McDonnell Distinguished Lecture Series Colloquium
Presolar grains are actual particles of stardust that can be identified in meteorites based on their isotopic compositions. They were produced in the mass outflows from dying stars, resided for millions of years in the interstellar medium, and eventually became part of the nascent nebula that collapsed to form the Sun and planets. Because they can be studied in detail in the laboratory by a wide variety of microanalytical techniques, presolar grains yield a wealth of information about the nucleosynthesis of the elements in stars and about the physical and chemical conditions prevailing during the formation of solid matter from gaseous stellar ejecta. In this talk, I will discuss what can be learned from one particular type of presolar grain, namely graphite spherules that were formed in stellar winds around asymptotic giant branch (AGB) carbon stars. I will show how we can use the size and composition of these grains, coupled with detailed dynamical models of AGB outflows, kinetic theory, and astronomical observations, to put constraints on the physical characteristics of the stellar winds in which the grains formed. In particular, it will emerge as necessary that these outflows are not generally spherically symmetric, but contain dense clumpy regions of small angular scale.