Trace-element concentrations in single circumstellar silicon carbide grains from the Murchison meteorite

Abstract— Concentrations of the trace elements Mg, Al, Ca, Ti, V, Fe, Sr, Y, Zr, Ba and Ce were determined by ion microprobe mass spectrometry in 60 individual silicon carbide (SiC) grains (in addition, Nb and Nd were determined in 20 of them), from separate KJH (size range 3.4–5.9 μm) of the Murchison carbonaceous meteorite, whose C-, N- and Si-isotopic compositions have been measured before (Hoppe et al., 1994) and provide evidence that these grains are of stellar origin. The selected SiC grains represent all previously recognized subgroups: mainstream (20 < ¹²C/¹³C < 120; 200 < ¹⁴N/¹⁵N; Si isotopes on slope 1.34 line), grains A (¹²C/¹³C < 3.5), grains B (3.5 < ¹²C/¹³C < 10), grains X (¹⁵N excesses, large ²⁸Si excesses) and grains Y (150 < ¹²C/¹³C < 260; Si isotopes on slope 0.35 line). Data on these grains are compared with measurements on fine-grained SiC fractions. Trace-element patterns reflect both the condensation behavior of individual elements and the source composition of the stellar atmospheres. A detailed discussion of the condensation of trace elements in SiC from C-rich stellar atmospheres is given in a companion paper by Lodders and Fegley (1995). Elements such as Mg, Al, Ca, Fe and Sr are depleted because their compounds are more volatile than SiC. Elements whose compounds are believed to be more refractory than SiC can also be depleted due to condensation and removal prior to SiC condensation. Among the refractory elements, however, the heavy elements from Y to Ce (and Nd) are systematically enriched relative to Ti and V, indicating enrichments by up to a factor of 14 of the s-process elements relative to elements lighter than Fe. Such enrichments are expected if N-type carbon stars (thermally pulsing AGB stars) are the main source of circumstellar SiC grains. Large grains are less enriched than small grains, possibly because they are from different AGB stars. The trace-element patterns of subgroups such as groups A and B and grains X can at least qualitatively be understood if grains A and B come from J-type carbon stars (known to be lacking in s-process enhancements shown by N-type carbon stars) or carbon stars that had not experienced much dredge-up of He-shell material and if grains X come from supernovae. However, a remaining puzzle is how stars become carbon stars without much accompanying dredge-up of s-process elements.     

Isotopic properties of silicon carbide X grains from the Murchison meteorite in the size range 0.5‐1.5 μm

Abstract— We report isotopic abundances for C, N, Mg‐Al, Si, Ca‐Ti, and Fe in 99 presolar silicon carbide (SiC) grains of type X (84 grains from this work and 15 grains from previous studies) from the Murchison CM2 meteorite, ranging in size from 0.5 to 1.5 μm. Carbon was measured in 41 X grains, n in 37 grains, Mg‐Al in 18 grains, Si in 87 grains, Ca‐Ti in 25 grains, and Fe in 8 grains. These X grains have ¹²C/¹³C ratios between 18 and 6800, ¹⁴N/¹⁵n ratios from 13 to 200, δ²⁹Si/²⁸Si between ?750 and +60%₀, δ³⁰Si/²⁸Si from ?770 to ?10%₀, and ⁵⁴Fe/⁵⁶Fe ratios that are compatible with solar within the analytical uncertainties of several tens of percent. Many X grains carry large amounts of radiogenic ²⁶Mg (from the radioactive decay of ²⁶Al, half‐life ? 7 times 10⁵ years) and radiogenic ⁴⁴Ca (from the radioactive decay of ⁴⁴Ti, half‐life = 60 years). While all X grains but one have radiogenic ²⁶Mg, only ～20% of them have detectable amounts of radiogenic ⁴⁴Ca. Initial ²⁶Al/²⁷Al ratios of up to 0.36 and initial ⁴⁴Ti/⁴⁸Ti ratios of up to 0.56 can be inferred. The isotopic data are compared with those expected from the potential stellar sources of SiC dust. Carbon stars, Wolf‐Rayet stars, and novae are ruled out as stellar sources of the X grains. The isotopic compositions of C and Fe and abundances of extinct ⁴⁴Ti are well explained both by type Ia and type II supernova (SN) models. The same holds for ²⁶Al/²⁷Al ratios, except for the highest ²⁶Al/²⁷Al ratios of >0.2 in some X grains. Silicon agrees qualitatively with SN model predictions, but the observed ²⁹Si/³⁰Si ratios in the X grains are in most cases too high, pointing to deficiencies in the current understanding of the production of Si in SN environments. The measured ¹⁴n/¹⁵n ratios are lower than those expected from SN mixing models. This problem can be overcome in a 15 M_odot; type II SN if rotational mixing, preferential trapping of N, or both from ¹⁵n‐rich regions in the ejecta are considered. The isotopic characteristics of C, N, Si, and initial ²⁶Al/²⁷Al ratios in small X grains are remarkably similar to those of large X grains (2–10 μm). Titanium‐44 concentrations are generally much higher in smaller grains, indicative of the presence of Ti‐bearing subgrains that might have served as condensation nuclei for SiC. The fraction of X grains among presolar SiC is largely independent of grain size. This implies similar grain‐size distributions for SiC from carbon stars (mainstream grains) and supernovae (X grains), a surprising conclusion in view of the different conditions for dust formation in these two types of stellar sources.     

Spallation recoil and age of presolar grains in meteorites

Abstract— We have determined the recoil losses from silicon carbide (SiC) grain‐size fractions of spallation Ne produced by irradiation with 1.6 GeV protons. During the irradiation, the SiC grains were dispersed in paraffin wax in order to avoid reimplantation into neighboring grains. Analysis for spallogenic ²¹Ne of grain‐size separates in the size range 0.3 to 6 μm and comparison with the ²²Na activity of the SiC + paraffin mixture indicates an effective recoil range of 2–3 μm with no apparent effect from acid treatments, which are routinely used in the isolation of meteoritic SiC grains. Our results indicate that the majority of presolar SiC grains in primitive meteorites, which are micrometer‐sized, will have lost essentially all spallogenic Ne produced by cosmic‐ray interaction in the interstellar medium. This argues against the validity of previously published presolar ages of Murchison SiC (～10 to ～130 Ma, increasing with grain size; Lewis et al., 1994), where recoil losses had been based on calculated recoil energies. It is argued that the observed variations in meteoritic SiC grain‐size fractions of ²¹Ne/²²Ne ratios are more likely due to the effects of nucleosynthesis in the He‐burning shell of the parent AGB stars which imposes new boundary conditions on nuclear parameters and stellar models. It is suggested that spallation‐Xe produced on the abundant Ba and REE in presolar SiC, rather than spallogenic Ne, may be a promising approach to the presolar age problem. There is a hint in the currently available Xe data (Lewis et al., 1994) that the large (>1 μm) grains may be younger than the smaller (<1 μm) grains. The retention of spallogenic ²¹Ne produced by the bombardment of SiC grains of different grain sizes with 1.6 GeV protons, avoiding reimplantation into neighboring grains by dispersing the SiC grains in paraffin wax, has been derived from a comparison of mass spectrometrically determined ²¹Ne, retained in the grains, with the ²²Na activity of the grains‐plus‐paraffin mixture. Compared to estimates of retention used in previous attempts to determine presolar ages for SiC (Tang and Anders, 1988b; Lewis et al., 1990, 1994), the results indicate significantly lower values. They do, however, agree with retention as expected from previous measurements of recoil ranges in similar systems (Nyquist et al., 1973; Steinberg and Winsberg, 1974). The prime reason for the discrepancy must lie in the energy of the recoiling nuclei entering in the calculation of retention by Tang and Anders (1988b), which is based on considerations by Ray and Völk (1983). Based on the results, it appears questionable that spallation contributes significantly to the observed variations of ²¹Ne/²²Ne ratios among various SiC grain‐size separates (Lewis et al., 1994). We rather suggest that the variations, just as it has been observed for Kr and Ba already (Lewis et al., 1994; Prombo et al., 1993), have a nucleosynthetic origin. Confirmation needs input of improved nuclear data and stellar models into new network calculations of the nucleosynthesis in AGB stars of elements in the Ne region. Finally we argue that, to determine presolar system irradiation effects, spallation Xe is more favorable than is Ne, primarily because of smaller recoil losses for Xe. Although preliminary estimates hint at the possibility that the larger (>1 μm) grains are younger than the smaller (<1 μm) ones, the major uncertainty for a quantitative evaluation lies in the exact composition of the Xe‐N component thought to originate from the envelope of the SiC grains' parent stars.     

Accretion and electrostatic interaction of interstellar dust grains; Interstellar grit

This work is divided into 13 sections and 2 appendices, and aims to elucidate the accretion mechanism, which operates via image-theory forces, whenever two interstellar dust grains come close together. Section 1 is an introduction. Section 2 proposes that the distribution of interstellar grains be taken asn(r) r ^–4 to avoid distortion of the 3K microwave background by radiation from spinning grains. Section 3 examines each of three types of image force accretion processes, finding them to be dominant compared to radiation or gravitational forces by at least a factor of 10¹⁹. Section 4 states that only grains made of conducting material (e.g., graphite, ice, iron) are involved in image theory. Section 5 presents reasons for believing that two grains should coalesce on impact. Section 6 examines the motion of charged interstellar grains in Hi and Hii regions. Section 7 demonstrates, by way of four examples involving dust grains ofr=10^–7 cm up tor=10^–4 cm, that the image effects on conducting grains are not trivial, and that the dynamics involved is not to be compared at all with elementary Coulomb interaction of two changes. Section 8 concludes that accretion with not take place in Hi clouds if thermal (equipartition) velocities prevail among the dust particles. section 9 examines grain interactions in Hii regions: here, following an argument due to Spitzer, consideration is given to the case of a population of dust grains all streaming in the direction of the local magnetic field B at velocities of order 0.1 km s^–1. It is shown that accretion takes place effectively, leading to the formation of interstellar grit, meaning grains of mass 10^–8 to 10^–7 gm, radius 0.1 mm; and leaving also a population ofr10^–6 cm grains, which are observed in polarization and extinction measurements. The existence of the latter is now a deduction and not an ad hoc postulate, as previously, and implies a distribution of the general formn(r) r _mean ^–3 , in approximate agreement with that of Section 2. Section 10 considers the accretion mechanism as a cascade process. Section 11 shows that the existence of grains in space ofr 10^–6 cm rules out an origin in supernova or galactic explosions, and supports a passive origin, perhaps in red giants or Mira variables. Section 12 discusses the implications of the results found for polarization observations and cosmogony, the latter being given a new foundation in which planets of different composition form automatically from a solar nebula. Section 13 is a conclusion.     

Populations of nanodiamond grains in meteorites from the data on isotopic composition and content of nitrogen

The present study has shown that the dependence of the isotopic composition of nitrogen on the N/C ratio, revealed from the data for bulk samples of meteoritic nanodiamond, can be obtained within the framework of the following model of the composition of populations of nanodiamond grains: (a) initial nanodiamond, i.e., the nanodiamond in the protoplanetary cloud before the accretion of the meteorite parent bodies, was composed mainly of grains of two populations (denoted as C_N and C_F), the ratio of which changed in meteorites depending on the degree of hydrothermal metamorphism; (b) only the grains of one of these populations (C_N) contain volume-bound nitrogen with δ¹⁵N = ?350‰; (c) the grains of both populations contain surface-bound nitrogen (δ¹⁵N ≡ 0). The calculations revealed the following properties of population grains in this model. (1) The grains of the C_N and C_F populations are most likely the same in isotopic composition of carbon and heterogeneous in distribution of its isotopes: the central part of grains is enriched with the δ¹²C isotope relative to the remainder of the grain. While the value of δ¹³C is ?37.3 ± 1.1‰ for carbon in the central part, it is ?32.8 ± 1.5‰ for the whole volume of the grains. (2) The noble gases of the HL component, specifically Xe-HL, are anomalous in isotopic composition and are most likely contained in the third population of nanodiamond grains (denoted as C_HL), the mass fraction of which is negligible relative to that for other grain populations. Only the grains of the C_HL population have an undoubtedly presolar origin, while the grains of the other nanodiamond populations could have formed at the early stages of the evolution of the protoplanetary cloud material before the accretion of the meteoritic parent bodies.     

Isotopic Composition and Abundance of Interstellar Neutral Helium Based on Direct Measurements

One of the important problems in astrophysics is the determination of the abundances of the helium isotopes ³He and ⁴He in various regions of the universe, since those abundances can provide evidence of the intensities of various possible processes of the production and decay of light elements and can thereby reflect their history. In this paper we describe the procedure and results of the first determination by a direct method of the abundances of helium isotopes in the local interstellar medium surrounding the solar system. The experiment was carried out on the piloted MIR station by the prolonged exposure in open space of specimens of metallic foil with their subsequent return to earth and detailed laboratory mass-spectrometric analysis. As a result, we were able to obtain estimates of the ⁴He density (about 7.5·10^-3 cm^-3) and the ³He/⁴He isotopic ratio (about 1.7·10^-4) for the local interst ellar medium.     

Isotopic composition of carbonates in the SNC meteorites Allan Hills 84001 and Nakhla

Abstract— We have measured the ¹³C/¹²C and ¹⁴C/¹²C ratios in CO₂ released by acid etching of the carbonate-bearing SNC meteorites Allan Hills 84001 and Nakhla. Most of the C released is strongly enriched in ¹³C. In 10 out of 12 samples, 15‰ <δ¹³C < 55‰. Terrestrial values of carbonateδ¹³C from weathering products are generally between ?10 and +10‰. Two leachate samples especially rich in ¹³C, ALH 84001,27 and Nakhla 25, have elemental Si/Mg ratios much lower than those of the bulk meteorites and ¹⁴C activities that are much lower than the values expected for terrestrial carbonates. The former observation indicates that these leachates consist primarily of carbonates and, less likely, phosphates. The latter observation implies that heavy C was introduced not by terrestrial weathering but by extraterrestrial processes. For ALH 84001,121 (sample 27) and Nakhla (BM 1913,26) δ¹³C = +41‰ and +35‰, respectively. The measured ¹⁸O/¹⁶O ratios in the leaches are similar: δ¹⁸O ～ 15 ± 5‰, contrasting with 4.2‰ in the bulk silicates. We infer that the C in the carbonates retains an extraterrestrial isotopic signature, but probably not O, due to its ease of isotopic exchange (Cole and Ohmoto, 1986).     

Physical and mechanical properties of stony meteorites

The method for experimental research of physical and mechanical properties of stony meteorites is considered. Experimental data on the physical and mechanical properties of samples of three ordinary chondrites are reported. Ordinary chondrites are characterized by a well-defined three-dimensional (spatial) anisotropy of physical and mechanical properties, when a compression strength in one of the directions significantly exceeds that in the other two directions. A measured compression strength of ordinary chondrites is in the range from 105 to 203 MPa, while a tensile strength is in the range from 18 to 31 MPa. As follows from the available published data on the strength of carbonaceous chondrites, they are drastically different in properties from ordinary chondrites. The observed critical aerodynamic loads do not exceed a measured tensile strength value of ordinary chondrites, which is actually the upper limit restricting the maximum aerodynamic load for ordinary chondrites.     

The 2200 Å extinction feature and the shape-distribution of graphite grains

The observed profile of the 2200 Å interstellar extinction feature is consistent with extinction by polydispersions in shape of small graphite spheroids.The National Radio Astronomy Observatory is operated by Associated Universities, Inc., under contract with the National Science Foundation.     

Catastrophic fragmentation of asteroids: evidence from meteorites

Meteorites are impact-derived fragments from ≈ 85 parent bodies. For seven of these bodies, the meteorites record evidence suggesting that they may have been catastrophically fragmented. We identify three types of catastrophic events: (a) impact and reassembly events > 4.4 Gy ago, involving molten or very hot parent bodies(> 1200°C); this affected the parent bodies of the ureilites, Shallowater, and the mesosiderites. In each case, the fragments cooled rapidly (≈ 1–1000°C day⁻¹) and then reassembled, (b) Later impacts involving cold bodies which, in some cases, reassembled; this occurred on the H and L ordinary chondrite parent bodies. The L parent body probably suffered another catastrophic event about 500 My ago. (c) Recent impacts of cold, multi-kilometer-sized bodies that generated meter-sized meteoroids; this occurred on the parent bodies of the IIIAB irons (650 My ago), the IVA irons (400 My ago), and the H ordinary chondrite (7 My ago).     

Asteroids and meteorites: Origin of stony-iron meteorites at mantle-core boundaries

Stony-iron meteorites formed at the core/mantle interfaces of small asteroidal parents. The mesosiderites formed when the thick crust of a largely molten parent body (100–200 km in diameter) foundered and sank through the mantle to the core. Pallasites formed in smaller parent bodies (50–100 km) in which olivine crystals from the partially molten mantle sank to the core/mantle interface and rafted there. Subsequent collisions stripped away the rocky mantles of both kinds of parent bodies, exposing the stony-iron surfaces of their cores to direct impacts, which continue to knock off meteorite fragments.     

Mean free path limitation of conduction electrons and extinction efficiencies of graphite grains

An extended Drude theory taking account of the limited free path of conduction electrons in small particles is used to calculate size dependent complex refractive indices of graphite spheres from bulk refractive index data. The size dependent refractive indices are then used in conjunction with the Mie theory to compute mean extinction efficiencies of graphite spheres. Significant departures from earlier results are found.     

The thermodynamics of dust formation: Evidence from meteorites

A substantial fraction of interstellar dust probably formed in the nebulae around protostars, a setting similar to that envisioned for meteoritic material. From studies of the mineralogy and composition of meteorites it is possible to obtain quantitative information on the conditions that prevailed in the nebula. For example, pressures in the range 10^–3 to 10^–6 atm are indicated. At these pressures the kinetics of nucleation and grain growth are favorable.The fact that the gas associated with interstellar dust has solar H/S ratios indicates that FeS, which forms at 680 K, independent of pressure, is not present in the dust. Since iron only becomes oxidized at even lower temperatures, also via pressure-independent reactions, oxidized iron is not expected in the dust. If most interstellar dust forms in nebulae and is ejected back into space, a relatively high temperature is implied, 700K. Dust formation around stars with high C/O ratios is expected to produce minerals found in the highly reduced enstatite chondrites.High-temperature fractionation processes ( 1000 K) played an important role in the nebula. Much of the Al, Ca, Ti, etc., evidently condensed and accreted into cm-sized objects, some of which are found in carbonaceous chondrites. These objects are explicable in terms of formation from a cooling neutral gas with cosmic composition. Their most important distinguishing characteristics are low volatile and low Si contents, coupled with high refractory element contents constrains formation via isothermal compression to grain temperature 1000 K.Invited contribution to the proceedings of a workshop onThermodynamics and Kinetics of Dust Formation in the Space Medium held at the Lunar and Planetary Institute, Houston, 6–8 September, 1978.     

CO adsorption and the optical properties of interstellar grains

Laboratory data on the spectra of CO adsorbed on small MgO particles show that CO absorption leads to a weakening of the 220 nm band together with a shift of this band to shorter wavelengths. CO adsorption also results in the formation of a cyclic CO carbon ion that absorbs at 2.15 m^–1. It is shown that this band provides a close match to a major component of the very broad structure seen in interstellar extinction at the same energy. Effects of CO adsorption on the 220 nm band and VUV extinction are discussed in light of recent observational data on stars with peculiar extinction curves.     

Interstellar Scintillation Studies of Pulsars and Distribution of Scattering Plasma in the Local Interstellar Medium

Pulsar scintillation measurements from the Ooty Radio Telescope (ORT) are used to investigate the distribution of scattering in the Local Interstellar Medium (LISM; region of ≲ 1 kpc of the Sun), specifically the region in and around the Local Bubble. A 3-componentmodel, where the Solar neighborhood is surrounded by a shell of enhanced plasma turbulence, is proposed for the LISM. Further, the Ooty data, along with those from Parkes and other telescopes are used for investigating thedistribution of scattering towards the nearby Loop I Superbubble. This revised version was published online in July 2006 with corrections to the Cover Date.     

Genesis of the IIICD iron meteorites: Evidence from silicate-bearing inclusions

Abstract— Our studies of the silicate-bearing inclusions in the IIICD iron meteorites Maltahöhe, Carlton and Dayton suggest that their mineralogy and mineral compositions are related to the composition of the metal in the host meteorites. An inclusion in the low-Ni Maltahöhe is similar in mineralogy to those in IAB irons, which contain olivine, pyroxene, plagioclase, graphite and troilite. With increasing Ni concentration of the metal, silicate inclusions become poorer in graphite, richer in phosphates, and the phosphate and silicate assemblages become more complex. Dayton contains pyroxene, plagioclase, SiO₂, brianite, panethite and whitlockite, without graphite. In addition, mafic silicates become more FeO-rich with increasing Ni concentration of the hosts. In contrast, silicates in IAB irons show no such correlation with host Ni concentration, nor do they have the complex mineral assemblages of Dayton. These trends in inclusion composition and mineralogy in IIICD iron meteorites have been established by reactions between the S-rich metallic magma and the silicates, but the physical setting is uncertain. Of the two processes invoked by other authors to account for groups IAB and IIICD, fractional crystallization of S-rich cores and impact generation of melt pools, we prefer core crystallization. However, the absence of relationships between silicate inclusion mineralogy and metal compositions among IAB irons analogous to those that we have discovered in IIICD irons suggests that the IAB and IIICD cores/metallic magmas evolved in rather different ways. We suggest that the solidification of the IIICD core may have been very complex, involving fractional crystallization, nucleation effects and, possibly, liquid immiscibility.     

IRTF observations of S complex and other asteroids: Implications for surface compositions,the presence of clinopyroxenes,and their relationship to meteorites

Abstract– We have obtained near‐infrared spectra for near‐Earth asteroids (NEA) and Main Belt asteroids by using NASA’s Infrared Telescope Facility. Most of the S complex classes of the Tholen‐Bus‐DeMeo scheme and the S(I)–S(VII) classes are represented. To help interpret the results, we examined visible/near‐IR spectra for ordinary chondrites. The unequilibrated ordinary chondrites (UOC) spectra contain a 2.3 μm feature which is absent in the spectra of the equilibrated ordinary chondrites (EOC). On the basis of literature data and new spectra low‐Ca clinopyroxenes, we suggest that the 2.3 μm in UOC is due to the presence of low‐Ca clinopyroxene in the UOC which is absent in EOC. While this difference can be seen in the raw spectra, we confirmed this observation using a modified Gaussian model (MGM) for spectral analysis. Both the UOC and the EOC plot in the S(IV) field of the band area ratio plot for asteroids. We suggest that many or most S(IV) asteroids have material resembling UOC on their surfaces. An internally heated ordinary chondrite parent object would have EOC material at depth and UOC material on the surface. Cosmic ray exposure ages, and K‐Ar ages for L chondrites, indicate that most EOC came from relatively few objects; however, the age distributions for UOC are unlike those of EOC. We suggest that while EOC come from the interiors of a limited number of S(IV) asteroids, the UOC come from the surfaces of a large number of S(IV) asteroids.     

3-D Structure of the Galactic Interstellar Matter: A Contribution from GAIA

We investigate the possibilities for tracing interstellar extinction with the ESA's astrometric space mission GAIA. The analysis is based on detailed simulations of the GAIA photometry, which are used to derive the distribution of interstellar matter in a modelled Galaxy. We find that `small' diffuse clouds (diameter D = 4 pc, E <sub>B-V</sub> = 0.06) will be easily traced with GAIA up to the distances of ∼ 800 pc. `Large' diffuse interstellar clouds (D = 10 pc, E _B-V = 0.13) will be located up to the distances of ∼ 2.5 kpc. This holds for the reddening tracers of spectral types O – K2 brighter than V = 17. Inmost cases, due to their low spatial density, the early type stars (O– A2) cannot provide reliable information about the distribution of interstellar matter. None of the reddening tracers measured by GAIA will provide reliable identification of the individual interstellar clouds beyond the distances of ∼ 3 kpc. Therefore, we conclude that the information available from photometric observations will be not sufficient for the detailed reconstruction of the 3-D distribution of Galactic interstellar matter. It is therefore extremely important to define the new strategies which would allow to combine all the available information, including the earlier space- and/or ground-based investigations, together with the information which will be provided by GAIA itself (parallaxes, E _B-V etc.). This revised version was published online in July 2006 with corrections to the Cover Date.     

The size distribution of dust grains in single clouds – II. The analysis of extinction using inhomogeneous grains

The extinction curves of single clouds, seen towards the stars HD 147165, 179406 and 202904, have been modelled using various mixtures containing both the bare and inhomogeneous (composite and/or multilayer) grains. It has been shown that the models composed of the bare graphite and silicate grains together with the multilayer grains containing silicates, organic refractory and water ice, are more useful in explaining extinction under the reduced cosmic abundances. The models based on Mathis' composite grains or on Greenberg & Li's core-mantle grains can also provide quite good fits of the extinction and the measured scattering parameters, but still require an excessive amount of carbon which results in too large a C/O ratio. The inhomogeneous grains essentially contribute to the extinction in practically the whole wavelength range of our extinction curves. As a rule, such grains have quite wide size distributions, centred at around 100 nm, although graphite grains are mainly of small sizes.     

Ultraviolet scattering properties and the size distribution of interstellar grains

Recent work related to the determination of the size distribution of interstellar grains is reviewed briefly. It is shown that the scattering properties of interstellar grains determined from ultraviolet observations of reflection nebulae point toward the existence of a bimodal size distribution of grains, consisting of classical wavelength sized grains and a class of much more numerous smaller particles, which scatter far ultraviolet light isotropically with near unit albedo. The implications of this result are discussed briefly.Invited contribution to the Proceedings of a Workshop onThermodynamics and Kinetics of Dust Formation in the Space Medium held at the Lunar and Planetary Institute, Houston, 6–8 September, 1978.