The impact of surface acetylene cyclotrimerization on the abundance of aromatic hydrocarbons in carbon-rich asymptotic giant branch stars

TL;DR

This work tackles the problem of how surface chemistry on dust grains affects aromatic hydrocarbon formation in the carbon-rich AGB envelope of IRC+10216. It introduces BRAHMA, a self-consistent model that couples gas-phase hydrocarbon chemistry with surface acetylene cyclotrimerization (CTM) on SiC and carbonaceous dust mantles and with dust growth, extending the network up to pyrene and accounting for pulsation-driven shocks. The key finding is that surface CTM can boost the total aromatic abundance by up to an order of magnitude, with the efficiency strongly controlled by the hydrocarbon desorption energy $E_{ m d}$ and the chemisorption barriers; the results also show that gas-phase and dust-surface processes are tightly linked and must be modeled together for realistic predictions. The study highlights that constraining uncertain parameters, particularly $E_{ m d}$ and adsorption barriers, is essential for future astrochemical modeling of evolved stellar systems and for understanding PAH formation and dust growth in AGB envelopes.

Abstract

This work investigates the catalytic role of dust grains in forming aromatic hydrocarbons via acetylene cyclotrimerization on their surfaces within the circumstellar envelopes of carbon-rich asymptotic giant branch (AGB) stars. We present a comprehensive computational astrochemical model coupling the gas-phase, gas-surface, and surface (cyclotrimerization) reactions, and the physical evolution of the dust grains (coagulation). The model expands upon the basic chemical network from previous models, enhancing them with updated reactions involving hydrocarbons up to pyrene. We applied this model to simulate the chemical evolution of the envelope of the prototypical AGB star IRC+10216, utilizing physical conditions derived from a hydrodynamical model available in literature. To quantify the impact of surface chemistry, we compared scenarios with and without the cyclotrimerization reaction, further testing the sensitivity of our results by varying the key parameter of hydrocarbon desorption energy. We find that surface-catalyzed cyclotrimerization is a viable pathway for aromatic formation in circumstellar environments, capable of enhancing the total abundance of aromatic species by up to an order of magnitude. Crucially, we show that gas-phase chemistry and dust surface processes are intrinsically linked; their synergistic evolution should be modeled self-consistently to accurately predict chemical abundances. This work underscores that constraining uncertain parameters, particularly desorption energies of hydrocarbons, is essential for future realistic modeling of astrochemical processes in evolved stellar systems.

Figures (7)

• Figure 1: Left: Gas-phase abundances of aromatic molecules versus distance from the star calculated with CTM (solid lines) and without it (dotted lines). Right: Gas-phase abundances of aromatic molecules at $R=2.7\;R_{\star}$. The red color indicates the model with CTM. The blue color shows the model without it.
• Figure 2: Same as in Fig. \ref{['abund_gas']} but abundances of aromatic molecules on surfaces of dust grains are presented.
• Figure 3: Left: Total abundance of all aromatic molecules obtained in the model with CTM. Gas-phase abundances are shown by dashed lines, solid-phase abundances are shown by dotted lines, and the sum abundance is shown by solid lines. Red, black, and blue colors illustrate the models with $E_{\rm d}=2$, 3 and 4 eV, respectively. Right: Ratio between the total abundance of all aromatic molecules obtained in the models with the surface acetylene CTM and without it. The designation "GR+SR" means the model with CTM, while the designation "GR" indicates the model without it. The ratio of gas-phase abundances is illustrated by solid lines, while the ratio of solid-phase abundances is shown by dotted lines. The colors correspond to the same models as in the left panel.
• Figure 4: Naphthalene formation scheme via reaction of benzyl and propargyl radicals, highlighting initial adducts and final product.
• Figure 5: Pressure-dependent naphthalene formation rate coefficients at various temperatures, showing convergence at low pressures and divergence at higher pressures.