for the Advancement of Science.
What were doing with the repetitive rp process is basically letting the rp-process happen a whole bunch of times. If we run rp and let those elements decay, and then run it again, we can add more protons. So repetitive rp allows us to circumvent the usual limitations of the rp process, Boyd said.
For repetitive rp to work, elements inside a star need to gather protons through the traditional rp-process at high temperatures, cool down, and then heat up enough to undergo rp-processing again. This doesnt happen in an ordinary star, but it may happen in a hypothetical two-star combination called a Thorne-Zytkow object.
A Thorne-Zytkow object starts out as a binary system, a close pair of stars that orbit around each other. The more massive of the two stars burns its fuel at a faster rate and eventually explodes as a supernova. All that remains of that star after the explosion is its core, a very dense clump of nuclear matter.
If the star was extremely massive to begin with, the leftover material may form a neutron star, an object that contains as much material as our sun, but measures only 10 kilometers across.
Meanwhile, the neutron stars companion continues to burn its fuel, uses up its supply of hydrogen, and starts to burn the next heaviest element, helium. Burning helium releases a great deal of energy, and the companion swells in size to become a red-giant star. When this happens, the giant may gobble up the tiny neutron star. This combination of stars with one inside the other is a Thorne-Zytkow object.
Gravity draws the neutron star to the center of the giant, where the high temperatures at its surface cause the rp-process to start happening. Like water in a boiling pot, atoms in the star heat up in the interior and rise to the surface, where they cool off and then fall back toward the center. In this way, the atoms undergo rp over and over.
We may well have observed a lot of Thorne-Zytkow objects, but theyre hard to identify, Boyd said. Its not clear how the appearance of a Thorne-Zytkow object would differ from a conventional red-giant.
Boyd said one way to identify a Thorne-Zytkow object may be to look for evidence of molybdenum atoms that rise to the surface, if such an object exists.
Its very difficult for any other mechanism that people have suggested to make these molybdenum and ruthenium isotopes, Boyd said. Thorne-Zytkow objects seem to hold more promise for us than anything else.
Contact: Richard Boyd, (614) 292-2874; Boyd@mps.Ohio-state.edu
Written by Pam Frost, (614) 292-9475; Frost.18@osu.edu