Cosmic ray interactions with indene and adamantane seeded water ice mantles

¹ Institut des Sciences Moléculaires d’Orsay (ISMO), UMR8214, CNRS, Université Paris-Saclay, 91405 Orsay, France
² Laboratoire de physique des deux infinis Irène Joliot-Curie, CNRS-IN2P3, Université Paris-Saclay, 91405 Orsay, France
³ Centre de Recherche sur les Ions, les Matériaux et la Photonique, CIMAP-CIRIL-GANIL, Normandie Université, ENSICAEN, UNICAEN, CEA, CNRS, 14000 Caen, France
⁴ Dipartimento di Fisica, Università di Trento, Trento 38123, Italy

^★ Corresponding author: eddy.abdo@universite-paris-saclay.fr

Received: 19 June 2025
Accepted: 16 September 2025

Abstract

Context. Indene is among the first polycyclic aromatic hydrocarbon (PAH) species detected in the gas phase in the Taurus molecular cloud (TMC-1), with high abundances for such a complex organic molecule (COM). Considering the conditions of these dense clouds, and the timescales of the processes at play, there have to be mechanisms driving the desorption of molecules to maintain a high level of gas phase species; otherwise, most of them should condense on dust grains when approaching the cloud core. On the other hand, detections of diamond features in young stellar objects have been reported, sparking interest in diamondoid species such as the cage molecule adamantane.

Aims. We investigate and quantify the importance of cosmic rays in the sputtering and radiolysis of COMs such as indene and adamantane and the potential release of species from the solid phase.

Methods. Ice films made of indene and adamantane mixed with water and deposited at 9 K were exposed to a 40 MeV ⁵⁶Fe¹⁰⁺ ion beam at the heavy-ion accelerator Grand Accélérateur National d’Ions Lourds (GANIL; Caen, France). Both infrared and quadrupole mass spectrometer measurements were recorded simultaneously throughout the irradiation, enabling the sputtering efficiencies and the radiolysis cross sections of the species to be determined.

Results. The intact-to-solid sputtered ratios and the radiolysis cross sections of indene and adamantane were determined. By combining these results with astrophysical models, the expected fraction of indene in the solid phase that would be required to explain the gas phase abundance by this process was calculated with respect to the depth in the cloud and compared to current gas phase abundances observed in TMC-1.