Steady state vibrational populations of SiO and CO in dilute black body radiation fields have been calculated as a function of total pressure, kinetic temperature and chemical composition of the gas. Approximate calculations for polyatomic molecules have also been presented. Vibrational disequilibrium becomes increasingly significant as total pressure and radiation density decrease. Many regions of postulated grain formation are found to be far from thermal equilibrium before the onset of nucleation. Calculations based upon classical nucleation theory or equilibrium thermodynamics are expected to be of dubious value in such regions. Laboratory measurements of the extinction of small iron and magnetite grains were made from 195nm to 830nm and found to be consistent with predictions based upon published optical constants. This implies that small iron particles are not responsible for the 220nm interstellar extinction feature. A feature which begins near 160nm in the extinction spectrum of HD44179 is identified as due to water ice. Measurements have been made of the critical partial pressure of SiO (P(,c)) necessary to initiate avalanche nucleation in the SiO-H(,2) system as a function of temperature (750K < T < 1000K). The condensate produced by this process is Si(,2)O(,3) rather than SiO(,2). Analysis of P(,c) versus T using classical nucleation theory yields a value of 500 ergs/cm('2) for the surface free energy of the initial clusters. Despite the fact that this value is in reasonable agreement with those from the literature, numerous inconsistencies in the analysis are noted. It is shown that classical nucleation theory is not applicable to this system. Measurements of P(,c) vs T for the Mg-SiO-H(,2) system have been obtained (750K < T < 1015K). These are compared to similar measurements for the SiO-H(,2) system. The presence of magnesium lowers the nucleation barrier for T < 925K but does not effect condensation at higher temperatures. Infrared spectra of both the initial condensate and samples annealed in vacuo at 1000K are presented and compared with infrared observations of OH26.5 + 0.6. Much of the previously ignored fine structure in both the 10 and 20 micron features could be modeled by laboratory produced amorphous silicate smokes.
- Pub Date:
- March 1982
- Physics: Astronomy and Astrophysics;
- Physics: Astronomy and Astrophysics