Hard X-ray irradiation of cosmic silicate analogs: structural evolution and astrophysical implications

¹ Institut d’Astrophysique Spatiale (IAS), CNRS, Univ. Paris Sud, Université Paris-Saclay, 91405 Orsay, France
e-mail: lisseth.gavilan@ias.u-psud.fr
² Laboratory Astrophysics and Cluster Physics Group of the Max Planck Institute for Astronomy at the Friedrich Schiller University & Institute of Solid State Physics, Helmholtzweg 3, 07743 Jena, Germany
³ Institut des Sciences de la Terre, Observatoire des Sciences de l’Univers de Grenoble, BP 53, 38041 Grenoble, France
⁴ Institut des Sciences Moléculaires d’Orsay (ISMO), CNRS, Univ. Paris Sud, Université Paris-Saclay, 91405 Orsay, France
⁵ LAPA-IRAMAT, NIMBE, CEA, CNRS, Université Paris-Saclay, CEA Saclay, 91191 Gif-sur-Yvette, France
⁶ LEEL, NIMBE, CEA, CNRS, Université Paris-Saclay, CEA Saclay, 91191 Gif-sur-Yvette, France
⁷ Max-Planck Institute for Astronomy Königstuhl 17, 69117 Heidelberg, Germany
⁸ European Synchrotron Radiation Facility, 71 avenue des Martyrs, 38000 Grenoble, France

Received: 6 November 2015
Accepted: 19 January 2016

Abstract

Context. Protoplanetary disks, interstellar clouds, and active galactic nuclei contain X-ray-dominated regions. X-rays interact with the dust and gas present in such environments. While a few laboratory X-ray irradiation experiments have been performed on ices, X-ray irradiation experiments on bare cosmic dust analogs have been scarce up to now.

Aims. Our goal is to study the effects of hard X-rays on cosmic dust analogs via in situ X-ray diffraction. By using a hard X-ray synchrotron nanobeam, we seek to simulate cumulative X-ray exposure on dust grains during their lifetime in these astrophysical environments and provide an upper limit on the effect of hard X-rays on dust grain structure.

Methods. We prepared enstatite (MgSiO₃) nanograins, which are analogs to cosmic silicates, via the melting-quenching technique. These amorphous grains were then annealed to obtain polycrystalline grains. These were characterized via scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) before irradiation. Powder samples were prepared in X-ray transparent substrates and were irradiated with hard X-rays nanobeams (29.4 keV) provided by beamline ID16B of the European Synchrotron Radiation Facility (Grenoble). X-ray diffraction images were recorded in transmission mode, and the ensuing diffractograms were analyzed as a function of the total X-ray exposure time.

Results. We detected the amorphization of polycrystalline silicates embedded in an organic matrix after an accumulated X-ray exposure of 6.4 × 10²⁷ eV cm^-2. Pure crystalline silicate grains (without resin) do not exhibit amorphization. None of the amorphous silicate samples (pure and embedded in resin) underwent crystallization. We analyze the evolution of the polycrystalline sample embedded in an organic matrix as a function of X-ray exposure.

Conclusions. Loss of diffraction peak intensity, peak broadening, and the disappearance of discrete spots and arcs reveal the amorphization of the resin embedded (originally polycrystalline) silicate sample. We explore the astrophysical implications of this laboratory result as an upper limit to the effect of X-rays on the structure of cosmic silicates.