Master equation modelling of non-equilibrium chemistry in stellar outflows.
John M C PlaneStruan H RobertsonPublished in: Faraday discussions (2022)
A current challenge in astrochemistry is to explain the formation of Fe-Mg silicate dust around evolved stars. The dust is observed to form within 2 to 3 stellar radii of oxygen-rich AGB stars, where the typical conditions are kinetic (translational) temperatures between 1200 and 1600 K, and total gas densities below 10 11 cm -3 . At these high temperatures, molecules with bond energies < 400 kJ mol -1 should be short-lived, and this results in kinetic bottlenecks in postulated mechanisms for converting the observed Fe, Mg, SiO and H 2 O into silicate. Here we show that, in the very low pressure regime of a stellar outflow, molecules can exhibit significant vibrational disequilibrium because optical transitions - both spontaneous and stimulated by the stellar radiation field - occur on a much faster timescale than collisions. As a result, relatively less stable molecules can form and survive long enough to provide building blocks to silicate formation. Here we use the molecule OSi(OH) 2 , formed by the recombination of SiO 2 and H 2 O, as an example. When vibrational disequilibrium is accounted for in a master equation treatment which includes optical transitions, the quantity of metal silicates produced in a low mass loss rate evolved star (R Dor) is increased by 6 orders of magnitude.
Keyphrases
- density functional theory
- molecular dynamics simulations
- high resolution
- molecular dynamics
- health risk assessment
- health risk
- human health
- high speed
- aqueous solution
- dna damage
- polycyclic aromatic hydrocarbons
- energy transfer
- metal organic framework
- risk assessment
- dna repair
- magnetic nanoparticles
- raman spectroscopy
- room temperature
- combination therapy
- heavy metals
- mass spectrometry
- radiation therapy
- carbon dioxide
- climate change