Production of linear alkanes via the solid-state hydrogenation of interstellar polyynes

¹ Xinjiang Astronomical Observatory, Chinese Academy of Sciences, Urumqi 830011, China
² Xinjiang Key Laboratory of Radio Astrophysics, Urumqi 830011, China
³ INAF – Osservatorio Astrofisico di Catania, via Santa Sofia 78, 95123 Catania, Italy
⁴ Laboratory for Astrophysics, Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands

^★ Corresponding author; gleb@xao.ac.cn

Received: 11 September 2024
Accepted: 2 December 2024

Abstract

Context. Highly unsaturated carbon chains, including polyynes, have been detected in many astronomical regions and planetary systems. With the success of the QUIJOTE survey of the Taurus Molecular Cloud-1 (TMC-1), the community has seen a “boom” in the number of detected carbon chains. On the other hand, the Rosetta mission revealed the release of fully saturated hydrocarbons, C₃H₈, C₄H₁₀, C₅H₁₂, and (under specific conditions) C₆H₁₄ with C₇H₁₆, from the comet 67P/Churyumov-Gerasimenko. The detection of the latter two is attributed to dust-rich events. Similarly, the analysis of samples returned from asteroid Ryugu by Hayabusa2 mission indicates the presence of long saturated aliphatic chains in Ryugu’s organic matter.

Aims. The surface chemistry of unsaturated carbon chains under conditions resembling those of molecular clouds can provide a possible link among these independent observations. However, laboratory-based investigations to validate such a chemistry is still lacking. In the present study, we aim to experimentally verify the formation of fully saturated hydrocarbons by the surface hydrogenation of C_2nH₂(n > 1) polyynes under ultra-high vacuum conditions at 10 K.

Methods. We undertook a two-step experimental technique. First, a thin layer of C₂H₂ ice was irradiated by UV-photons (≥121 nm) to achieve a partial conversion of C₂H₂ into larger polyynes: C₄H₂ and C₆H₂. Afterwards, the obtained photoprocessed ice was exposed to H atoms to verify the formation of various saturated hydrocarbons.

Results. In addition to C₂H₆, which was investigated previously, the formation of larger alkanes, including C₄H₁₀ and (tentatively) C₆H₁₄, is confirmed by our study. A qualitative analysis of the obtained kinetic data indicates that hydrogenation of HCCH and HCCCCH triple bonds proceeds at comparable rates, given a surface temperature of 10 K. This can occur on the timescales typical for the dark cloud stage. A general pathway resulting in formation of other various aliphatic organic compounds by surface hydrogenation of N- and O-bearing polyynes is also proposed. We also discuss the astrobiological implications and the possibility of identifying alkanes with JWST.