Gas-phase Condensation of Carbonated Silicate Grains

Abstract

Reports on the detection of carbonates in planetary nebulae (PNe) and protostars have suggested the existence of a mechanism that produces these compounds in stellar winds and outflows. A subsequent laboratory study has reported a possible mechanism by presenting the non-thermodynamic-equilibrium (TE), gas-phase condensation of amorphous silicate grains with amorphous calcium carbonate inclusions. The authors concluded that water vapor was necessary for the formation of the carbonates. We present a laboratory study with pulsed laser ablation of a MgSi target in O2 and CO2 gases and report, in the absence of water vapor, the non-TE, gas-phase condensation of amorphous carbonated magnesium silicate dust. It consists of amorphous silicate grains with the formula MgSiO3, which comprise carbonate groups homogeneously dispersed in their structure. The IR spectra of the grains show the characteristic bands of amorphous silicates and two bands at ∼6.3 and ∼7.0 μm, which we assign to the carbonate groups. The silicate bands are not significantly affected at an estimated Si:C ratio of 9:1–9:2. Such grains could form in winds and outflows of evolved stars and PNe if C atoms are present during silicate condensation. Additionally, we find that Lyα radiation dissociates the carbonate groups at the surface of the carbonated silicate grains and we estimate the corresponding photodissociation cross section of (0.04 ± 0.02) ×10−16 cm2. Therefore, photodissociation would limit the formation of carbonate groups on grains in winds and outflows of stars emitting vacuum ultraviolet photons, and the carbonates observed in protostars have not formed by gas-phase condensation.