Bottom-up dust nucleation theory in oxygen-rich evolved stars

Abstract

Context. Aluminium oxide (alumina; Al2O3) is a promising candidate as a primary dust condensate in the atmospheres of oxygen-rich evolved stars. Therefore, alumina ‘seed’ particles might trigger the onset of stellar dust formation and of stellar mass loss in the wind. However, the formation of alumina dust grains is not well understood. Aims. We aim to shed light on the initial steps of cosmic dust formation (i.e. nucleation) in oxygen-rich environments via a quantum-chemical bottom-up approach. Methods. Starting with an elemental gas-phase composition, we construct a detailed chemical-kinetic network that describes the formation and destruction of aluminium-bearing molecules and dust-forming (Al2O3)n clusters up to the size of dimers (n = 2) coagulating to tetramers (n = 4). Intermediary species include the prevalent gas-phase molecules AlO and AlOH as well as AlxOy clusters with x = 1–5, y = 1–6. The resulting extensive network is applied to two model stars, which represent a semi-regular variable and a Mira type, and to different circumstellar gas trajectories, including a non-pulsating outflow and a pulsating model. The growth of larger-sized (Al2O3)n clusters with n = 4–10 is described by the temperature-dependent Gibbs free energies of the most favourable structures (i.e. the global minima clusters) as derived from global optimisation techniques and calculated via density functional theory. We provide energies, bond characteristics, electrostatic properties, and vibrational spectra of the clusters as a function of size, n, and compare these to corundum, which corresponds to the crystalline bulk limit (n →∞). Results. The circumstellar aluminium gas-phase chemistry in oxygen-rich giants is primarily controlled by AlOH and AlO, which are tightly coupled by the reactions AlO+H2, AlO+H2O, and their reverse. Models of semi-regular variables show comparatively higher AlO abundances, as well as a later onset and a lower efficiency of alumina cluster formation when compared to Mira-like models. The Mira-like models exhibit an efficient cluster production that accounts for more than 90% of the available aluminium content, which is in agreement with the most recent ALMA observations. Chemical equilibrium calculations fail to predict both the alumina cluster formation and the abundance trends of AlO and AlOH in the asymptotic giant branch dust formation zone. Furthermore, we report the discovery of hitherto unreported global minimum candidates and low-energy isomers for cluster sizes n = 7, 9, and 10. A homogeneous nucleation scenario, where Al2O3 monomers are successively added, is energetically viable. However, the formation of the Al2O3 monomer itself represents an energetic bottleneck. Therefore, we provide a bottom-up interpolation of the cluster characteristics towards the bulk limit by excluding the monomer, approximately following an n−1∕3 dependence.

Experimental study of the removal of excited state phosphorus atoms by H2O and H2: implications for the formation of PO in stellar winds

“…We use here a model that we applied recently to circumstellar gas trajectories, including nonpulsating and pulsating outflows, for two late-type stars with a different evolutionary stage: a semi-regular (SRV) AGB star, and a Mira-type (MIRA) AGB star (Gobrecht et al 2022). In the present study, we use the gas trajectories of the MIRA model star in oxygen-rich conditions (C/O = 0.75), characterised by a total density of ngas = 4 ✁ 10 14 cm -3 and a temperature of 2000 K at the stellar surface.…”

Dust Formation in Astrophysical Environments: The Importance of Kinetics

“…Consider again the fiducial reaction (1). Based on quantum chemical calculations (Gobrecht et al, 2022), the chemical routes involved in the formation of the monomer and the dimer of corundum are shown in Figure 4. As this figure illustrates, AlO is the main aluminum-bearing species involved and extra oxygen that is required is provided by H 2 O and OH, where the latter is part of the (kinetic) H-H 2 -O-OH-H 2 O system.…”

“…This can be modeled by following the Lagrangian trajectory of a gas parcel through its shock compression and heating and adiabatic expansion and cooling phases with a chemical kinetics network. This approach-pioneered by Cherchneff and coworkers (Cherchneff, 2006)-has now been applied to the formation of molecules and dust formation in oxygen-rich and carbon-rich AGB ejecta (Cherchneff, 2011(Cherchneff, , 2012Decin et al, 2008;Gobrecht et al, 2016Gobrecht et al, , 2022. Figure 7 shows abundances calculated after one complete pulsation as a function of distance and efficient cluster formation in the wind occurs already at one stellar radius.…”

“…Note the rapid onset of the Al 8 O 12 cluster when the temperature reaches ≃1400 K. Bottom panel: Calculated abundances for a model where pulsations of the atmosphere are taken into account. Figure adapted from(Gobrecht et al, 2022).…”

The Photoionization Dynamics, Electronic Spectroscopy, and Excited State Photochemistry of AlCO and AlOC

“…One such reaction that is likely is on the surface of aluminum (Al 2 O 3 ) dust grains that are condensed in the inner CSE of evolved stars (dust acceleration zone ;Ziurys 2006) and in the center of dense interstellar clouds (Mangan et al 2021;Gobrecht et al 2022). Casarin et al performed a density functional theory (DFT) based chemisorption study of the interaction of CO with an Al 2 O 3 surface and found that the preferred orientation consists of the C atom reacting perpendicular with the surface to form a chemical bond (Casarin et al 2000).…”