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

Abstract

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. 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. We prepared enstatite nanograins, 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 and high-resolution transmission electron microscopy 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. X-ray diffraction images were recorded in transmission mode. We detected the amorphization of polycrystalline silicates embedded in an organic matrix after an accumulated X-ray exposure of 6.4 x 10${}^{27}$ eV cm${}^{-2}$. Pure crystalline silicate grains (without resin) did not exhibit amorphization. None of the amorphous silicate samples (pure and embedded in resin) underwent crystallization. We analyzed the evolution of the polycrystalline sample embedded in an organic matrix as a function of X-ray exposure. Loss of diffraction peak intensity, peak broadening, and the disappearance of discrete spots and arcs, revealed the amorphization of the resin embedded (originally polycrystalline) silicate sample.

arXiv commentAccepted for publication in Astronomy & Astrophysics on 19/01/2016

References in corpus (16)

• Cold Dark Clouds: The Initial Conditions for Star Formation
• Evolution of the Solar Activity over Time and Effects on Planetary Atmospheres: I. High-energy Irradiances (1-1700 A)
• The absence of crystalline silicates in the diffuse interstellar medium
• The Spectrum of Diffuse Cosmic Hard X-Rays Measured with HEAO-1
• Evidence for Accretion: High-Resolution X-ray Spectroscopy of the Classical T Tauri Star TW Hydrae
• Photoevaporation of Circumstellar Disks by FUV, EUV and X-ray Radiation from the Central Star
• Dust evolution in protoplanetary disks around Herbig Ae/Be stars - The Spitzer view
• Crystalline silicates and dust processing in the protoplanetary disks of the Taurus young cluster
• X-rays in the Orion Nebula Cluster: Constraints on the origins of magnetic activity in pre-main sequence stars
• Models of Chemistry, Thermal Balance and Infrared Spectra from Intermediate-Aged Disks around G and K stars
• Exclusion of Cosmic Rays in Protoplanetary Disks. II. Chemical Gradients and Observational Signatures
• Ionization by Impact Electrons in Solids: Electron Mean Free Path Fitted Over a Wide Energy Range
• Forsterite Amorphisation by Ion Irradiation: Monitoring by Infrared Spectroscopy
• The infrared emission spectra of compositionally inhomogeneous aggregates composed of irregularly shaped constituents
• Abundant crystalline silicates in the disk of a very low mass star
• Dust amorphization in protoplanetary disks

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• Soft X-ray Irradiation of Silicates: Implications on Dust Evolution in Protoplanetary Disks
• X-ray induced deuterium enrichment on N-rich organics in protoplanetary disks: an experimental investigation using synchrotron light
• Spitzer/IRS full spectral modeling to characterize mineralogical properties of silicate dust in heavily obscured AGNs