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

¹ CY Cergy Paris Université, Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, CNRS, LIRA, 95000 Cergy, France
² Center for Astrochemical Studies, Max-Planck-Institut für Extraterrestrische Physik (MPE), Gießenbachstr. 1, 85741 Garching, Germany

^★ Corresponding authors: bhusquin@mpe.mpg.de; francois.dulieu@cyu.fr

Received: 7 March 2025
Accepted: 9 September 2025

Abstract

Context. 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. This might affect the formation of molecules.

Aims. We investigated the effect of Ne on the production of formaldehyde (H₂CO) and methanol (CH₃OH) through carbon monoxide (CO) hydrogenation on different cold surfaces. We highlight its role in conditions corresponding to pre-stellar cores.

Methods. 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 products were analysed using a quadrupole mass spectrometer. 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. We measured the evolution of the CO layer through Fourier-transform infrared absorption spectroscopy. Additionally, we used a gas-grain chemical code to simulate a pre-stellar core and determine where Ne can affect the chemistry.

Results. 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 H₂CO 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 CH₃OH 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. Gas-grain chemical modelling showed that the first layer of Ne condenses in the central area of a pre-stellar core, typically within 5000 AU, where CO molecules completely freeze out onto grains.

Conclusions. Ne inhibits the hydrogenation process in the very central part of pre-stellar cores, and in general, where temperatures drop below 9 K and the density increases above 10⁴ cm⁻³.