Experimental sticking coefficients of CO and N_2 on sub-micrometric cosmic grain analogs

Abstract

Measuring the sticking coefficient of molecules pertinent to astrochemistry - such as CO - on substrates that mimic interstellar dust grains is crucial for the comprehensive understanding of gas-grain chemical processes. Although astrochemical models assume a sticking coefficient of 1, recent laboratory experiments on H2O and CO2 have revealed significantly lower values when measured on small grain analogs. As the effect of grain size on molecular adsorption has been largely ignored to date, further experiments are needed to determine the accretion rates of species known to freeze out on dust grains. Our aim is to determine the sticking coefficients of CO and N2 on sub-micrometric silicate and carbon grains. By quantifying realistic sticking coefficients on these dust grain analogs, we can improve the accuracy of astrochemists' predictions of molecular abundances as affected by gas-grain interactions. The molecules of interest were added to various substrates at 10 K in an ultra-high vacuum. The amount of adsorbate that stuck to the substrate was quantified using X-ray photoelectron spectroscopy. These quantities were compared to a reference with a sticking coefficient of 1, allowing the deduction of the sticking coefficient for each substrate. The average sticking coefficients of CO and N2 on grain analogs are 0.17 for CO and 0.14 for N2 on olivine powder, and 0.05 for CO and 0.07 on N2 on soot, instead of the presumed 1. This is in line with the low values previously reported for H2O and CO2 These laboratory results indicate that CO and N2 in addition to H2O and CO2 also exhibit a low sticking coefficient on dust grain analogs. It is thus necessary to reconsider the interactions between gaseous species and dust particles as a low-efficiency process. This reduction in accretion and reaction rates has important implications for how we understand astrochemistry.