Seeding grain nucleation and dust growth: Ionisation, epoxidation and charge disproportionation effects

Abstract

This work studies the likely dust seeding processes arising from alkali metal and alkaline earth ionisation, epoxidation (epoxide bond formation via oxygen atom insertion into C=C bonds), and grain charge disproportionation (the existence around the uncharged state of oxidised cationic and reduced anionic states) at (sub-)nanometre size scales. The chemical, physical, and photon-initiated processes leading to dust seeding are explored within the framework of the size-dependent physical, optical, and photoelectric properties of the THEMIS carbonaceous nanoparticles. The critical grain charge states at (sub-)nanometre size scales are derived as a function of the interstellar and circumstellar physical conditions. Photo-initiated low-energy ionisation, epoxide reactions, and disproportionation-driven electrostatic effects could play key roles in seeding dust nucleation and growth. The size-dependent seed cluster and nanograin charge distribution is shown to encompass both positive and negative charges where the ionisation is driven by low ionisation metals or by weak attenuation. Cluster seeding via ionisation and epoxidation could help to explain the co-spatial and contemporaneous nucleation and growth of both carbon-rich and oxygen-rich dust in the same regions. This may be enhanced by electrostatic effects, driven by charge disproportionation, between negatively-charged, nucleation-seeding, polyatomic clusters and positively-charged ions or larger (nano)particles. Such processes could occur in the dust-forming regions in novae, Wolf-Rayet, and Luminous Blue Variable systems and electrostatic effects may also aid the accretion of nanoparticles in the outer regions of molecular clouds.

Figures (6)

• Figure 1: Size-dependent nanoparticle IP (red line, Eq. \ref{['eq_IP']}), EA (blue line, Eq. \ref{['eq_EA']}), work function ($W$, cobalt line, Eq. \ref{['eq_Wa']}) and band gap ($E_{\rm g}$, black line, Eq. \ref{['eq_Eg']}). The atomic IPs and the carbon atom EA (dashed and dotted horizontal lines) are also shown. All quantities are in eV.
• Figure 2: Equilibrium charge states for particles as a function of radius, for the parameter sets indicated in Table \ref{['table_conds']} (purple to blue to cobalt). The squares indicate the derived carbon dust radii, $\sim 1$ and $30-50$ nm in WR 140 2023ApJ...951...89L and the small diamond marks a seed cluster of radius = 0.3nm. The vertical dashed lines indicate the minimum grain radius and the approximate peak of the large grain size distribution in THEMIS. The red line shows the CNM charge states and the orange lines those for dense cloud edges with $A_{\rm V}$ of 0, 0.25, 0.5, 1, 2, and 3 (thin to thick orange lines, respectively).
• Figure 3: Schematic view of seed cluster formation, charge states and evolution (black lines, see Fig. \ref{['fig_params']} for seed cluster IPs) during dust nucleation in a hardening radiation field (bottom to top). The ionisation stages (K|Na|Li|Ca, Mg|Fe|Si, C, and H|O), neutral-neutral reactions, epoxidation, epoxide ring opening-driven chemistry, cation accretion and electrostatic agglomeration are indicated. For simplicity hydrogen atoms are not indicated.
• Figure 4: The photon attenuation lengths (dashed lines) for spherical a-C(:H) grains of radii 0.37 (black), 1 (red), 10 (green), and 100 nm (blue). The solid blue line shows the electron mean free path length as a function of energy, which is the same for all grain sizes.
• Figure 5: The photoelectric yields (solid lines) for spherical a-C(:H) grains of radii 0.37 (black), 1 (red), 10 (green), and 100 nm (blue). The dashed lines show the photoelectric yields tabulated in 2016MNRAS.459.2751K for 1, 10, and 100 nm carbon grains.