Gas-phase condensation of carbonated silicate grains

Reports on the detection of carbonates in planetary nebulae (PNe) and protostars suggested the existence of a mechanism that produce these compounds in stellar winds and outflows. A consecutive laboratory study reported a possible mechanism by observing the non-thermodynamic equilibrium (TE), gas-phase condensation of amorphous silicate grains with amorphous calcium carbonate inclusions. It concluded that water vapor was necessary to the formation of the carbonates. We present a laboratory study with pulsed laser ablation of an 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 formula MgSiO₃ that comprise carbonate groups homogeneously dispersed in their structure. The infrared spectra of the grains show the characteristic bands of amorphous silicates and two bands at ~6.3 and ~7.0 μm that we assign to the carbonate groups. The silicate bands are not significantly affected at an estimated Si:C ratio of 9:1 to 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 Lyman-α 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 VUV photons and the carbonates observed in protostars have not formed by gas-phase condensation.