Short-lived nuclides in the early solar system

Isotopic records in meteorites provide evidence for the presence of several short-lived nuclides in the early solar system with half-lives varying fro m 10(5) to similar to 8 x 10(7) years. Most of the nuclides with longer hal f-life (> 10(7) years) are considered to be products of stellar nucleosynth esis taking place over long time scales in our galaxy. However, for the rel atively shorter-lived nuclides, two possibilities exist; they could be prod ucts of energetic particle interactions taking place in a presolar or early solar environment, or, they could have been produced in a stellar source a nd injected into the protosolar molecular cloud just prior to its collapse. The presently available data appear to support the latter case and put a s tringent constraint of less than a million years for the time scale for the collapse of the protosolar molecular cloud to form the Sun and some of the first solar system solids. This short time scale also suggests the possibi lity of a triggered origin for the solar system with the very process of in jection of the short-lived nuclides acting as the trigger for the collapse of the protosolar molecular cloud. Fossil records of the short-lived nuclid es in meteorites also provide very useful chronological information on the early solar system processes like the time scale for nebular processing, th e time scales for differentiation and for metal/silicate fractionation with in planetesimals. The currently available data suggest a time scale of a fe w million years for nebular processing and a relatively short time scale of about ten million years within which differentiation, melting and recrysta llization in some of the planetesimals took place.