Seed particle formation for silicate dust condensation by SiO nucleation

TL;DR

This study revises the vapour pressure data of SiO, showing that SiO nucleation can trigger silicate dust formation at higher temperatures in stellar outflows than previously thought, aligning better with observations.

Contribution

The paper provides new vapour pressure measurements of solid SiO and re-calibrates nucleation data, improving understanding of dust formation in stellar environments.

Findings

Nucleation begins at higher temperatures than previously estimated.

Condensation temperatures are about 100 K lower than observed, possibly due to silicate greenhouse effects.

Silicate dust formation triggered by SiO clustering is consistent with IR observations.

Abstract

Clustering of the abundant SiO molecules has been discussed as a possible mechanism of seed particle formation for silicate dust in stellar outflows with an oxygen rich element mixture. Previous results indicated that condensation temperatures based on this mechanism are significant lower than what is really observed. This negative result strongly rests on experimental data on vapour pressure of SiO. New determinations show the older data to be seriously in error. Here we aim to check with improved data the possibility that SiO nucleation triggers the cosmic silicate dust formation. First we present results of our measurements of vapour pressure of solid SiO. Second, we use the improved vapour pressure data to re-calibrate existing experimental data on SiO nucleation from the literature. Third, we use the re-calibrated data on SiO nucleation in a simple model for dust-driven winds to determine the condensation temperature of silicate in stellar outflows from AGB stars. We show that onset of nucleation under circumstellar conditions commences at higher temperature than was previously found. Calculated condensation temperatures are still by about 100 K lower than observed ones, but this may be due to the greenhouse effect of silicate dust temperatures. The assumption that the onset of silicate dust formation in late-type M stars is triggered by cluster formation of SiO is compatible with dust condensation temperatures derived from IR observations.