The Birth of the Solar System in a Molecular Cloud: Evidence from the Isotopic Pattern of Short-lived Nuclides in the Early Solar System

A good positive correlation between the initial solar abundances of shortlived (now extinct) nuclides (when normalized to their nucleosynthetic production ratios) and their mean lifetimes on a logarithmic plot has been well known for some time. Here I show that: (i) the slope for short-lived nuclides in the average interstellar medium in such a diagram is always 1. (ii) for molecular clouds, the slope is expected to be 2 or slightly less than 2 for a model where the molecular clouds are at a steady state and slowly exchange matter with the remaining interstellar medium. The existing data suggest a residence time of ∼6 x10 yrs for the matter present in molecular clouds. (iii) the intercept depends on (1) the residence time of matter in molecular clouds, (2) the mass fraction of the interstellar medium that is in molecular clouds, (3) the age of the galaxy and (4) the ratio of the time-average nucleosynthtic production rate and the production rate at the time of solar system formation. (iv) the abundances of Mn, Hf, Pu and Sm in the early solar system are likely formed by the same type of supernova sources (SNII?) over the history of our galaxy, while I (and possibly Pd) were produced in a different type of supernova sources (SNIa?) with the production rate skewed toward the early history of our galaxy. The abundances of these nuclides most likely characterize the average ISM values modified during their residence in the molecular cloud complex where the solar system formed. The abundances of Al, Ca and Fe are too high to be of galactic production; these must be a contamination from young stellar sources that formed within the proto-Solar molecular cloud. These young sources could not have contributed significant quantities of Mn, Hf, Pu and Sm or I and thus were dissimilar to typical supernova sources.