Presolar Grains - Information Carried By Presolar Grains

Information Carried By Presolar Grains

The study of presolar grains provides information about nucleosynthesis and stellar evolution. Grains bearing the isotopic signature of rapid-process and alpha capture nucleosynthesis are useful in testing models of supernovae explosions. For example, some presolar grains supernova grains have very large excesses of calcium-44, a stable isotope of calcium which normally composes only 2% of calcium abundance. The calcium in some presolar grains is composed primarily of Ca-44, which is presumably the remains of the extinct radionuclide Ti-44, a titanium isotope composed of equal numbers (22 each) of neutrons and protons which is formed in abundance in Type IIa supernovae after rapid capture of eight alpha particles to Si-28, after the process of silicon burning normally begins, prior to supernova explosion. However, this isotope has a day half life of 59 years, and thus in the supernova grains derived from such explosians, is soon converted entirely to calcium-44. Excesses of the decay products of the longer lived but still extinct nuclides Ca-41 (half life 350,000 years) and Al-26 (730,000 years) have also been detected in such grains. The r-process isotopic anomalies of these grains include relative excesses of N-15 and O-18 relative to solar system abundances, as well as excesses of neutron-rich Ca-42, Ca-42, and Ti-49.

Other presolar grains (AGB star grains) provide isotopic and physical information on asymptotic giant branch stars, which have manufactured the lion's share of the refractory elements lighter than iron in the galaxy. Because the elements in these particles were made at different times (and places) in the early Milky Way, the set of collected particles further provides insight into galactic evolution prior to formation of our solar system.

Along with providing information on nucleosynthesis, solid grains provide information on the physico-chemical conditions under which they formed, and on events subsequent to their formation. For example consider red giants - which produce much of the carbon in our galaxy. Their atmospheres are cool enough for condensation processes to take place - resulting in the precipitation of solid particles (i.e. multiple atom agglomerations of elements - such as carbon) - in their atmosphere. This is unlike the atmosphere of our sun, which is too hot to allow build-up of atoms to form more complex molecules. These solid fragments of matter, are then injected into the interstellar medium by radiation pressure. Hence particles bearing the signature of stellar nucleosynthesis are providing us with information on: (i) condensation processes in red giant atmospheres, (ii) radiation and heating processes in the interstellar medium, and (iii) the types of particles that carried the elements of which we are made across the galaxy to our Solar system.