X-Ray-induced Deuterium Enrichment of N-rich Organics in Protoplanetary Disks: An Experimental Investigation Using Synchrotron Light

1Lisseth Gavilan, 2Laurent Remusat, 2Mathieu Roskosz, 3Horia Popescu, 3Nicolas Jaouen, 4Christophe Sandt, 5Cornelia Jäger, 6Thomas Henning, 7Alexandre Simionovici, 8Jean Louis Lemaire
The Astrophysical Journal 840, 35 Link to Article [https://doi.org/10.3847/1538-4357/aa6bfc]
1LATMOS, Université Versailles St Quentin, UPMC Université Paris 06, CNRS, 11 blvd d’Alembert, F-78280 Guyancourt, France
2IMPMC, CNRS UMR 7590; Sorbonne Universités, UPMC Université Paris 06; IRD, Muséum National d’Histoire Naturelle, CP 52, 57 rue Cuvier, Paris F-75231, France
3SEXTANTS beamline, SOLEIL synchrotron, L’Orme des Merisiers, F-91190 Saint-Aubin, France
4SMIS beamline, SOLEIL synchrotron, L’Orme des Merisiers, F-91190 Saint-Aubin, France
5Laboratory Astrophysics and Cluster Physics Group of the Max Planck Institute for Astronomy at the Friedrich Schiller University & Institute of Solid State Physics, Helmholtzweg 3, D-07743 Jena, Germany
6Max-Planck Institute for Astronomy Königstuhl 17, D-69117 Heidelberg, Germany
7Institut des Sciences de la Terre, Observatoire des Sciences de l’Univers de Grenoble, BP 53, F-38041 Grenoble, France
8Institut des Sciences Moléculaires d’Orsay (ISMO), CNRS, Univ. Paris Sud, Université Paris-Saclay, F-91405 Orsay, France
9Institut Jean Lamour, CNRS, Université de Lorraine, F-54011 Nancy, France

The deuterium enrichment of organics in the interstellar medium, protoplanetary disks, and meteorites has been proposed to be the result of ionizing radiation. The goal of this study is to simulate and quantify the effects of soft X-rays (0.1–2 keV), an important component of stellar radiation fields illuminating protoplanetary disks, on the refractory organics present in the disks. We prepared tholins, nitrogen-rich organic analogs to solids found in several astrophysical environments, e.g., Titan’s atmosphere, cometary surfaces, and protoplanetary disks, via plasma deposition. Controlled irradiation experiments with soft X-rays at 0.5 and 1.3 keV were performed at the SEXTANTS beamline of the SOLEIL synchrotron, and were immediately followed by ex-situ infrared, Raman, and isotopic diagnostics. Infrared spectroscopy revealed the preferential loss of singly bonded groups (N–H, C–H, and R–N≡C) and the formation of sp3 carbon defects with signatures at ~1250–1300 cm−1. Raman analysis revealed that, while the length of polyaromatic units is only slightly modified, the introduction of defects leads to structural amorphization. Finally, tholins were measured via secondary ion mass spectrometry to quantify the D, H, and C elemental abundances in the irradiated versus non-irradiated areas. Isotopic analysis revealed that significant D-enrichment is induced by X-ray irradiation. Our results are compared to previous experimental studies involving the thermal degradation and electron irradiation of organics. The penetration depth of soft X-rays in μm-sized tholins leads to volume rather than surface modifications: lower-energy X-rays (0.5 keV) induce a larger D-enrichment than 1.3 keV X-rays, reaching a plateau for doses larger than 5 × 1027 eV cm−3. Synchrotron fluences fall within the expected soft X-ray fluences in protoplanetary disks, and thus provide evidence of a new non-thermal pathway to deuterium fractionation of organic matter.

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