grain growth

Spectroscopic sizing of interstellar icy grains with JWST

E. Dartois, J. A. Noble et al., Nature Astronomy, 9 Jan 2024

Clouds of gas and dust in the Galaxy are the birthplace of stars and planetary systems. On their journey from the diffuse interstellar medium, through molecular clouds, to protoplanetary disks, grains of cold dust acquire a molecular mantle through accretion and surface chemistry. These "icy grains" thus formed are constantly evolving, and can grow under the influence of collision and aggregation mechanisms in dense, cold regions, influencing the conditions leading to subsequent planetary formation. These observations were made as part of the Early Release Science (ERS) "Ice Age" program, using the James Webb Space Telescope (JWST)1 , by an international team led by Leiden Observatory (Netherlands) and involving scientists from the Physics of Ionic and Molecular Interactions laboratory (PIIM, CNRS/Aix-Marseille Université), the Institut des sciences moléculaires d'Orsay (ISMO, CNRS/Université Paris-Saclay), the Laboratoire d'astrophysique de Bordeaux (LAB, CNRS/Université de Bordeaux) and the Laboratoire d'études du rayonnement et de la matière en astrophysique et atmosphères (LERMA, CNRS/Sorbonne Université/Observatoire de Paris/CY Cergy Paris Université). The data collected from the dense Chameleon I cloud demonstrate that the growth of these grains can be observed at a distance, and begins very early in the life of interstellar clouds, before the so-called protostellar phase that will lead to the formation of new stars and their possible retinue of planets. This magnification significantly changes the way light interacts with these grains, which begin to scatter light in a significant, wavelength-selective way. The spectroscopic profiles of the ice bands observed in the infrared range are then modified, making them markers of grain size evolution. A fine spectral analysis of these profiles, probed by looking at the extinction of light from stars in our Galaxy located in the background of the dense Chameleon cloud, confirms that icy grains are reaching micron sizes. These grains, larger than those found in the classical interstellar medium, are larger than the grain size hitherto measured in these environments. This implies a number of changes in the local microphysics, including a transfer of mass from smaller to larger grains, a potential reduction in the surface area of grains available for chemical reactions, and changes in the penetration and propagation of stellar radiation fields outside the cloud. The distortion of the observed profiles also complicates the determination of chemical abundances. The observation of significant growth of icy grains in dense clouds quantitatively constrains the evolution of grain size prior to star and planet formation. These JWST observations probe the growth of grains at the heart of dense clouds. The spectroscopic profiles of the ice bands observed are so distorted that it is no longer possible to ignore physical effects when interpreting the chemical information contained in these spectra. The spectral range covered from near- to mid-infrared allows several ice bands to be probed simultaneously, considerably increasing the level of constraint that can be placed on the grain size distribution. Magnification to micron sizes combined with the high ratio of ice volume to more refractory material observed along these sightlines indicates that both aggregation and gas phase condensation are occurring concomitantly. Such observations of significant grain growth in the dense cloud phase therefore constrain the time dependence of grain size along the evolutionary trajectory of star-forming regions.

CO2 ice profiles

Influence of grain growth on the CO2 ice spectroscopic profiles

Modelling for dense cores and disks in view of JWST ice observations
E. Dartois, J. Noble, N. Ysard, K. Demyk, and M. Chabot, Astronomy and Astrophysics, 666, October 2022, A153

Interstellar dust grain growth in dense clouds and protoplanetary disks, even moderate, affect the observed interstellar ice profiles as soon as a significant fraction of dust grains are in the size range close to the wave vector at the considered wavelength. The continuum baseline extraction made prior to analysing ice profiles influences the subsequent analysis and estimated ice composition, that is most often based on deconvolution using thin film ice mixture spectra. Using the Discrete Dipole Approximation for Scattering and Absorption of Light the mass absorption coefficient of several distri- butions of ellipsoidal silicates core, water and carbon dioxide ice mantle grains are calculated. A few models include additionally amorphous carbon in the core and pure carbon monoxide in the ice mantle. We explore the evolution of the size distribution starting in the dense core phases to simulate the first steps of grain growth up to three microns in size. The resulting mass absorption coefficient are injected in RADMC3D radiative transfer models of spherical dense core and protoplanetary disk templates to retrieve the observable spectral energy distribution. Calculations are performed using the full scattering capabilities of the radiative transfer code. We then focus on the particularly relevant carbon dioxide ice calculated profile. The carbon dioxide antisymmetric stretching mode profile is a meaningful indicator of grain growth. The observed profile toward dense core with the Infrared space observatory and Akari satellites already showed profiles possibly indicative of moderate grain growth. Conclusions. The observation of true protoplanetary disks at high inclination with the JWST should present deformed profiles that will allow constraining the extent of dust growth. Extracting ice mantles composition will require to understand and take grain growth into account.

Ryugu chemical characterisation from synchrotron spectroscopy in the mid to Far-IR of Hayabusa2 samples

Emmanuel Dartois, Yoko Kebukawa, Cécile Engrand, Jean Duprat, Laure Bejach, Jérémie Mathurin, Alexandre Dazzi, Ariane Deniset-Besseau, Lydie Bonal, Eric Quirico, C. Sandt, F. Borondics, Hikaru Yabuta, Hisayoshi Yurimoto, Tomoki Nakamura, Takaaki Noguchi, Ryuji Okazaki, Hiroshi Naraoka, Kanako Sakamoto, Shogo Tachibana, Sei-ichiro Watanabe and Yuichi Tsuda, Tomoyo Morita, Mizuha Kikuiri, Kana Amano, Eiichi Kagawa & the Hayabusa2-initial-analysis IOM and Stone teams. LPSC 2022

The 6th December 2020, the Hayabusa 2 mission from the Japanese space agency (JAXA) returned samples collected on the Cb-type dark asteroid Ryugu to Earth [1]. For the first time, Hayabusa 2 brought back samples from the surface of a known carbonaceous asteroid. InfraRed (IR) analyses of these samples provide unprecedented information on the formation and early evolution of Solar System. A key goal of this study between the IOM [2] and Stone [3] Initial Analysis Teams is to elucidate the distributions and chemical characteristics of macromolecular organic materials and minerals in a C-type asteroid. In this work, we provide hyperspectral analyses showing the characteristics content and distribution of both the organics and the minerals of Hayabusa 2 particles that will enable the comparison with other types of asteroid and cometary samples available in the laboratory.

Electronic sputtering of solid N2

E. Dartois, M. Chabot, T. Id Barkach, H. Rothard, P. Boduch, B. Augé, J. Duprat, J. Rojas, Nuclear Instruments and Methods in Physics Research B 485 (2020) 13–19

Most sputtering yield measurements for solid N2 are reported for stopping powers lower than 10−13 eV cm2/molecule. We measured the sputtering yield for solid N2 at stopping powers, in the electronic regime, above 10−12 eV cm2/molecule, extending the range of such measurements by more than an order of magnitude, using a 33 MeV58Ni9+ swift heavy ions beam. The evolution of the thin N2 ice films was monitored in-situ by mid-infrared spectroscopy (FTIR) during irradiation. As N2 is only weakly infrared active, and can be hardly monitored directly via an infrared absorption mode in such experiments, we use the Fabry–Perot interference fringes of the ice film to evaluate, via an optical model, the erosion of the N2 film as a function of ion fluence. A sputtering model including several sputtering crater shapes is developed and tested against experimental data. We derive the sputtering yield for a semi-infinite N2 ice film and its dependence with the ice thickness for thin film conditions, monitoring the N2 ice sputtering depth. We combine the results with previous measurements at lower stopping powers to derive the electronic sputtering of solid N2 over a large stopping power range.

Mechanochemical synthesis of aromatic infrared band carriers

The top-down chemistry of interstellar carbonaceous dust grain analogues
E. Dartois, E. Charon, C. Engrand, T. Pino, and C. Sandt, A&A 637, A82 (2020)

Interstellar space hosts nanometre- to micron-sized dust grains, which are responsible for the reddening of stars in the visible. The carbonaceous-rich component of these grain populations emits in infrared bands that have been observed remotely for decades with telescopes and satellites. They are a key ingredient of Galactic radiative transfer models and astrochemical dust evolution. However, except for C60 and its cation, the precise carriers for most of these bands are still unknown and not well reproduced in the laboratory. Aims. In this work, we aim to show the high-energy mechanochemical synthesis of disordered aromatic and aliphatic analogues provides interstellar relevant dust particles. Methods. The mechanochemical milling of carbon-based solids under a hydrogen atmosphere produces particles with a pertinent spectroscopic match to astrophysical observations of aromatic infrared band (AIB) emission, linked to the so-called astrophysical polycyclic aromatic hydrocarbon (PAH) hypothesis. The H/C ratio for the analogues that best reproduce these astronomical infrared observations lies in the 5±2% range, potentially setting a constraint on astrophysical models. This value happens to be much lower than diffuse interstellar hydrogenated amorphous carbons, another Galactic dust grain component observed in absorption, and it most probably provides a constraint on the hydrogenation degree of the most aromatic carbonaceous dust grain carriers. A broad band, observed in AIBs, evolving in the 1350-1200 cm−1 (7.4-8.3 μm) range is correlated to the hydrogen content, and thus the structural evolution in the analogues produced. Results. Our results demonstrate that the mechanochemical process, which does not take place in space, can be seen as an experi- mental reactor to stimulate very local energetic chemical reactions. It introduces bond disorder and hydrogen chemical attachment on the produced defects, with a net effect similar to the interstellar space very localised chemical reactions with solids. From the vantage point of astrophysics, these laboratory interstellar dust analogues will be used to predict dust grain evolution under simulated interstellar conditions, including harsh radiative environments. Such interstellar analogues offer an opportunity to derive a global view on the cycling of matter in other star forming systems.

Nanometer scale infrared chemical imaging of organic matter in Ultracarbonaceous Antarctic micrometeorites (UCAMM)

J. Mathurin, E Dartois, T. Pino, C. Engrand, J. Duprat, A. Deniset-Besseau, F. Borondics, C. Sandt, and A. Dazzi, Astronomy & Astrophysics 622, A160 (2019)

The composition of comets and asteroids sheds light on the formation and early evolution of the solar system. The study of micrometeorites containing large concentrations of carbonaceous material (i.e. ultra-carbonaceous antarctic micrometeorites, UCAMMs) allows for unique information on the association of minerals and organics at surface of icy objects (comets) to be obtained. Methods. In this work we map the organic matter of UCAMMs collected in the Antarctic snow at sub-wavelength spatial scales using the Atomic Force Microscope InfraRed (AFMIR) technique. The sample preparation did not involve any chemical pretreatment to extract organic matter. The AFMIR measurements were performed on a limited spectral coverage (1900–1350 cm−1) allowing chemical functional groups to be imaged at spatial scales relevant to the study of micrometeorites. The AFMIR images reveal the variability of the functional groups at very small scales and the intimate association of carbon- and oxygen-bearing chemical bonds. We demonstrate the possibility to potentially separate the olefinic and aromatic C=C bonding in the subcomponents of the UCAMM fragment. These variations probably originate in the early mixing of the different reservoirs of organic matter constituting these dust particles. The measurements demonstrate the potential for analysing such complex organic- matter – mineral association at scales below the diffraction limit. The development of such studies and extension to the full infrared range spectral coverage will drive a new view on the vibrational infrared analysis of interplanetary material.

Swift heavy ion irradiation of interstellar dust analogues

Small carbonaceous species released by cosmic rays

Interstellar dust grain particles are immersed in vacuum ultraviolet (VUV) and cosmic ray radiation environments influencing their physicochemical composition. Owing to the energetic ionizing interactions, carbonaceous dust particles release fragments that have direct impact on the gas phase chemistry. The exposure of carbonaceous dust analogues to cosmic rays is simulated in the laboratory by irradiating films of hydrogenated amorphous carbon interstellar analogues with energetic ions. New species formed and released into the gas phase are explored. Methods. Thin carbonaceous interstellar dust analogues were irradiated with gold (950 MeV), xenon (630 MeV), and carbon (43 MeV) ions at the GSI UNILAC accelerator. The evolution of the dust analogues is monitored in situ as a function of fluence at 40, 100, and 300 K. Effects on the solid phase are studied by means of infrared spectroscopy complemented by simultaneously recording mass spectrometry of species released into the gas phase. Specific species produced and released under the ion beam are analyzed. Cross sections derived from ion-solid interaction processes are implemented in an astrophysical context.

Dome C ultracarbonaceous Antarctic micrometeorites

Infrared and Raman fingerprints

UltraCarbonaceous Antarctic MicroMeteorites (UCAMMs) represent a small fraction of interplanetary dust particles reaching the Earth’s surface and contain large amounts of an organic component not found elsewhere. They are most probably sampling a contribution from the outer regions of the solar system to the local interplanetary dust particle (IDP) flux. Aims. We characterize UCAMMs composition focusing on the organic matter, and compare the results to the insoluble organic matter (IOM) from primitive meteorites, IDPs, and the Earth.
We acquired synchrotron infrared microspectroscopy (μFTIR) and μRaman spectra of eight UCAMMs from the Concordia/CSNSM collection, as well as N/C atomic ratios determined with an electron microprobe.
The spectra are dominated by an organic component with a low aliphatic CH versus aromatic C=C ratio, and a higher nitrogen fraction and lower oxygen fraction compared to carbonaceous chondrites and IDPs. The UCAMMs carbonyl absorption band is in agreement with a ketone or aldehyde functional group. Some of the IR and Raman spectra show a C≡N band corresponding to a nitrile. The absorption band profile from 1400 to 1100 cm−1 is compatible with the presence of C-N bondings in the carbonaceous network, and is spectrally different from that reported in meteorite IOM. We confirm that the silicate-to-carbon content in UCAMMs is well below that reported in IDPs and meteorites. Together with the high nitrogen abundance relative to carbon building the organic matter matrix, the most likely scenario for the formation of UCAMMs occurs via physicochemical mechanisms taking place in a cold nitrogen rich environment, like the surface of icy parent bodies in the outer solar system. The composition of UCAMMs provides an additional hint of the presence of a heliocentric positive gradient in the C/Si and N/C abundance ratios in the solar system protoplanetary disc evolution.

Cosmic ray sputtering yield of interstellar H2O ice mantles

Ice mantle thickness dependence

Interstellar grain mantles present in dense interstellar clouds are in constant exchange with the gas phase via accretion and desorption mechanisms (UV, X-ray photodesorption, cosmic ray induced sputtering, grain thermal fluctuations, chemical-reaction energy-release). The relative importance of the various desorption mechanisms is of uttermost importance for astrophysical models, in order to constrain the chemical evolution in such high density dense cloud regions. The sputtering yields for swift ions simulating the effects of cosmic rays are most often measured in the semi-infinite limit using thick ice targets, with the determination of the effective yield per incident ion. In this experimental work we investigate the sputtering yield as a function of ice mantle thickness, exposed to Xe ions at 95MeV. The ion induced ice phase transformation and the sputtering yield were simultaneously monitored by infrared spectroscopy and mass spectrometry, respectively. The sputtering yield is constant above a characteristic ice layer thickness and then starts to decrease below. An estimate of the typical sputtering depth corresponding to this length can be evaluated by comparing the infinite thickness yield to the column density where the onset of the sputtering yield decrease occurs. In these experiments the measured characteristic desorption depth corresponds to ≈ 30 ice layers. Assuming an "effective" cylindrical shape for the volume of sputtered molecules, the aspect ratio (diameter to height of the cylinder in the semi-infinite ice film case) is close to unity. It shows that most ejected molecules are arising from a rather compact volume. The measured infinite thickness sputtering yield for water ice mantles scales as the square of the ion electronic stopping power (Se, deposited energy per unit path length). Considering the experiments on insulators, we expect that the desorption depth dependence varies with Sαe , with α∼0.5. Astrophysical models should take into account this ice mantles thickness dependence constraints in the interface regions when ices are close to their extinction threshold. In the very dense cloud regions, most of the water ice mantles are above this limit for the bulk of the cosmic rays.