Laboratory studies of silicate smokes: Analog studies of circumstellar materials

Abstract

Grains entering our solar system at the heliopause or encountered by an interstellar probe will be dominated by materials formed in carbon-rich and oxygen-rich outflows from high mass-loss-rate AGB stars with moderate contributions from novae and supernovae. Laboratory studies of the condensation and thermal evolution of silicate grains have greatly increased our understanding of grain formation processes and may provide the basis for theoretical prediction of chemical speciation and spectral evolution of grains produced in specific stellar outflows. These grains will be modified by long-term exposure to high-energy cosmic rays, thus rendering them amorphous, and will potentially be coated by one or more layers of refractory carbonaceous material. This coating results when radiation-damaged organic-rich water ices are allowed to sublime in vacuum during the transition from the interior of a dense molecular cloud to the warm interstellar medium. Measurement of the chemical compositions of large numbers of individual grains in the local interstellar medium could lead to a better understanding of the life cycle of grains in the general interstellar medium. Such measurements might also serve as a diagnostic indicator of the primary source of interstellar grains: origin in stellar outflows or in the interstellar medium itself. These measurements would be possible using an impact-ionization, time-of-flight, mass spectrometer during the cruise phase of an interstellar probe mission. Copyright 2000 by the American Geophysical Union.