Efficient Interstellar Grain Growth from High Sticking Coefficients on Amorphous Carbon Dust

Cosmic dust is the solid phase of the interstellar medium (ISM), classically assumed to be composed of carbonaceous and silicate grains with size distributions spanning about 5 to about 1 mu m (Weingartner & Draine 2001, Draine & Li 2007, Hensley & Draine 2023). While it constitutes at most order-of-magnitude 1% of the ISM mass, dust is second only to stars in importance for the observable properties of galaxies (Zavala et al. 2021). Large uncertainties in the efficiency of grain growth obfuscate the relative contribution of the two dominant sources of dust in the Universe: direct production from evolved stars versus gas-phase accretion in the ambient ISM (Feldmann 2015, Esmerian & Gnedin 2022, Esmerian & Gnedin 2024). Advances in supercomputers have only recently allowed us to move beyond simple, idealized predictions of dust grain growth efficiencies (Leitch-Devlin & Williams 1985) with atomistic dynamical calculations (Bossion et al. 2024). We show that small carbon dust grains can grow significantly on timescales much shorter than the age of the universe and, in some ISM phases, comparable to the lifetimes of giant molecular clouds. Specifically, we perform molecular dynamics simulations of an amorphous carbon (a-C) grain surface impacted by gas-phase atoms of cosmologically abundant elements with realistic interstellar conditions, finding high (0.2) sticking coefficients for all non-inert elements at all relevant gas and grain temperatures. We present the results of experiments conducted on similar dust candidate materials that support our theoretical calculations. Our results therefore confirm that the process of gas-phase accretion onto grains is likely an efficient mechanism for the growth of interstellar dust mass on astrophysical timescales, and plausibly central to the evolutionary life-cycle of interstellar grains at all cosmic epochs. (abridged)