Gas-phase condensation of carbonated silicate grains

Abstract

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 O${}_2$ and CO${}_2$ 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 MgSiO${}_3$ 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 $\sim$6.3 and $\sim$7.0 $\mu$m 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-$\alpha$ radiation dissociates the carbonate groups at the surface of the carbonated silicate grains and we estimate the corresponding photodissociation cross section of (0.04 $\pm$ 0.02) $\times$ 10${}^{-16}$ cm${}^2$. 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.

arXiv comment19 pages, 16 figures, accepted for publication in The Astrophysical Journal

References in corpus (13)

• Observations of the Icy Universe
• A Unified Representation of Gas-Phase Element Depletions in the Interstellar Medium
• The c2d Spitzer Spectroscopic Survey of Ices Around Low-Mass Young Stellar Objects: I. H2O and the 5-8 um Bands
• Dust formation in the oxygen-rich AGB star IK Tau
• Oxygen Depletion in the Interstellar Medium: Implications for Grain Models and the Distribution of Elemental Oxygen
• The complete ISO spectrum of NGC 6302
• The mineral composition and spatial distribution of the dust ejecta of NGC 6302
• The essential elements of dust evolution: A viable solution to the interstellar oxygen depletion problem?
• Separate silicate and carbonaceous solids formed from mixed atomic and molecular species diffusing in neon ice
• A Closer Look at Some Gas-Phase Depletions in the ISM: Trends for O, Ge, and Kr vs. $F_{\ast }$, $f({\mathrm{H}}_2)$, and Starlight Intensity
• Soft X-ray Irradiation of Silicates: Implications on Dust Evolution in Protoplanetary Disks
• Sakurai's Object revisited: new laboratory data for carbonates and melilites suggest the carrier of 6.9 $\mu$m excess absorption is a carbonate
• Carbonates and ices in the $z=0.89$ galaxy-absorber towards PKS 1830-211 and within star-forming regions of the Milky Way