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Gas-phase condensation of carbonated silicate grains
- Source :
- Astrophys. J. 966 (2), 191 (2024)
- Publication Year :
- 2024
-
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.<br />Comment: 19 pages, 16 figures, accepted for publication in The Astrophysical Journal
- Subjects :
- Astrophysics - Astrophysics of Galaxies
Condensed Matter - Materials Science
Subjects
Details
- Database :
- arXiv
- Journal :
- Astrophys. J. 966 (2), 191 (2024)
- Publication Type :
- Report
- Accession number :
- edsarx.2403.16925
- Document Type :
- Working Paper
- Full Text :
- https://doi.org/10.3847/1538-4357/ad381d