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.     

Normal Nearby Galaxies

Following on from IRAS, ISO has provided a huge advancement in our knowledge of the phenomenology of the infrared (IR) emission of normal galaxies and the underlying physical processes. Highlights include the discovery of an extended cold dust emission component, present in all types of gas-rich galaxies and carrying the bulk of the dust luminosity; the definitive characterisation of the spectral energy distribution in the IR, revealing the channels through which stars power the IR light; the derivation of realistic geometries for stars and dust from ISO imaging; the discovery of cold dust associated with H I extending beyond the optical body of galaxies; the remarkable similarity of the near-IR (NIR)/mid-IR (MIR) SEDs for spiral galaxies, revealing the importance of the photo-dissociation regions in the energy budget for that wavelength range; the importance of the emission from the central regions in shaping up the intensity and the colour of the global MIR luminosity; the discovery of the “hot” NIR continuum emission component of interstellar dust; the predominance of the diffuse cold neutral medium as the origin for the main interstellar cooling line, [C II] 158 μm, in normal galaxies. 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 Local Bubble and Interstellar Material Near the Sun

The properties of interstellar matter at the Sun are regulated by our location with respect to a void in the local matter distribution, known as the Local Bubble. The Local Bubble (LB) is bounded by associations of massive stars and fossil supernovae that have disrupted dense interstellar matter (ISM), driving low density intermediate velocity ISM into the void. The Sun appears to be located in one of these flows of low density material. This nearby interstellar matter, dubbed the Local Fluff, has a bulk velocity of ∼19 km s⁻¹ in the local standard of rest. The flow is coming from the direction of the gas and dust ring formed where the Loop I supernova remnant merges into the LB. Optical polarization data suggest that the local interstellar magnetic field lines are draped over the heliosphere. A longstanding discrepancy between the high thermal pressure of plasma filling the LB and low thermal pressures in the embedded Local Fluff cloudlets is partially mitigated when the ram pressure component parallel to the cloudlet flow direction is included.     

Molecular Hydrogen

Observations of H₂ line emission in galactic and extragalactic environments obtained with the Infrared Space Observatory (ISO) are reviewed. The diagnostic capability of H₂ observations is illustrated. We discuss what one has learned about such diverse astrophysical sources as photon-dominated regions, shocks, young stellar objects, planetary nebulae and starburst galaxies from ISO observations of H₂ emission. In this context, we emphasise use of measured H₂ line intensities to infer important physical quantities such as the gas temperature, gas density and radiation field and we discuss the different possible excitation mechanisms of H₂. We also briefly consider future prospects for observation of H₂ from space and from the ground. 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 Solar System d/h Ratio: Observations and Theories

The measured D/H ratios in interstellar environments and in the solar system are reviewed. The two extreme D/H ratios in solar system water - (720±120)×10⁻⁶ in clay minerals and (88±11)×10⁻⁶ in chondrules, both from LL3 chondritic meteorites - are interpreted as the result of a progressive isotopic exchange in the solar nebula between deuterium-rich interstellar water and protosolar H₂. According to a turbulent model describing the evolution of the nebula (Drouart et al., 1999), water in the solar system cannot be a product of thermal (neutral) reactions occurring in the solar nebula. Taking 720×10⁻⁶ as a face value for the isotopic composition of the interstellar water that predates the formation of the solar nebula, numerical simulations show that the water D/H ratio decreases via an isotopic exchange with H₂. During the course of this process, a D/H gradient was established in the nebula. This gradient was smoothed with time and the isotopic homogenization of the solar nebula was completed in 10⁶ years, reaching a D/H ratio of 88×10⁻⁶. In this model, cometary water should have also suffered a partial isotopic re-equilibration with H₂. The isotopic heterogeneity observed in chondrites result from the turbulent mixing of grains, condensed at different epochs and locations in the solar nebula. Recent isotopic determinations of water ice in cold interstellar clouds are in agreement with these chondritic data and their interpretation (Texeira et al., 1999). This revised version was published online in June 2006 with corrections to the Cover Date.     

The Composition of Cosmic Rays and the Mixing of the Interstellar Medium

The differences between the composition of Galactic cosmic rays and that of the interstellar medium are manifold, and they contain a wealth of information about the varying processes that created them. These differences reveal much about the initial mixing of freshly synthesized matter, the chemistry and differentiation of the interstellar medium, and the mechanisms and environment of ion injection and acceleration. Here we briefly explore these processes and show how they combine to create the peculiar, but potentially universal, composition of the cosmic rays and how measurements of the composition can provide a unique measure of the mixing ratio of the fresh supernova ejecta and the old interstellar medium in this initial phase of interstellar mixing. In particular, we show that the major abundance differences between the cosmic rays and the average interstellar medium can all result from cosmic ray ion injection by sputtering and scattering from fast refractory oxide grains in a mix of fresh supernova ejecta and old interstellar material. Since the bulk of the Galactic supernovae occur in the cores of superbubbles, the bulk of the cosmic rays are accelerated there out of such a mix. We show that the major abundance differences all imply a mixing ratio of the total masses of fresh supernova ejecta and old interstellar material in such cores is roughly 1 to 4. That means that the metallicity of ∼3 times solar, since the ejecta has a metallicity of ∼8 times that of the present interstellar medium.     

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.     

Interstellar Dust in the Solar System

The Ulysses spacecraft has been orbiting the Sun on a highly inclined ellipse almost perpendicular to the ecliptic plane (inclination 79°, perihelion distance 1.3 AU, aphelion distance 5.4 AU) since it encountered Jupiter in 1992. The in situ dust detector on board continuously measured interstellar dust grains with masses up to 10⁻¹³ kg, penetrating deep into the solar system. The flow direction is close to the mean apex of the Sun’s motion through the solar system and the grains act as tracers of the physical conditions in the local interstellar cloud (LIC). While Ulysses monitored the interstellar dust stream at high ecliptic latitudes between 3 and 5 AU, interstellar impactors were also measured with the in situ dust detectors on board Cassini, Galileo and Helios, covering a heliocentric distance range between 0.3 and 3 AU in the ecliptic plane. The interstellar dust stream in the inner solar system is altered by the solar radiation pressure force, gravitational focussing and interaction of charged grains with the time varying interplanetary magnetic field. We review the results from in situ interstellar dust measurements in the solar system and present Ulysses’ latest interstellar dust data. These data indicate a 30° shift in the impact direction of interstellar grains w.r.t. the interstellar helium flow direction, the reason of which is presently unknown.     

Clusters of Galaxies: Setting the Stage

Clusters of galaxies are self-gravitating systems of mass ∼10¹⁴–10¹⁵ h ⁻¹ M_⊙ and size ∼1–3h ⁻¹ Mpc. Their mass budget consists of dark matter (∼80%, on average), hot diffuse intracluster plasma (≲20%) and a small fraction of stars, dust, and cold gas, mostly locked in galaxies. In most clusters, scaling relations between their properties, like mass, galaxy velocity dispersion, X-ray luminosity and temperature, testify that the cluster components are in approximate dynamical equilibrium within the cluster gravitational potential well. However, spatially inhomogeneous thermal and non-thermal emission of the intracluster medium (ICM), observed in some clusters in the X-ray and radio bands, and the kinematic and morphological segregation of galaxies are a signature of non-gravitational processes, ongoing cluster merging and interactions. Both the fraction of clusters with these features, and the correlation between the dynamical and morphological properties of irregular clusters and the surrounding large-scale structure increase with redshift. In the current bottom-up scenario for the formation of cosmic structure, where tiny fluctuations of the otherwise homogeneous primordial density field are amplified by gravity, clusters are the most massive nodes of the filamentary large-scale structure of the cosmic web and form by anisotropic and episodic accretion of mass, in agreement with most of the observational evidence. In this model of the universe dominated by cold dark matter, at the present time most baryons are expected to be in a diffuse component rather than in stars and galaxies; moreover, ∼50% of this diffuse component has temperature ∼0.01–1 keV and permeates the filamentary distribution of the dark matter. The temperature of this Warm-Hot Intergalactic Medium (WHIM) increases with the local density and its search in the outer regions of clusters and lower density regions has been the quest of much recent observational effort. Over the last thirty years, an impressive coherent picture of the formation and evolution of cosmic structures has emerged from the intense interplay between observations, theory and numerical experiments. Future efforts will continue to test whether this picture keeps being valid, needs corrections or suffers dramatic failures in its predictive power.     

An Overview of the Origin of Galactic Cosmic Rays as Inferred from Observations of Heavy Ion Composition and Spectra

The galactic cosmic rays arriving near Earth, which include both stable and long-lived nuclides from throughout the periodic table, consist of a mix of stellar nucleosynthesis products accelerated by shocks in the interstellar medium (ISM) and fragmentation products made by high-energy collisions during propagation through the ISM. Through the study of the composition and spectra of a variety of elements and isotopes in this diverse sample, models have been developed for the origin, acceleration, and transport of galactic cosmic rays. We present an overview of the current understanding of these topics emphasizing the insights that have been gained through investigations in the charge and energy ranges Z≲30 and E/M≲1 GeV/nuc, and particularly those using data obtained from the Cosmic Ray Isotope Spectrometer on NASA’s Advanced Composition Explorer mission.     

The Milky Way 3-Helium Abundance

We are making precise determinations of the abundance of the light isotope of helium, ³He. The ³He abundance in Milky Way sources impacts stellar evolution, chemical evolution, and cosmology. The abundance of ³He is derived from measurements of the hyperfine transition of ³He⁺ which has a rest wavelength of 3.46 cm (8.665 GHz). As with all the light elements, the present interstellar ³He abundance results from a combination of Big Bang Nucleosynthesis (BBNS) and stellar nucleosynthesis. We are measuring the ³He abundance in Milky Way H ii regions and planetary nebulae (PNe). The source sample is currently comprised of 60 H ii regions and 12 PNe. H ii regions are examples of zero-age objects that are young relative to the age of the Galaxy. Therefore their abundances chronicle the results of billions of years of Galactic chemical evolution. PNe probe material that has been ejected from low-mass (M≤ 2M _⊙) to intermediate-mass (M∼2–5M _⊙) stars to be further processed by future stellar generations. Because the Milky Way ISM is optically thin at centimeter wavelengths, our source sample probes a larger volume of the Galactic disk than does any other light element tracer of Galactic chemical evolution. The sources in our sample possess a wide range of physical properties (including object type, size, temperature, excitation, etc.). The ³He abundances we derive have led to what has been called “The ³He Problem”.     

Observations of the local interstellar medium with the Extreme Ultraviolet Explorer

Because of the strong absorption of extreme ultraviolet radiation by hydrogen and helium, almost every observation with the Extreme Ultraviolet Explorer (EUVE) satellite is affected by the diffuse clouds of neutral gas in the local interstellar medium (LISM). This paper reviews some of the highlights of the EUVE results on the distribution and physical state of the LISM and the implications of these results with respect to the interface of the LISM and the heliosphere. The distribution of sources found with the EUVE all-sky surveys shows an enhancement in absorption toward the galactic center. Individual spectra toward nearby continuum sources provide evidence of a greater ionization of helium than hydrogen in the Local Cloud with an mean ratio of H I/He I of 14.7. The spectral distribution of the EUV stellar radiation field has been measured, which provides a lower limit to local H II and He II densities, but this radiation field alone cannot explain the local helium ionization. A combination of EUVE measurements of H I, He I, and He II columns plus the measurement of the local He I density with interplanetary probes can place constraints on the local values of the H I density outside the heliosphere to lie between 0.15 and 0.34 cm^–3 while the H II density ranges between 0.0 and 0.14 cm^–3. The thermal pressure (P/k = nT) of the Local Cloud is derived to be between 1700 and 2300 cm^–3 K, a factor of 2 to 3 above previous estimates.     

In situ measurements of interstellar dust with the Ulysses and Galileo spaceprobes

Interstellar dust was first identified by the dust sensor onboard Ulysses after the Jupiter flyby in February 1992. These findings were confirmed by the Galileo experiment on its outbound orbit from Earth to Jupiter. Although modeling results show that interstellar dust is also present at the Earth orbit, a direct identification of interstellar grains from geometrical arguments is only possible outside of 2.5 AU. The flux of interstellar dust with masses greater than 6 · 10^–14 g is about 1 · 10^–4 m ^–2 s ^–1 at ecliptic latitudes and at heliocentric distances greater than 1AU. The mean mass of the interstellar particles is 3 · 10^–13 g. The flux arrives from a direction which is compatible with the influx direction of the interstellar neutral Helium of 252° longitude and 5.2° latitude but it may deviate from this direction by 15 – 20°.     

Alfvén waves in the context of solar-like star formation: accretion columns and disks

In this work we examine the damping of Alfvén waves as a source of plasma heating in disks and magnetic funnels of young solar like stars, the T Tauri stars. We apply four different damping mechanisms in this study: viscous-resistive, collisional, nonlinear and turbulent, exploring a wide range of wave frequencies, from 10⁻⁵Ω_i to 10⁻¹Ω_i (where Ω_i is the ion-cyclotron frequency). The results show that Alfvénic heating can increase the ionization rate of accretion disks and elevate the temperature of magnetic funnels of T Tauri stars opening possibilities to explain some observational features of these objects. This revised version was published online in August 2006 with corrections to the Cover Date.     

The local interstellar medium

The local interstellar medium can be probed in different ways: by analyzing low energy X-ray data in the range 0.1–0.4 keV, where the radiation is absorbed by the interstellar gas at column densities in excess of about 10²⁰ cm^-2 — and can therefore be regarded as local, by determining the absorption of stellar emission spectra from nearby stars along their lines of sight by intervening gas and by directin situ measurements of those components which penetrate the heliosphere sufficiently far, provided they can be distinguished from interplanetary material. The current status of these different investigations gives the following picture: the solar system is surrounded by a bubble of hot gas (density 0.005cm^-3, temperature 10⁶ K) out to several tens of parsecs. More locally it is embedded in a small warm cloud of density 0.07cm^-3, temperature 7000 K, column density 5 × 10¹⁷ cm^-2 — which gives a mass of about 0.1M . The transition to the heliosphere is governed by solar UV ionization, snowploughing of the interstellar gas by the outwardly expanding solar wind and the bow shock. The heliosphere is the region inside the solar wind terminal shock. Classically it would be regarded as not yet affected by (or aware of) the obstacle ahead. Practically, the existence of the interstellar medium makes itself felt even far inside the heliosphere by the penetration of neutral gas, dust, plasma waves, shock accelerated particles and cosmic rays. These are the local probes of the interstellar medium.     

The Effects of Transverse Magnetic Field and Density Variations on the Particle Energy Spectra in a Reconnecting 3D Current Sheet

Electron and proton acceleration by a super-Dreicer electric field is further investigated in a non-neutral reconnecting current sheet (RCS) with a variable plasma density. The tangential B _z and transverse magnetic field components B _x are assumed to vary with the distances x and z from the X nullpoint linearly and exponentially, respectively; the longitudinal component (a ‘guiding field’) is accepted constant. Particles are found to gain a bulk of their energy in a thin region close to the X nullpoint where the RCS density increases with z exponentially with the index λ and the tangential magnetic field B _x also increases with z exponentially with the index α. For the RCS with a constant density (λ = 0), the variations of the tangential magnetic field lead to particle power-law energy spectra with the spectral indices γ₁ being dependent on the exponent α as: for protons and for electrons in a strong guiding field (β > 10⁻²) and for electrons in a moderate or weak guiding field (β > 10⁻⁴). For the RCS with an exponential density increase in the vicinity of the X nullpoint (λ≥ 0) there is a further increase of the resulting spectral indices γ that depends on the density exponent index λ as for protons and for electrons in weaker guiding fields and as for electrons in stronger guiding fields. These dependencies can explain a wide variety (1.5–10) of particle spectral indices observed in solar flares by the variations of a magnetic field topology and physical conditions in a reconnecting region. This can be used as a diagnostic tool for the investigation of the RCS dynamics from the accelerated particle spectra found from hard X-ray and microwave emission.     

Numerical Simulations of the Warm-Hot Intergalactic Medium

In this paper we review the current predictions of numerical simulations for the origin and observability of the warm hot intergalactic medium (WHIM), the diffuse gas that contains up to 50 per cent of the baryons at z∼0. During structure formation, gravitational accretion shocks emerging from collapsing regions gradually heat the intergalactic medium (IGM) to temperatures in the range T∼10⁵–10⁷ K. The WHIM is predicted to radiate most of its energy in the ultraviolet (UV) and X-ray bands and to contribute a significant fraction of the soft X-ray background emission. While O vi and C iv absorption systems arising in the cooler fraction of the WHIM with T∼10⁵–10^5.5 K are seen in FUSE and Hubble Space Telescope observations, models agree that current X-ray telescopes such as Chandra and XMM-Newton do not have enough sensitivity to detect the hotter WHIM. However, future missions such as Constellation-X and XEUS might be able to detect both emission lines and absorption systems from highly ionised atoms such as O vii, O viii and Fe xvii.     

D/H and Nearby Interstellar Cloud Structures

Analysis of UV spectra obtained with the HST, FUSE and other satellites provides a new understanding of the deuterium abundance in the local region of the galactic disk. The wide range of gas-phase D/H measurements obtained outside of the Local Bubble can now be explained as due to different amounts of deuterium depletion on carbonaceous grains. The total D/H ratio including deuterium in the gas and dust phases is at least 23 parts per million of hydrogen, which is providing a challenge to models of galactic chemical evolution. Analysis of HST and ground-based spectra of many lines of sight to stars within the Local Bubble have identified interstellar velocity components that are consistent with more than 15 velocity vectors. We have identified the structures of 15 nearby warm interstellar clouds on the basis of these velocity vectors and common temperatures and depletions. We estimate the distances and masses of these clouds and compare their locations with cold interstellar clouds.     

Atomic Deuterium/Hydrogen in the Galaxy

An accurate value of the D/H ratio in the local interstellar medium (LISM) and a better understanding of the D/H variations with position in the Galactic disk and halo are vitally important questions as they provide information on the primordial D/H ratio in the Galaxy at the time of the protosolar nebula, and the amount of astration and mixing in the Galaxy over time. Recent measurements have been obtained with UV spectrographs on FUSE, HST, and IMAPS using hot white dwarfs, OB stars, and late-type stars as background light sources against which to measure absorption by D and H in the interstellar medium along the lines of sight. Recent analyses of FUSE observations of seven white dwarfs and subdwarfs provide a weighted mean value of D/H = (1.52±0.08) × 10⁻⁵ (15.2 ± 0.8 ppm), consistent with the value of (1.50 ± 0.10) × 10⁻⁵ (15.0 ± 1.0 ppm) obtained from analysis of lines of sight toward nearby late-type stars. Both numbers refer to the ISM within about 100 pc of the Sun, which samples warm clouds located within the Local Bubble. Outside of the Local Bubble at distances of 200 to 500 pc, analyses of far-UV spectra obtained with the IMAPS instrument indicate a much wider range of D/H ratios between 0.8 to 2.2 ppm. This portion of the Galactic disk provides information on inhomogeneous astration in the Galaxy. This revised version was published online in August 2006 with corrections to the Cover Date.     

Dust Properties in the Local Interstellar Medium

Ulysses measurements yield reliable in-situdetection of large dust particles which stem from the interstellar medium (ISM) and which are not observed in interstellar extinction data. Both current models of large grains in the ISM: core-mantle grains as well as composite grains, are in agreement with dust properties implied by the Ulysses results. However, the size of particles detected by Ulysses still exceeds the size of the large grains that are predicted for the ISM. This revised version was published online in August 2006 with corrections to the Cover Date.