Origins of Elements
Origins of Elements
The study of nuclear astrophysics is a linkage between the centres of atoms and the centres of stars. It spans an enormous range in distances and time scales. It joins astronomers at telescopes on mountains with physicists in accelerators underground. It is an all-encompasing field, and tries to answer fundamental questions about the origins of all matter on Earth and throughout the universe.
When the universe formed, there were no atoms, just a very hot and dense soup of exotic particles. After a little while, perhaps 3 minutes or so, the universe cooled enough that "normal" particles such as protons, electrons, and neutrons could form. And then some of them fused with each other, producing some of the elements we know today: hydrogen, deuterium, helium, and lithium.
Every other element, however, was not formed in the early universe. They were formed in the centres of stars, or during stellar explosions. To quote the famous astronomer Carl Sagan, "We are all star stuff". Starlight is a result of nuclear fusion going on in the hot, dense interiors of stars -- hydrogen fuses to form helium, helium fuses to form carbon and oxygen, and gradually heavier and heavier elements are formed. Elements even heavier than iron form during even more energetic events, such as novae and supernovae.
There are two main thrusts of nuclear astrophysics. The first is to understand the individual nuclear reactions that take the original light elements and form everything heavier. Experiments at places like TRIUMF mimic the conditions in the cores of stars, and actually measure what nuclear reactions take place, under what conditions, and determine what is produced. We also need to understand the structure of atomic nuclei in detail, so that we can accurately predict results of experiments that are currently impossible.
The second thrust is on the astrophysics side. We need to understand what physical conditions exist in stars, to determine which of these reactions occur in the universe, to what extent and in what kinds of stars. We need to understand how stars are born and then change with time, and how the nuclear-processed material leaves the centre of the star and enriches the rest of the Galaxy. We need to understand how the gas in the galaxy moves around and forms new stars. And we need to know detailed chemical abundances of a wide variety of stars, to determine whether any of our models are correct.
Recent advances in nuclear astrophysics:
- ISAC at TRIUMF: The advent of laboratories capable of producing and accelerating unstable nuclei has allowed researchers to study directly, for the first time, the properties of such nuclei and their reactions. These studies are probing our understanding of the energy generation and nucleosynthesis in the hottest stellar environments, where these unstable nuclei play a central role.
- Pre-solar grains obtained from ancient meteorites and from the recently completed NASA Stardust mission are "star stuff", dust grains formed in the envelopes of red giants and in the cooling ejecta from supernovae. The compositions of the pre-solar grains provide valuable constraints to stellar models and nuclear astrophysics, because the errors on the measurements are so small, and because we can obtain detailed information for many more elements AND isotopes, including rare heavy elements like barium, xenon, krypton, and rubidium - this information is not available from stellar spectra.
- The increase in speed and complexity of computers, such as those available to Origins members through SHARCNet and CITA, have allowed a substantial increase in the complexity, and hence realism, of stellar modeling and reaction rate network modeling.
- As telescopes increase in size, we can obtain spectra for fainter and fainter stars, giving us better probes of chemical abundances in distant stars and low mass systems which form the bulk of the stellar population of the galaxy.
Overview of OI program in Nuclear Astrophysics
Abundances in Globular Clusters
Stars that we observe today in galactic globular clusters are amongst the oldest in the Galaxy. Their abundances are a fossil record of the nuclear reactions that occurred in the stars that died before the cluster was formed. Understanding the history of globular cluster stars poses one of the greatest challenges to astronomers because the abundances we observe in these stars are different from those we observe anywhere else, and so far remain an unexplained mystery. Amanda Karakas and Alison Sills are working to try to understand the chemical history of globular cluster stars by first making accurate theoretical predictions of the nucleosynthesis (the synthesis of elements) of now long-dead red giant stars. With Yeshe Fenner's help we then fold these predictions into a "chemical evolution model" which determines how the cluster gas changes with time. One of the parameters of the chemical evolution model is the distribution of stellar masses and we can change this distribution to test different assumptions. The abundance predictions along with the chemical evolution model is the state-of-the-art when it comes to understanding the mystery of globular cluster abundance anomalies!
The tools used by Amanda Karakas to determine how the elements are formed inside red giant stars are also currently being updated to follow many more elements than ever before. We do this by increasing the number of elements, or more accurately, nuclear species (neutrons, protons, isotopes of elements) in a "nuclear network". A nuclear network contains just the right number of nuclear reactions which describes how every species in the network is produced by fusion or neutron capture (for elements heavier than iron) or destroyed by the same processes, depending on the temperature in the interior of the star. Along with help from Alan Chen, Amanda is currently adding more species to the network and checking that the nuclear reactions used are the most accurate ones available.
Unstable Nuclei in Astrophysics
Our current experimental efforts are centered at the TRIUMF-ISAC facility. Alan Chen's group has been carrying out experiments aimed at understanding the production of nuclei of interest to gamma-ray astronomy -- such as Sodium-22, Aluminum-26, and Titanium-44 -- in hot, explosive, stellar environments. These experiments will determine directly the rates of the key reactions involved in the production of such nuclei. On the technical front, we are also developing new particle detectors for use in future experiments at the upgrade to the present ISAC facility, called ISAC-II. At ISAC-II, we will be measuring the properties and structure of heavier, highly-unstable nuclei of importance to the synthesis of elements heavier than iron in the r-process; and to the energy generation in the rp-process, thought to be the power source in x-ray bursts occurring on the surface of accreting neutron stars. Our group has also recently initiated research programs at the RIKEN-CNS Laboratory in Japan and at the National Superconducting Cyclotron Laboratory in the US, where we are performing experiments complementary to the ones at ISAC and that probe the same scientific questions.