Solid Solution Model for Interstellar Dust Grains and Their Organics

Abstract

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 in the gas-rich outflow of late-stage stars or expanding gas shells of supernova explosions, do not consist of just high melting point oxides or silicates. Instead they form solid solutions with gas-phase components H 2 O, CO, and CO 2 that are omnipresent in environments where the grains condense. Through a series of thermo-dynamically well-understood solid-state processes, these solid solutions become "parents" of organic matter that precipitates inside the grains. Thus, the mineral dust grains and their organics become part of the same thermodynamically defined solid phase and, hence, physically inseparable. This model can account for many astronomical observations, which no prior model can adequately address, specifically: (1) Organics in the diffuse ISM are identified by a 3.4 μm IR band, characteristic of aliphatic hydrocarbons composed of-CH 2 - and of-CH 3 groups. (2) The methylene-to-methyl ratio is nearly constant, implying a CH 2 :CH 3 ratio of ∼5:2. (3) The intensity ratio between the 9.7 and the 3.4 μm band is nearly constant, implying a silicate-to-organics ratio of ∼ 10:1. (4) In dense clouds the complex 3.4 μm band is replaced by a weak, featureless 3.47 μm band. (5) Whereas silicate grains identified by their 9.7 μm band tend to align in magnetic fields, grains with a strong 3.4 μm organic signature do not tend to align. © 2006. The American Astronomical Society. All rights reserved.