Neon is an inhibitor of CO hydrogenation in pre-stellar core conditions

TL;DR

This study reveals that neon, despite its chemical inertness, can condense onto interstellar grain surfaces in pre-stellar core conditions and substantially inhibit CO hydrogenation to formaldehyde (H2CO) and methanol (CH3OH). Using co-deposition and layered-ice experiments at 9–12 K in ultra-high vacuum, the authors show that increasing Ne coverage reduces product yields, with the strongest suppression at lower temperatures, and that Ne can diffuse to the surface to form an insulating layer that isolates CO from H atoms. Complementary gas-grain modelling locates Ne condensation to the central regions of cores (within several thousand AU) and demonstrates that Ne can modify hydrogenation chemistry where T ≤ 9 K and densities exceed 10^4 cm^-3, depending on the Ne binding energy on ice. The results provide actionable parameters for incorporating Ne surface processes into astrochemical models, with implications for the formation of formaldehyde, methanol, and potentially more complex organic molecules in dense, cold regions of the interstellar medium.

Abstract

Neon (Ne) is the fifth most abundant element in the Universe. Because it is chemically inert, it has never been considered in astrochemical models that studied molecular evolution. In the cold dark environments of pre-stellar cores, where the temperatures are below 10 K, Ne can condense onto the surface of interstellar grains. We investigated the effect of Ne on the production of formaldehyde (H$_2$CO) and methanol (CH$_3$OH) through carbon monoxide (CO) hydrogenation on different cold surfaces. We highlight its role in conditions corresponding to pre-stellar cores. In an ultra-high vacuum system, we conducted two types of experiments. The first experiment involved the co-deposition of CO and H atoms with or without Ne. The second experiment involved depositing a monolayer of CO and separately a monolayer of Ne (or vice versa), followed by bombarding the layers with hydrogen atoms. Additionally, we used a gas-grain chemical code to simulate a pre-stellar core and determine where Ne can affect the chemistry. The presence of Ne on the surface significantly inhibits CO hydrogenation at temperatures below 12 K. In the co-deposition experiments, we observed a 38% decrease in the H$_2$CO production at 11 K when the quantity of Ne in the mixture was lower than a monolayer. At 10 K and with one monolayer in the mixture, the production decreased to 77%, and it reached 91% for a few monolayers of Ne in the mixture at 9 K. While the decrease in CH$_3$OH formation is still notable, it is less pronounced: 43% at 11 K, 61% at 10 K, and 77% at 9 K. Experiments with stacked layers revealed that the CO layer decay varies slightly when the Ne layer is positioned above or below it. This observation indicates that Ne and CO create a mixture in which Ne can diffuse and stabilize at the surface, which isolates CO molecules from the accreting H atoms.

Figures (14)

• Figure 1: TPD curves for H2CO ($m/z$=30) on a gold substrate at four different temperatures: 9, 10, 11 and 12 K. The blue line represents the co-deposition of CO and H, and the dashed orange line represents the co-deposition of CO, H, and Ne.
• Figure 2: As in Fig. \ref{['figNe: TPD H2CO']}, but for CH3OH ($m/z$=31).
• Figure 3: Evolution of the surface coverage of a layer of Ne according to the deposition time for different temperatures and substrates.
• Figure 4: Integrated infrared-band area of CO in the case of H atom bombardment at 9 K of pure CO ice (1 ML, black circles), CO ice deposited onto Ne (1 ML, 1.43 ML, orange squares), and CO ice deposited under Ne (1.43 ML, 1 ML, blue triangles). The inset shows the linear slope of the fit from each experiment.
• Figure 5: Schematic view of the physical process of Ne blocking hydrogenation as represented in Figure \ref{['figNe: CO decroissance']}. When CO molecules accrete onto the surface, they diffuse below the neon layer. In contrast to this, neon acts to prevent hydrogen from diffusing into the bulk, thus retaining it on the surface. This means that the H atom cannot react with CO.