OB Associations, Wolf–Rayet Stars, and the Origin of Galactic Cosmic Rays

We have measured the isotopic abundances of neon and a number of other species in the galactic cosmic rays (GCRs) using the Cosmic Ray Isotope Spectrometer (CRIS) aboard the ACE spacecraft. Our data are compared to recent results from two-component (Wolf–Rayet material plus solar-like mixtures) Wolf–Rayet (WR) models. The three largest deviations of galactic cosmic ray isotope ratios from solar-system ratios predicted by these models, ¹²C/¹⁶O, ²²Ne/²⁰Ne, and ⁵⁸Fe/⁵⁶Fe, are very close to those observed. All of the isotopic ratios that we have measured are consistent with a GCR source consisting of ∼20% of WR material mixed with ∼80% material with solar-system composition. Since WR stars are evolutionary products of OB stars, and most OB stars exist in OB associations that form superbubbles, the good agreement of our data with WR models suggests that OB associations within superbubbles are the likely source of at least a substantial fraction of GCRs. In previous work it has been shown that the primary ⁵⁹Ni (which decays only by electron-capture) in GCRs has decayed, indicating a time interval between nucleosynthesis and acceleration of >10⁵ y. It has been suggested that in the OB association environment, ejecta from supernovae might be accelerated by the high velocity WR winds on a time scale that is short compared to the half-life of ⁵⁹Ni. Thus the ⁵⁹Ni might not have time to decay and this would cast doubt upon the OB association origin of cosmic rays. In this paper we suggest a scenario that should allow much of the ⁵⁹Ni to decay in the OB association environment and conclude that the hypothesis of the OB association origin of cosmic rays appears to be viable.     

Rocky Cometary Particulates: Their Elemental,Isotopic and Mineralogical Ingredients

The determination of the chemical composition of solid cometary dust particles was one of the prime objectives of the three missions to Comet Halley in 1986. The dust analysis was performed by time-of-flight mass-spectrometry. Within the experimental uncertainty the mean abundances of the rock-forming elements in cometary dust particles are comparable to their abundances in CI-chondrites and in the solar photosphere, i.e. they are cosmic. H, C, and N, on the other hand, in cometary dust are significantly more abundant than in CI-chondrites, approach solar abundances, are to some extent related to O, and reside in an omnipresent refractory organic component dubbed CHON. Element variations between individual dust grains are characterized by correlations of Mg, Si, and O, and to a lesser extent of Fe and S. From particle-to-particle variations of the rock forming elements information on the mineralogy of cometary dust can be obtained. Cluster analysis revealed certain groups that partly match the classifications of stratospheric interplanetary dust particles. About half of Halley's analyzed particles are characterized by anhydrous Fe-poor Mg-silicates, Fe-sulfides, and rarely Fe metal. The Fe-poor Mg-silicates link Halley's dust to that of Hale-Bopp as shown by recent IR observations. No significant deviation from normal of the isotopic composition of the elements is unequivocally present with the notable exception carbon: ¹²C-rich grains with ¹²C/¹³C-ratios up to ≈ 5,000 link cometary dust to presolar circumstellar grains identified in certain chondrites. This revised version was published online in June 2006 with corrections to the Cover Date.     

The Origin of Primary Cosmic Rays: Constraints from ACE Elemental and Isotopic Composition Observations

Cosmic-ray isotope observations from NASAs Advanced Composition Explorer (ACE) mission have been used to investigate the composition of cosmic-ray source material. Source abundances relative to ⁵⁶Fe are reported for eleven isotopes of Ca, Fe, Co, and Ni, including the very rare isotopes ⁴⁸Ca and ⁶⁴Ni. Although the source abundances range over a factor 10⁴, most of the ratios to ⁵⁶Fe are consistent with solar-system values to within 20%. However, there are some notable differences, the most significant being an excess of (70±30)% relative to the solar system for the cosmic-ray source ratio ⁵⁸Fe/⁵⁶Fe. The possible association of such an excess with a contribution to the cosmic-ray source from Wolf–Rayet star ejecta is discussed.     

Fe/O Ratios in Interplanetary Shock Accelerated Particles

It is widely accepted that diffusive shock acceleration is an important process in the heliosphere, in particular in producing the energetic particles associated with interplanetary shocks driven by coronal mass ejections. In its simplest formulation shock acceleration is expected to accelerate ions with higher mass to charge ratios less efficiently than those with lower mass to charge. Thus it is anticipated that the Fe/O ratio in shock-accelerated ion populations will decrease with increasing energy above some energy. We examine the circumstances of five interplanetary shocks that have been reported to have associated populations in which Fe/O increases with increasing energy. In each event, the situation is complex, with particle contributions from other sources in addition to the shock. Furthermore, we show that the Fe/O ratio in shock-accelerated ions can decrease even when the shock is traveling through an Fe-rich ambient ion population. Thus, although shock acceleration of an Fe-rich suprathermal population has been proposed to explain large Fe-rich solar particle events, we find no support for this proposal in these observations.     

The Solar Wind Charge-eXchange Contribution to the Local Soft X-ray Background

The major sources of the Soft X-ray Background (SXRB), besides distinct structures as supernovae and superbubbles (e.g. Loop I), are: (i) an absorbed extragalactic emission following a power law, (ii) an absorbed thermal component (～2×10⁶ K) from the galactic disk and halo, (iii) an unabsorbed thermal component, supposedly at 10⁶ K, attributed to the Local Bubble and (iv) the very recently identified unabsorbed Solar Wind Charge-eXchange (SWCX) emission from the heliosphere and the geocorona. We study the SWCX heliospheric component and its contribution to observed data. In a first part, we apply a SWCX heliospheric simulation to model the oxygen lines (3/4 keV) local intensities during shadowing observations of the MBM 12 molecular cloud and a dense filament in the south galactic hemisphere with Chandra, XMM-Newton, and Suzaku telescopes. In a second part, we present a preliminary comparison of SWCX model results with ROSAT and Wisconsin surveys data in the 1/4 keV band. We conclude that, in the 3/4 keV band, the total local intensity is entirely heliospheric, while in the 1/4 keV band, the heliospheric component seems to contribute significantly to the local SXRB intensity and has potentially a strong influence on the interpretation of the ROSAT and Wisconsin surveys data in terms of Local Bubble hot gas temperature.     

The Cool Interstellar Medium

Infrared spectroscopy and photometry with ISO covering most of the emission range of the interstellar medium has led to important progress in the understanding of the physics and chemistry of the gas, the nature and evolution of the dust grains and also the coupling between the gas and the grains. We review here the ISO results on the cool and low-excitation regions of the interstellar medium, where T _gas≲ 500 K, n _H∼ 100–10⁵ cm⁻³ and the electron density is a few 10⁻⁴. JEL codes: D24, L60, 047 Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, The Netherlands, and the United Kingdom), and with the participation of ISAS and NASA.     

Stable Isotope Ratios as a Biomarker on Mars

As both Earth and Mars have had similar environmental conditions at least for some extended time early in their history (Jakosky and Phillips in Nature 412:237–244, 2001), the intriguing question arises whether life originated and evolved on Mars as it did on Earth (McKay and Stoker in Rev. Geophys. 27:189–214, 1989). Conceivably, early autotrophic life on Mars, like early life on Earth, used irreversible enzymatically enhanced metabolic processes that would have fractionated stable isotopes of the elements C, N, S, and Fe. Several important assumptions are made when such isotope fractionations are used as a biomarker. The purpose of this article is two-fold: (1) to discuss these assumptions for the case of carbon and to summarize new insights in abiologic reactions, and (2) to discuss the use of other stable isotope systems as a potential biomarker. It is concluded that isotopic biomarker studies on Mars will encounter several important obstacles. In the case of carbon isotopes, the most important obstacle is the absence of a contemporary abiologic carbon reservoir (such as carbonate deposits on Earth) to act as isotopic standard. The presence of a contemporary abiologic sulfate reservoir (evaporite deposits) suggests that sulfur isotopes can be used as a potential biomarker for sulfate-reducing bacteria. The best approach for tracing ancient life on Mars will be to combine several biomarker approaches; to search for complexity, and to combine small-scale isotopic variations with chemical, mineralogical, and morphological observations. An example of such a study can be a layer-specific correlation between δ ¹³C and δ ³⁴S within an ancient Martian evaporite, which morphologically resembles the typical setting of a shallow marine microbial mat.     

The Chemical Composition of Interstellar Matter at the Solar Location

The Local Interstellar Cloud (LIC) surrounds the Solar System and sets the boundary conditions for the heliosphere. Using both in situ and absorption line data towards ε CMa we are able to constrain both the ionization and the gas phase abundances of the LIC gas at the Solar Location. We find that the abundances are consistent with all of the carbonaceous dust grains having been destroyed, and in fact with a supersolar abundance of C. The constituents of silicate grains, Si, Mg, and Fe, appear to be sub-solar, indicating that silicate dust is present in the LIC. N, O and S are close to the solar values.     

Origin and Evolution of the Light Nuclides

After a short historical (and highly subjective) introduction to the field, I discuss our current understanding of the origin and evolution of the light nuclides D, ³He, ⁴He, ⁶Li, ⁷Li, ⁹Be, ¹⁰B and ¹¹B. Despite considerable observational and theoretical progress, important uncertainties still persist for each and every one of those nuclides. The present-day abundance of D in the local interstellar medium is currently uncertain, making it difficult to infer the recent chemical evolution of the solar neighborhood. To account for the observed quasi-constancy of ³He abundance from the Big Bang to our days, the stellar production of that nuclide must be negligible; however, the scarce observations of its abundance in planetary nebulae seem to contradict this idea. The observed Be and B evolution as primaries suggests that the source composition of cosmic rays has remained ∼constant since the early days of the Galaxy, a suggestion with far reaching implications for the origin of cosmic rays; however, the main idea proposed to account for that constancy, namely that superbubbles are at the source of cosmic rays, encounters some serious difficulties. The best explanation for the mismatch between primordial Li and the observed “Spite-plateau” in halo stars appears to be depletion of Li in stellar envelopes, by some yet poorly understood mechanism. But this explanation impacts on the level of the recently discovered early “⁶Li plateau”, which (if confirmed), seriously challenges current ideas of cosmic ray nucleosynthesis.     

The Galileo Dust Detector

The Galileo Dust Detector is intended to provide direct observations of dust grains with masses between 10^-19 and 10^-9 kg in interplanetary space and in the Jovian system, to investigate their physical and dynamical properties as functions of the distances to the Sun, to Jupiter and to its satellites, to study its interaction with the Galilean satellites and the Jovian magnetosphere. Surface phenomena of the satellites (like albedo variations), which might be effects of meteoroid impacts will be compared with the dust environment. Electric charges of particulate matter in the magnetosphere and its consequences will be studied; e.g., the effects of the magnetic field on the trajectories of dust particles and fragmentation of particles due to electrostatic disruption. The investigation is performed with an instrument that measures the mass, speed, flight direction and electric charge of individual dust particles. It is a multicoincidence detector with a mass sensitivity 10⁶ times higher than that of previous in-situ experiments which measured dust in the outer solar system. The instrument weighs 4.2 kg, consumes 2.4 W, and has a normal data transmission rate of 24 bits s^-1 in nominal spacecraft tracking mode. On December 29, 1989 the instrument was switched-on. After the instrument had been configured to flight conditions cruise science data collection started immediately. In the period to May 18, 1990 at least 168 dust impacts have been recorded. For 81 of these dust grains masses and impact speeds have been determined. First flux values are given.     

Processes that Promote and Deplete the Exosphere of Mercury

It has been speculated that the composition of the exosphere is related to the composition of Mercury’s crustal materials. If this relationship is true, then inferences regarding the bulk chemistry of the planet might be made from a thorough exospheric study. The most vexing of all unsolved problems is the uncertainty in the source of each component. Historically, it has been believed that H and He come primarily from the solar wind (Goldstein, B.E., et al. in J. Geophys. Res. 86:5485–5499, 1981), Na and K come from volatilized materials partitioned between Mercury’s crust and meteoritic impactors (Hunten, D.M., et al. in Mercury, pp. 562–612, 1988; Morgan, T.H., et al. in Icarus 74:156–170, 1988; Killen, R.M., et al. in Icarus 171:1–19, 2004b). The processes that eject atoms and molecules into the exosphere of Mercury are generally considered to be thermal vaporization, photon-stimulated desorption (PSD), impact vaporization, and ion sputtering. Each of these processes has its own temporal and spatial dependence. The exosphere is strongly influenced by Mercury’s highly elliptical orbit and rapid orbital speed. As a consequence the surface undergoes large fluctuations in temperature and experiences differences of insolation with longitude. Because there is no inclination of the orbital axis, there are regions at extreme northern and southern latitudes that are never exposed to direct sunlight. These cold regions may serve as traps for exospheric constituents or for material that is brought in by exogenic sources such as comets, interplanetary dust, or solar wind, etc. The source rates are dependent not only on temperature and composition of the surface, but also on such factors as porosity, mineralogy, and space weathering. They are not independent of each other. For instance, ion impact may create crystal defects which enhance diffusion of atoms through the grain, and in turn enhance the efficiency of PSD. The impact flux and the size distribution of impactors affects regolith turnover rates (gardening) and the depth dependence of vaporization rates. Gardening serves both as a sink for material and as a source for fresh material. This is extremely important in bounding the rates of the other processes. Space weathering effects, such as the creation of needle-like structures in the regolith, will limit the ejection of atoms by such processes as PSD and ion-sputtering. Therefore, the use of laboratory rates in estimates of exospheric source rates can be helpful but also are often inaccurate if not modified appropriately. Porosity effects may reduce yields by a factor of three (Cassidy, T.A., and Johnson, R.E. in Icarus 176:499–507, 2005). The loss of all atomic species from Mercury’s exosphere other than H and He must be by non-thermal escape. The relative rates of photo-ionization, loss of photo-ions to the solar wind, entrainment of ions in the magnetosphere and direct impact of photo-ions to the surface are an area of active research. These source and loss processes will be discussed in this chapter.     

The Ice Survey Opportunity of ISO

The instruments on board the Infrared Space Observatory have for the first time allowed a complete low (PHOT, CVF) to medium resolution (SWS) spectroscopic harvest, from 2.5 to 45 μm, of interstellar dust. Amongst the detected solids present in starless molecular clouds surrounding recently born stellar and still embedded objects or products of the chemistry in some mass loss envelopes, the so-called “ice mantles” are of specific interest. They represent an interface between the very refractory carbonaceous and silicates materials that built the first grains with the rich chemistry taking place in the gas phase. Molecules condense, react on ices, are subjected to UV and cosmic ray irradiation at low temperatures, participating efficiently to the evolution toward more complex molecules, being in constant interaction in an ice layer. They also play an important role in the radiative transfer of molecular clouds and strongly affect the gas phase chemistry. ISO results shed light on many other species than H₂O ice. The detection of these van der Waal's solids is mainly performed in absorption. Each ice feature observed by ISO spectrometer is an important species, with abundance in the 10⁻⁴–10⁻⁷ range with respect to H₂. Such high abundances represent a substantial reservoir of matter that, once released later on, replenishes the gas phase and feeds the ladder of molecular complexity. Medium resolution spectroscopy also offers the opportunity to look at individual line profiles of the ice features, and therefore to progressively reveal the interactions taking place in the mantles. This article will give a view on selected results to avoid to overlap with the numerous reviews the reader is invited to consult (e.g. van Dishoeck, in press; Gibb et al., 2004.). Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, The Netherlands, and the United Kingdom), and with the participation of ISAS and NASA.     

The Cassini Cosmic Dust Analyzer

The Cassini-Huygens Cosmic Dust Analyzer (CDA) is intended to provide direct observations of dust grains with masses between 10⁻¹⁹ and 10⁻⁹ kg in interplanetary space and in the jovian and saturnian systems, to investigate their physical, chemical and dynamical properties as functions of the distances to the Sun, to Jupiter and to Saturn and its satellites and rings, to study their interaction with the saturnian rings, satellites and magnetosphere. Chemical composition of interplanetary meteoroids will be compared with asteroidal and cometary dust, as well as with Saturn dust, ejecta from rings and satellites. Ring and satellites phenomena which might be effects of meteoroid impacts will be compared with the interplanetary dust environment. Electrical charges of particulate matter in the magnetosphere and its consequences will be studied, e.g. the effects of the ambient plasma and the magnetic field on the trajectories of dust particles as well as fragmentation of particles due to electrostatic disruption.The investigation will be performed with an instrument that measures the mass, composition, electric charge, speed, and flight direction of individual dust particles. It is a highly reliable and versatile instrument with a mass sensitivity 10⁶ times higher than that of the Pioneer 10 and 11 dust detectors which measured dust in the saturnian system. The Cosmic Dust Analyzer has significant inheritance from former space instrumentation developed for the VEGA, Giotto, Galileo, and Ulysses missions. It will reliably measure impacts from as low as 1 impact per month up to 10⁴ impacts per second. The instrument weighs 17 kg and consumes 12 W, the integrated time-of-flight mass spectrometer has a mass resolution of up to 50. The nominal data transmission rate is 524 bits/s and varies between 50 and 4192 bps.This revised version was published online in July 2005 with a corrected cover date.     

Dust in Supernovae and Supernova Remnants II: Processing and Survival

Observations have recently shown that supernovae are efficient dust factories, as predicted for a long time by theoretical models. The rapid evolution of their stellar progenitors combined with their efficiency in precipitating refractory elements from the gas phase into dust grains make supernovae the major potential suppliers of dust in the early Universe, where more conventional sources like Asymptotic Giant Branch (AGB) stars did not have time to evolve. However, dust yields inferred from observations of young supernovae or derived from models do not reflect the net amount of supernova-condensed dust able to be expelled from the remnants and reach the interstellar medium. The cavity where the dust is formed and initially resides is crossed by the high velocity reverse shock which is generated by the pressure of the circumstellar material shocked by the expanding supernova blast wave. Depending on grain composition and initial size, processing by the reverse shock may lead to substantial dust erosion and even complete destruction. The goal of this review is to present the state of the art about processing and survival of dust inside supernova remnants, in terms of theoretical modelling and comparison to observations.     

Isotopic Composition of the Solar Wind Inferred from In-Situ Spacecraft Measurements

The Sun is the largest reservoir of matter in the solar system, which formed 4.6 Gyr ago from the protosolar nebula. Data from space missions and theoretical models indicate that the solar wind carries a nearly unfractionated sample of heavy isotopes at energies of about 1 keV/amu from the Sun into interplanetary space. In anticipation of results from the Genesis mission’s solar-wind implanted samples, we revisit solar wind isotopic abundance data from the high-resolution CELIAS/MTOF spectrometer on board SOHO. In particular, we evaluate the isotopic abundance ratios ¹⁵N/¹⁴N, ¹⁷O/¹⁶O, and ¹⁸O/¹⁶O in the solar wind, which are reference values for isotopic fractionation processes during the formation of terrestrial planets as well as for the Galactic chemical evolution. We also give isotopic abundance ratios for He, Ne, Ar, Mg, Si, Ca, and Fe measured in situ in the solar wind.     

Nonthermal Radiation Mechanisms

In this paper we review the possible radiation mechanisms for the observed non-thermal emission in clusters of galaxies, with a primary focus on the radio and hard X-ray emission. We show that the difficulty with the non-thermal, non-relativistic Bremsstrahlung model for the hard X-ray emission, first pointed out by Petrosian (Astrophys. J. 557, 560, 2001) using a cold target approximation, is somewhat alleviated when one treats the problem more exactly by including the fact that the background plasma particle energies are on average a factor of 10 below the energy of the non-thermal particles. This increases the lifetime of the non-thermal particles, and as a result decreases the extreme energy requirement, but at most by a factor of three. We then review the synchrotron and so-called inverse Compton emission by relativistic electrons, which when compared with observations can constrain the value of the magnetic field and energy of relativistic electrons. This model requires a low value of the magnetic field which is far from the equipartition value. We briefly review the possibilities of gamma-ray emission and prospects for GLAST observations. We also present a toy model of the non-thermal electron spectra that are produced by the acceleration mechanisms discussed in an accompanying paper Petrosian and Bykov (Space Sci. Rev., 2008, this issue, Chap. 11).     

New Experimental Data and What It Tells Us About the Sources and Acceleration of Cosmic Rays

This paper summarizes new data in several fields of astronomy that relate to the origin and acceleration of cosmic rays in our galaxy and similar nearby galaxies. Data from radio astronomy shows that supernova remnants, both in our galaxy and neighboring galaxies, appear to be the sources of most of the accelerated electrons observed in these galaxies. -ray measurements also reveal several strong sources associated with supernova remnants in our galaxy. These sources have -ray spectra that are suggestive of the acceleration of cosmic-ray nuclei. Cosmic-ray observations from the Voyager and Ulysses spacecraft suggest a source composition that is very similar to the solar composition but with distinctive differences in the ⁴He, ¹²C,¹⁴ N and ²²Ne abundances that are the imprint of giant W-R star nucleosynthesis. Injection effects which depend on the first ionization potential (FIP) of the elements involved are also observed, in a manner similar to the fractionization observed between the solar photosphere and corona and also analogous to the preferential acceleration observed for high FIP elements at the heliospheric solar wind termination shock. Most of the ⁵⁹Ni produced in the nucleosynthesis of Fe peak nuclei just prior to a SN explosion appears to have decayed to ⁵⁹Co before the cosmic rays have been accelerated, suggesting that the⁵⁹ Ni is accelerated at least 10⁵ yr after it is produced. The decay of certain K capture isotopes produced during cosmic-ray propagation has also been observed for the first time. These measurements suggest that re-acceleration after an initial principal acceleration cannot be large. The high energy spectral indices of cosmic-ray nuclei show a significant charge dependent trend with the index of hydrogen being -2.76 and that of Fe -2.61. The escape length dependence of cosmic rays from our galaxy can now be measured up to ~300 GeV nucl^-1 using the Fe sec/Fe ratio. This escape length is P ^-0.05 above 10 GeV nucl^-1 leading to a typical source spectral index of (2.70±0.10) -0.50 = -2.20 for nuclei. This is similar to the source index of -2.3 inferred for electrons within the errors of ±0.1 in the index for both components. Spacecraft measurements in the outer heliosphere suggest that the local cosmic-ray energy density is ~2eV cm^-3 – larger than previously assumed. Gamma-ray measurements of electron bremsstrahlung below 50 MeV from the Comptel experiment on CGRO show that fully 20–30% of this energy is in electrons, several times that previously assumed. New estimates of the amount of matter traversed by cosmic rays using measurements of the B/C ratio are also higher than earlier estimates – this value is now ~10 g cm^-2 at 1 GeV nucl^-1. Thus altogether cosmic rays are energetically a more important component of our galaxy than previously assumed. This has implications both for the types of sources that are capable of accelerating cosmic rays and also for the role that cosmic rays may play in ionizing the diffuse interstellar medium.     

Distributed Sources in Comets

The distribution of some molecules and radicals (H₂CO, CO, HNC, CN,?…) in the atmosphere of several comets cannot be explained only by a direct sublimation from the nucleus, or by gas phase processes in the coma. Such molecules are in part the result of a distributed source in the coma, which could be the photo and thermal degradation of dust. We present a review of the degradation processes and discuss possible interpretations of the observations in which the degradation of solid complex organic material in dust particles seems to play a major role. The knowledge of such gas production mechanisms provides important clues on the chemical nature of the refractory organic material contained in comet nuclei.     

Isotopic Abundances in Comets

Isotopic ratios in comets provide keys for the understanding of the origin of cometary material, and the physical and chemical conditions in the early Solar Nebula. We review here measurements acquired on the D/H, ¹²C/¹³C, ¹⁶O/¹⁸O, ¹⁴N/¹⁵N, ³²S/³⁴S ratios in dust and gases, and discuss their cosmogonic implications. The prospects for future measurements from cometary space missions and remote sensing observations at millimeter and submillimeter wavelengths are presented. This revised version was published online in August 2006 with corrections to the Cover Date.