Sputtering of grains in C-type shocks

Abstract

A B S T R A C TSputtering 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.A grid of C-type shock models has been computed, using our new sputtering yields, for pre-shock densities in the range 10 4 < n H nH 1 2nH 2 < 10 6 cm 23 and shock speeds 20 < v s < 45 km s 21 X Sputtered fractions can be high, exceeding 50 per cent for shock speeds in excess of approximately 40 km s 21 . The column densities of Si and SiO were also computed, for comparison with observations. Key words: shock waves ± ISM: jets and outflows. I N T R O D U C T I O NIn an earlier work (Field et al. 1997; hereafter FMFF), sputtering yields were reported for amorphous carbon and amorphous SiO 2 targets, for sputtering of C, Si and O from these targets by impact of He, C, O, Si and Fe. The consequences for the chemistry in C-type shocks were very briefly explored with special reference to the production of gas-phase SiO. Our purpose in the present work is twofold. In the first place, we seek to make available values of sputtering yields for targets that mimic more closely the composition of interstellar grains. To this end, the sputtering of grains composed of Mg-and Fe-bearing silicates is studied here. The results are relevant to the oxygen-rich environments in which outflows, associated with star formation, are typically found. In the second place, the new sputtering data are used in a sophisticated C-type shock model to yield a grid of results for a range of shock parameters, notably the shock velocity, pre-shock gas density and magnetic field. These models determine, for example, the budget of refractory material in the gas phase, in particular of SiO. These results may be used in conjunction with observational data (e.g. Dutrey, Guilloteau & Bachiller 1997; Lefloch et al. 1998) as a guide to the shock conditions in any particular region.In Section 2 the technical means for calculating sputtering yields are described, with particular reference to the energetics of the processes involved in sputtering. Sputtering yields are presented in Section 3 for magnesium silicate (Mg 2 SiO 4 ), or forsterite', iron silicate (Fe 2 SiO 4 ), or`fayalite' and an`olivine', MgFeSiO 4 , for a variety of projectile species. In Section 4, the grid of shock models is presented and brief reference is made to...

A re-evaluation of dust processing in supernova shock waves

“…the energy needed to displace an atom within the lattice. May et al (2000) estimated these parameters for forsterite-type materials, (Fe, Mg) 2 SiO 4 . In particular, they used the heat of atomisation as an estimate of the surface binding energy and performed calculations within the framework of the ionic model in order to quantify the lattice binding energy.…”

“…However, while the surface and lattice binding energy are experimentally or theoretically rather well constrained, the value of the displacement energy is still uncertain. May et al (2000) took the displacement energy of Mg and O atoms in MgO compounds as a guide (E D = 52 and 54 eV for Mg and O respectively) and assumed E D = 50 eV for all the atoms in the silicate. However, the uncertainty on this parameter may be a factor of two and, for example, Wang et al (1999) assumed a displacement energy of E D ≈ 25 eV for similar forsterite-type materials.…”

“…Furthermore, since in an amorphous silicate, Fe and Mg atoms will tend to have no preferred sites, we assume the same values for the lattice binding energy for both atoms (E B ≈ 1.9 eV). We estimate the surface binding energy with the same approach as per May et al (2000) and we assume the same silicate density as in the Jones et al (2013) dust model. We report the assumed values in Table 8.…”

“…However, the SRIM results are sensitive to the assumed value of the displacement energy and a higher displacement energy (e.g. E D = 50 eV as estimated by May et al 2000) will lead to a higher threshold energy and a lower sputtering yield over a large range of energies (Leroux, priv. comm.).…”

Relating jet structure to photometric variability: the Herbig Ae star HD 163296

“…The gas-phase abundance of these species is strongly depleted in the ISM with respect to solar abundances because their atoms are locked onto dust grains (Savage & Sembach 1996). When dust grains evaporate in the launching region or are sputtered in shocks along the jet, these species are released into the gas-phase (Jones et al 1994;Jones 2000;May et al 2000;Draine 2004;Guillet et al 2009Guillet et al , 2011. Gas phase depletion of refractory elements has been observed by Nisini et al (2005) and Podio et al (2006Podio et al ( , 2009Podio et al ( , 2011 in HH jets from Class I sources at large distances from their driving source.…”