Friedemann T. Freund

Solid Solution Model for Interstellar Dust Grains and Their Organics

We present a dust grain model based on the fundamental principle of solid solutions. The model is applicable to the mineral (silicate) component of the dust in the interstellar medium (ISM). We show that nanometer-sized mineral grains,which condense inthe gas-rich outflowof late-stage stars or expandinggas shellsofsupernova explosions,do notconsistof justhighmeltingpointoxidesorsilicates. Insteadtheyformsolidsolutionswithgas-phasecomponents H2O, CO, and CO2 that are omnipresent in environments where the grains condense. Through a series of thermodynamically well-understood solid-state processes, these solid solutions beco me ‘‘parents’’ of organic matter that precipitates inside the grains. Thus, the mineral dust grains and their organics become part of the same thermodynamicallydefinedsolidphaseand,hence,physicallyinseparable.Thismodelcanaccountformanyastronomical observations,whichnopriormodelcanadequatelyaddress,specifically:(1)OrganicsinthediffuseISMareidentified