Search

Quantifying the effect of anharmonicity on the infrared spectrum of large molecules such as polycyclic aromatic hydrocarbons (PAHs) at high temperatures is the focus of a number of theoretical and experimental studies, many of them motivated by astrophysical applications. We recorded the IR spectrum of pyrene C16H10microcrystals embedded in KBr pellets over a wide range of temperatures (14–723 K) and studied the evolution of band positions, widths, and integrated intensities with temperature. We identified jumps for some of the spectral characteristics of some bands in the 423–473 K range. These were attributed to a change of phase from crystal to molten in condensed pyrene, which appears to affect more strongly bands involving large CH motions. Empirical anharmonicity factors that quantify the linear evolution of band positions and widths with temperature for values larger than ∼150–250 K, depending on the band, were retrieved from both phases and averaged to provide recommended values for these anharmonicity factors. The derived values were found to be consistent with available gas phase data. We conclude about the relevance of the methodology to produce data that can be compared with calculated anharmonic IR spectra and provide input for models that simulate the IR emission of astro-PAHs.

Massive stars disrupt their natal molecular cloud material through radiative and mechanical feedback processes. These processes have profound effects on the evolution of interstellar matter in our Galaxy and throughout the universe, from the era of vigorous star formation at redshifts of 1–3 to the present day. The dominant feedback processes can be probed by observations of the Photo-Dissociation Regions (PDRs) where the far-ultraviolet photons of massive stars create warm regions of gas and dust in the neutral atomic and molecular gas. PDR emission provides a unique tool to study in detail the physical and chemical processes that are relevant for most of the mass in inter- and circumstellar media including diffuse clouds, proto-planetary disks, and molecular cloud surfaces, globules, planetary nebulae, and star-forming regions. PDR emission dominates the infrared (IR) spectra of star-forming galaxies. Most of the Galactic and extragalactic observations obtained with the James Webb Space Telescope (JWST) will therefore arise in PDR emission. In this paper we present an Early Release Science program using the MIRI, NIRSpec, and NIRCam instruments dedicated to the observations of an emblematic and nearby PDR: the Orion Bar. These early JWST observations will provide template data sets designed to identify key PDR characteristics in JWST observations. These data will serve to benchmark PDR models and extend them into the JWST era. We also present the Science-Enabling products that we will provide to the community. These template data sets and Science-Enabling products will guide the preparation of future proposals on star-forming regions in our Galaxy and beyond and will facilitate data analysis and interpretation of forthcoming JWST observations.

Abstract—Synthetic MgSiO3glasses were irradiated at room temperature by 300 keV electrons in a transmission electron microscope (TEM). One of the samples had been previously irradiated by 50 keV He+ ions. Electron irradiation induces the nucleation and growth of randomly oriented nanometer‐sized crystallites. The crystallites first consist of MgO and subsequently of forsterite (Mg2SiO4). Both are seen to form within an amorphous SiO2matrix. The rate of crystallization of the samples has been monitored by conventional TEM imaging and electron diffraction. The sample that had been pre‐irradiated with He+ ions is found to transform faster than the as‐quenched glass. The crystallization of metastable MgSiO3glasses is explained by ionizing radiation‐induced elemental diffusion that allows the reorganization of matter into a more favourable thermodynamic state. These results show that ionizing radiation interactions could account for crystal formation as observed in infrared spectroscopy in some young stellar environments.

Abstract—We present the results of irradiation experiments aimed at understanding the structural and chemical evolution of silicate grains in the interstellar medium. A series of He+ irradiation experiments have been performed on ultra‐thin olivine, (Mg,Fe)2SiO4, samples having a high surface/volume (S/V) ratio, comparable to the expected S/V ratio of interstellar dust. The energies and fluences of the helium ions used in this study have been chosen to simulate the irradiation of interstellar dust grains in supernovae shock waves. The samples were mainly studied using analytical transmission electron microscopy. Our results show that olivine is amorphized by low‐energy ion irradiation. Changes in composition are also observed. In particular, irradiation leads to a decrease of the atomic ratios O/Si and Mg/Si as determined by x‐ray photoelectron spectroscopy and by x‐ray energy dispersive spectroscopy. This chemical evolution is due to the differential sputtering of atoms near the surfaces. We also observe a reduction process resulting in the formation of metallic iron. The use of very thin samples emphasizes the role of surface/volume ratio and thus the importance of the particle size in the irradiation‐induced effects. These results allow us to account qualitatively for the observed properties of interstellar grains in different environments, that is, at different stages of their evolution: chemical and structural evolution in the interstellar medium, from olivine to pyroxene‐type and from crystalline to amorphous silicates, porosity of cometary grains as well as the formation of metallic inclusions in silicates.