Star formation and extinct radioactivities

Abstract

A supernova trigger for star formation, such as was discussed by Cameron and Truran a few years ago, lacks observational confirmation, and the nucleosynthesis arguments for suggesting its relevance to the formation of the solar system no longer seem compelling. Observations of star formation currently point toward nearly isolated individual events taking place in the interiors of dense molecular clouds, in which cores are formed in the clouds and collapse to form stars. If the formation of the solar system proceeded in this manner, then a viable theory of cloud core formation must provide circumstances in which it is not uncommon for the cloud cores to be formed with a significant supply of fresh short-lived radioactivities. A review is made of the evidence for the existence of now extinct radioactivities in primitive solar system material and an examination is made of the implications for the early stages of formation of the Sun and solar system. The characteristics of possible disturbances in dense molecular clouds which can initiate the formation of cloud cores is discussed,and in particular those disturbances which can produce fresh radioactivities are considered. A red giant star of roughly one solar mass on the asymptotic giant branch appears to have been the best candidate to account for the short-lived extinct radioactivities in the early solar system. Star formation itself can generate disturbances through the T Tauri stellar winds and bipolar outflows, and it appears likely that several such disturbances in a chain of star-forming events were needed before the radioactive material was diluted enough to be consistent with the abundances seen in early solar system material.