Sputtering of the refractory cores of interstellar grains (original) (raw)

Energetic Processing of Interstellar Silicate Grains by Cosmic Rays

The Astrophysical Journal, 2007

While a significant fraction of silicate dust in stellar winds has a crystalline structure, in the interstellar medium nearly all of it is amorphous. One possible explanation for this observation is the amorphization of crystalline silicates by relatively ''low'' energy, heavy-ion cosmic rays. Here we present the results of multiple laboratory experiments showing that single-crystal synthetic forsterite (Mg 2 SiO 4 ) amorphizes when irradiated by 10 MeV Xe ions at large enough fluences. Using modeling, we extrapolate these results to show that 0.1Y5.0 GeV heavy-ion cosmic rays can rapidly ($70 Myr) amorphize crystalline silicate grains ejected by stars into the interstellar medium.

On the nature of interstellar grains

Astrophysics and Space Science, 1979

Data on inter:~tellar extinction are interpreted to imply an identification of interstellar grains with naturally freeze-dried bacteria and algae. The total mass of such bacterial and algal cells in the galaxy is enormous, ~ 1040 g. The identification is based on Mie scattering calculations for an experimentally determined size distribution of bacteria. Agreement between our model calculations and astronomical data is remarkably precise over the wavelength intervals 1 ~-1 < A-z < 1.94 ~-1 and 2.5 t,-z < t-1 < 3.0 p-1. Over the more restricted waveband 4000-5000 ,~ an excess interstellar absorption is found which is in uncannily close agreement with the absorption properties of phytoplankton pigments. The strongest of the diffuse interstellar bands are provisionally assigned to carotenoid-chlorophyll pigment complexes such as exist in algae and pigmented bacteria. The 12200 A interstellar absorption feature could be due to 'degraded' cellulose strands which form spherical graphitic particles, but could equally well be due to protein-lipid-nucleic acid complexes in bacteria and viruses. Interstellar extinction at wavelengths A < 1800 ,~ could be due to scattering by virus particles. Ever since the existence of interstellar grains was first recognized over half a century ago, astronomers have striven unceasingly to understand their properties-sizes, shapes and composition-and how they might be formed. These questions have assumed a growing importance over the past decade mainly from a conviction that grains play a crucial role in controlling many astrophysical processes. A vigorous effort on the part of many astronomers has led to a welter of new observational data, to much theorizing and heated controversy, but little in the way of agreement or understanding. The nature of interstellar grains remains to this day a major unsolved problem in astronomy (see, for example, Hoyle and Wickramasinghe, 1962; Wickramasinghe, 1967; Martin, 1978). The wavelength dependence' of interstellar extinction is perhaps the most direct observational test of interstellar grain models. The observed interstellar extinction at visual wavelengths has an average value of ~2 mug kpc-1 in directions close to the plane of the galaxy. This datum combined with other scattering and polarization observations leads to the result that grains are strongly dielectric, with radii ~ I 0-5 cm, and that they make up a few percent by mass of all interstellar matter. With an overall composition of interstellar matter similar to the composition of the Sun's outer layers, we thus find that a significant fraction of all C, N, O atoms in interstellar clouds is in the form of grains. About 104o g of interstellar grains exist throughout the galaxy. Since the typical turnover time of interstellar matter into stars is ~ l09 yr, grains must be re-supplied at an average rate of ~ 1031 g yr-z * Honorary Professorial Fellow.

The cosmic-ray induced sputtering process on icy grains

Monthly Notices of the Royal Astronomical Society

In molecular cloud cores, the cosmic ray (CR) induced sputtering via CR ion-icy grain collision is one of the desorption processes for ice molecules from mantles around dust grains. The efficiency of this process depends on the incident CR ion properties as well as the physicochemical character of the ice mantle. Our main objective is the examination of the sputtering efficiency for H2O and CO ices found in molecular cloud cores. In the calculation routine, we consider a multidimensional parameter space that consists of 30 CR ion types, 5 different CR ion energy flux distributions, 2 separate ice mantle components (pure H2O and CO), 3 ice formation states, and 2 sputtering regimes (linear and quadratic). We find that the sputtering behaviour of H2O and CO ices is dominated by the quadratic regime rather than the linear regime, especially for CO sputtering. The sputtering rate coefficients for H2O and CO ices show distinct variations with respect to the adopted CR ion energy flux as ...

IGM metal enrichment through dust sputtering

We study the motion of dust grains into the Intergalactic Medium (IGM) around redshift z = 3, to test the hypothesis that grains can efficiently pollute the gas with metals through sputtering. We use the results available in the literature for radiation-driven dust ejection from galaxies as initial conditions, and follow the motion onward. Via this mechanism, grains are ejected into the IGM with velocities > 100 km s −1 ; as they move supersonically, grains can be efficiently eroded by non-thermal sputtering. However, Coulomb and collisional drag forces effectively reduce the charged grain velocity. Up-to-date sputtering yields for graphite and silicate (olivine) grains have been derived using the code TRIM, for which we provide analytic fits. After training our method on a homogeneous density case, we analyze the grain motion and sputtering in the IGM density field as derived from a ΛCDM cosmological simulation at z = 3.27. We found that only large (a 0.1-µm) grains can travel up to considerable distances (few ×100 kpc physical) before being stopped. Resulting metallicities show a well defined trend with overdensity δ. The maximum metallicities are reached for 10 < δ < 100 (corresponding to systems, in QSO absorption spectra, with 14.5 < log N (H i) < 16). However the distribution of sputtered metals is very inhomogeneous, with only a small fraction of the IGM volume polluted by dust sputtering (filling factors of 18 per cent for Si and 6 per cent for C). For the adopted size distribution, grains are never completely destroyed; nevertheless, the extinction and gas photo-electric heating effects due to this population of intergalactic grains are well below current detection limits.

THE EFFECT OF HEAVY COSMIC-RAY IONS ON SILICATE GRAINS IN THE INTERSTELLAR DUST

The Astrophysical Journal, 2010

Electronmicroscopic samples of crystalline Mg 2 SiO 4 forsterite were irradiated by energetic Ar, Fe, Kr, and Xe ions at room temperature. Tracks with a mean radius R e = 1.36 nm were observed after irradiation by 56 MeV Fe ions, while no tracks were induced by a 48 MeV Ar beam. Amorphization of forsterite grains by cosmic-ray (CR) Fe ions are discussed, including the effects of low temperature, ion velocity, and ion-induced crystallization. CR Fe ions induce amorphous tracks in crystalline forsterite only in the range 40-140 MeV, and the period of time for complete amorphization is τ cr ≈ 13,400 Myr. Our estimate is τ cr ≈ 1300 Myr for enstatite. Thus, heavy CR particles do not reduce the crystallinity of silicate grains within a reasonable time, as supposed previously. However, energetic ions can induce crystallization in amorphous solids, and this may be partially or fully responsible for the estimated 0.2% crystallinity of silicates in the interstellar medium.

Modification of dust-grain structure by sputtering

Monthly Notices of the Royal Astronomical Society, 2004

We have applied the SRIM computer code to study the sputtering of some likely astrophysical grain materials, and we have shown that selective embedding of metallic projectiles offers a partial explanation of gas-phase depletions. We show that supernova shockwaves sweep a significantly larger mass of interstellar gas per unit time than the shockwaves generated by outflows in star-forming regions. We apply our sputtering model to the bombardment levels expected in a supernova shock, and show that net embedding may dominate over net sputtering, leading to grain growth under some circumstances, particularly when the bombarding gas is enriched with metals from the supernova progenitor star. A combination of short cooling times and net embedding mean that it is possible for a type II supernova to generate more dust that it destroys, and we conclude that, in general, the sputtering process often leads to a compositional change in the grain material rather than simply to grain erosion.

Sputtering of grains in C-type shocks

Monthly Notices of the Royal Astronomical Society, 2000

Sputtering yields are reported for the release of Mg, Fe, Si and O under impact of He, C, O, Si and Fe on grain material composed of Mg-and Fe-bearing silicates. The yields were derived using the trim code, which simulates the results of the transport of ions in matter by means of classical Monte Carlo techniques. The energetics of the sputtering process are a key factor in the sputtering calculations, and so detailed determinations have been made of the energy with which atoms are bound to the lattice, using solid-state simulation programs. The probability of ejection of an atom is computed at a given energy, for a number of angles of incidence, and integrated to obtain the mean yield at that energy. These numerical results are then fitted with a simple function of energy for convenience in subsequent applications.

The shape and composition of interstellar silicate grains

Astronomy and Astrophysics, 2008

We investigate the composition and shape distribution of silicate dust grains in the interstellar medium. The effects of the amount of magnesium and iron in the silicate lattice are studied in detail. We fit the spectral shape of the interstellar 10 µm extinction feature as observed towards the galactic center using various particle shapes and dust materials. We use very irregularly shaped coated and non-coated porous Gaussian Random Field particles as well as a statistical approach to model shape effects. For the dust materials we use amorphous and crystalline silicates with various composition as well as silicon carbide (SiC). The results of our analysis of the 10 µm feature are used to compute the shape of the 20 µm silicate feature and to compare this with observations of this feature towards the galactic center. By using realistic particle shapes to fit the interstellar extinction spectrum we are, for the first time, able to derive the magnesium fraction in interstellar silicates. We find that the interstellar silicates are highly magnesium rich (Mg/(Fe + Mg) > 0.9) and that the stoichiometry lies between pyroxene and olivine type silicates (O/Si ≈ 3.5). This composition is not consistent with that of the glassy material found in GEMS in interplanetary dust particles indicating that the amorphous silicates found in the Solar system are, in general, not unprocessed remnants from the interstellar medium. Also, we find that a significant fraction of silicon carbide (∼3%) is present in the interstellar dust grains. We discuss the implications of our results for the formation and evolutionary history of cometary and circumstellar dust. We argue that the fact that crystalline silicates in cometary and circumstellar grains are almost purely magnesium silicates is a natural consequence of our findings that the amorphous silicates from which they were formed were already magnesium rich.

Comment on “The shape and composition of interstellar silicate grains”

Astronomy & Astrophysics, 2008

In the paper entitled "The shape and composition of interstellar silicate grains" (A&A, 462, 667-676 (2007)), Min et al. explore non-spherical grain shape and composition in modeling the interstellar 10 and 20 μm extinction features. This progression towards more physically realistic models is vitally important to enabling valid comparisons between dust observations and laboratory measurements. Min et al. proceed to compare their model results with GEMS (glass with embedded metal and sulfides) from interplanetary dust particles (IDPs) and to discuss the nature and origin of GEMS. Specifically, they evaluate the hypothesis of Bradley (1994) that GEMS are remnant interstellar (IS) amorphous silicates. From a comparison of the mineralogy, chemical compositions, and infrared (IR) spectral properties of GEMS with their modeling results, Min et al. conclude that "the composition of interstellar medium (ISM) silicates is not consistent with that of GEMS" and that "GEMS are, in general, not unprocessed leftovers from the diffuse ISM". These original conclusions were based, however, on erroneous GEMS data.

Grains in photo-ionized environments

2001

Ever since the pioneering study of , it has been widely recognized that grains play an important role in the heating and cooling of photo-ionized environments. This includes the diffuse ISM, as well as H II regions, planetary nebulae, and photo-dissociation regions. A detailed code is necessary to model grains in a photo-ionized medium since the interactions of grains with their environment include a host of microphysical processes, and their importance can only be judged by performing a complete simulation. In this paper we will use the spectral synthesis code Cloudy for this purpose. A comprehensive upgrade of the grain model has been recently incorporated in Cloudy, and certain aspects of this upgrade will be discussed. Special emphasis will be on the new grain charge model. We will consider in detail the physics of grains in both ionized and neutral environments, and will present a calculation of photo-electric heating and collisional cooling rates for a range of physical conditions and grain materials and for a range of grain sizes (including a realistic size distribution). We conclude with a brief discussion of the problems currently hampering progress in this field. The new grain model will be used to model the silicate emission in the Ney-Allen nebula, and will help us better understand the nature of the grains in that part of the Orion complex.