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.     

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.     

Deuterium Abundance in the Local ISM and Possible Spatial Variations

Excellent HST/GHRS spectra of interstellar hydrogen and deuterium Lyman- absorption toward nearby stars allow us to identify systematic errors that have plagued earlier work and to measure accurate values of the D/H ratio in local interstellar gas. Analysis of 12 sightlines through the Local Interstellar Cloud leads to a mean value of D/H = (1.50 ± 0.10) × 10-5 with all data points lying within ± 1 of the mean. Whether or not the D/H ratio has different values elsewhere in the Galaxy and beyond is a very important open question that will be one of the major objectives of the Far Ultraviolet Spectroscopic Explorer (FUSE) mission.     

The Local Interstellar Medium: Peculiar or Not?

The local Interstellar Medium (ISM) at the 500 pc scale is by many respects a typical place in our Galaxy made of hot and tenuous gas cavities blown by stellar winds and supernovae, that includes the 100 pc wide “Local Hot Bubble (LHB)”, dense and cold clouds forming the cavity “walls”, and finally diffuse and warm clouds embedded within the hot gas, such as the Local Interstellar Cloud (LIC) presently surrounding the Sun. A number of measurements however, including abundance data, have contradicted this “normality” of our interstellar environment. Some contradictions have been explained, some not. I review recent observations at different spatial scales and discuss those peculiarities. At all scales Johannes Geiss has played a major role. At the scale of the first hundred parsecs, there are at least three “anomalies”: (i) the peculiar Gould Belt (GB), (ii) the recently measured peculiar Deuterium abundance pattern, (iii) the low value of the local O, N and ³He gas phase abundances. I discuss here the possibility of a historical link between these three observations: the large scale phenomenon which has generated the Belt, a giant cloud impact or an explosive event could be the common origin. At the 50–100 parsec scale, some of the unexplained or contradictory measurements of the Local Bubble hot gas, including its EUV/soft X ray emissions, ion column-densities and gas pressure may at least partially be elucidated in the light of the newly discovered X-ray emission mechanism following charge transfer between solar wind high ions and solar system neutrals. The Local Bubble hot gas pressure and temperature may be lower than previously inferred. Finally, at the smaller scale of the local diffuse cloudlets (a few parsecs), the knowledge of their structures and physical states has constantly progressed by means of nearby star absorption spectroscopy. On the other hand, thanks to anomalous cosmic rays and pickup ions measurements, local abundances of ISM neutral species are now precisely derived and may be compared with the absorption data. Interestingly these comparisons are now accurate enough to reveal other (noninterstellar) sources of pickup ions. However the actual physical state of the ISM 10–20,000 A.U. ahead along the Sun trajectory, which will be the ambient interstellar medium in a few thousands years, remains unknown. Local Bubble hot gas or warm LIC-type gas? More EUV/UV spectroscopic data are needed to answer this question.     

Deuterium Observations in the Galaxy

An accurate measurement of the primordial value of D/H would provide one of the best tests of nucleosynthesis models for the early Universe and the baryon density. Such evaluations have been traditionally made using present estimations of the deuterium abundance in the interstellar medium, extrapolated backwards in time with the use of galactic evolution models. Direct estimations of the primordial deuterium abundance have been carried out only recently in QSOs absorbers at high redshift.We will summarize galactic observations of deuterium and suggest that, perhaps, a single D/H value for the interstellar medium is not representative. These evaluations mainly came from observations completed in the far UV with first the Copernicus satellite over the Lyman lines series followed then by H and D Lyman-alpha lines observations with both the IUE and the GHRS on the Hubble Space Telescope. We discuss different known systematics and show that the situation is not yet clear. It is not possible today to claim that we know "the" D/H value in the interstellar medium, if any.Overall and in the context of additional D observations made in the solar system, we conclude that the actual evolution of deuterium from Big-Bang nucleosynthesis to now is not yet understood. More observations, recently made with IMAPS (the Interstellar Medium Absorption Profile Spectrograph) and hopefully to be made with FUSE (the Far Ultraviolet Spectroscopic Explorer to be launched in the fall of 1998), at higher spectral resolution or in many different galactic sites are certainly needed to help us reach a better global view of the evolution of that key element, and thus better constrain any evaluation of its primordial abundance.     

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.     

GHRS observations of the LISM

The GHRS has obtained high-resolution spectra of interstellar gas toward 19 nearby stars. These excellent data show that the Sun is located inside the Local Interstellar Cloud (LIC) with other warm clouds nearby. I will summarize the physical properties of these clouds and the three-dimensional structure of this warm interstellar gas. There is now clear evidence that the Sun and other late-type stars are surrounded by hydrogen walls in the upwind direction. The D/H ratio probably has a constant value in the LIC, (1.6 ± 0.2) × 10^–5, consistent with the measured values for all LIC lines of sight.     

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.     

Isotopic Fractionation by Ion-Molecule Reactions

Isotopic fractionation in interstellar clouds can occur by ion-molecule reactions at low temperatures. The major effect is not kinetic but thermodynamic in origin in that it arises from the difference in rate coefficients between forward and backward directions in reactions which exchange isotopic atoms. In this article, we concentrate on deuterium fractionation in interstellar clouds; this effect enhances the relative abundances of deuterated isotopomers to their normal counterparts by up to four orders of magnitude as compared with the basic D/H elemental abundance ratio. We also discuss the fractionation of ¹⁵N and ¹³C. This revised version was published online in August 2006 with corrections to the Cover Date.     

The Galactic Environment of the Sun: Interstellar Material Inside and Outside of the Heliosphere

Interstellar material (ISMa) is observed both inside and outside of the heliosphere. Relating these diverse sets of ISMa data provides a richer understanding of both the interstellar medium and the heliosphere. The galactic environment of the Sun is dominated by warm, low-density, partially ionized interstellar material consisting of atoms and dust grains. The properties of the heliosphere are dependent on the pressure, composition, radiation field, ionization, and magnetic field of ambient ISMa. The very low-density interior of the Local Bubble, combined with an expanding superbubble shell associated with star formation in the Scorpius-Centaurus Association, dominate the properties of the local interstellar medium (LISM). Once the heliosphere boundaries and interaction mechanisms are understood, interstellar gas, dust, pickup ions, and anomalous cosmic rays inside of the heliosphere can be directly compared to ISMa outside of the heliosphere. Our understanding of ISMa at the Sun is further enriched when the circumheliospheric interstellar material is compared to observations of other nearby ISMa and the overall context of our galactic environment. The IBEX mission will map the interaction region between the heliosphere and ISMa, and improve the accuracy of comparisons between ISMa inside and outside the heliosphere.     

Interstellar Matter and the Boundary Conditions of the Heliosphere

The interstellar cloud surrounding the solar system regulates the galactic environment of the Sun, and determines the boundary conditions of the heliosphere. Both the Sun and interstellar clouds move through space, so these boundary conditions change with time. Data and theoretical models now support densities in the cloud surrounding the solar system of n(H0)=0.22±0.06 cm−3, and n(e−)∼0.1 cm−3, with larger values allowed for n(H0) by radiative transfer considerations. Ulysses and Extreme Ultraviolet Explorer satellite He0 data yield a cloud temperature of 6400 K. Nearby interstellar gas appears to be structured and inhomogeneous. The interstellar gas in the Local Fluff cloud complex exhibits elemental abundance patterns in which refractory elements are enhanced over the depleted abundances found in cold disk gas. Within a few parsecs of the Sun, inconclusive evidence for factors of 2–5 variation in Mg+ and Fe+ gas phase abundances is found, providing evidence for variable grain destruction. In principle, photoionization calculations for the surrounding cloud can be compared with elemental abundances found in the pickup ion and anomalous cosmic-ray populations to model cloud properties, including ionization, reference abundances, and radiation field. Observations of the hydrogen pile up at the nose of the heliosphere are consistent with a barely subsonic motion of the heliosphere with respect to the surrounding interstellar cloud. Uncertainties on the velocity vector of the cloud that surrounds the solar system indicate that it is uncertain as to whether the Sun and α Cen are or are not immersed in the same interstellar cloud. This revised version was published online in June 2006 with corrections to the Cover Date.     

Relations between ISM inside and outside the heliosphere

Thanks to remarkable new tools, such as the Goddard High Resolution Spectrograph (GHRS) on board the HST and the EUVE spectrometer on the interstellar side, and Ulysses particle detectors on the heliospheric side, it is possible now to begin to compare abundances and physical properties of the interstellar matter outside the heliosphere (from absorption features in the stellar spectra), and inside the heliosphere (from in situ or remote detection of the interstellar neutrals or their derivatives, the pick-up ions or the Anomalous Cosmic Rays detected by the two Voyager spacecraft).Ground-based and UV spectra of nearby stars show that the Sun is located between two volumes of gas of different heliocentric velocities V and temperatures T (see also Linsky et al, this issue). One of these clouds has the same velocity (V= 25.6 km s^–1 from = 255 and =8) and temperature (6700 K) as the heliospheric helium of interstellar origin probed by Ulysses, and is certainly surrounding our star (and then the Local Interstellar Cloud or LIC). This Identification allows comparisons between interstellar constituents on both sides of the heliospheric interface.Ly-alpha background data (absorption cell and recent HST-GHRS spectra) suggest that the heliospheric neutral H velocity is smaller by 5–6 km s^–1 than the local cloud velocity, and therefore that H is decelerated at its entrance into the heliosphere, in agreement with interaction models between the heliosphere and the ISM which include the coupling with the plasma. This is in favor of a non negligible electron density (at least 0.05 cm³). There are other indications of a rather large ionization of the ambient ISM, such as the ionization equilibrium of interstellar magnesium and of sodium. However the resulting range for the plasma density is still broad.The heliospheric neutral hydrogen number density (0.08–0.16 cm^–3) is now less precisely determined than the helium density (0.013–0.017 cm^–3, see Gloeckler, Witte et al, Mobius, this issue). The comparison between the neutral hydrogen to neutral helium ratios in the ISM (recent EUVE findings) and in the heliosphere, suggests that 15 to 70% of H does not enter the heliosphere. The comparison between the interstellar oxygen relative abundance (with respect to H and He) in the ISM and the heliospheric abundance deduced from pick-up ions is also in favor of some filtration, and thus of a non-negligible ionization.For a significant ISM plasma density, one expects a Hydrogen wall to be present as an intermediate state of the interstellar H around the interface between inside and outside. Since 1993, the two UVS instruments on board Voyager 1 and 2 indeed reveal clearly the existence of an additional Ly-alpha emission, probably due to a combination of light from the compressed H wall, and from a galactic source. On the other hand, the decelerated and heated neutral hydrogen of this H wall has recently been detected in absorption in the spectra of nearby stars (see Linsky, this issue).     

Cosmic-Ray Energy Spectra and Time Variations in the Local Interstellar Medium: Constraints and Uncertainties

The spectra of galactic cosmic rays that are observed inside the heliosphere result from the interaction of the spectra present in the local interstellar medium with the structured but turbulent magnetic field carried by the solar wind. Observational tests of solar modulation theory depend on comparisons between spectra inside and outside the heliosphere. Our knowledge of the local interstellar spectra are indirect, using extrapolations of interplanetary spectra measured at high energies where solar modulation effects are minimal and modeling of the physical processes that occur during particle acceleration and transport in the interstellar medium. The resulting estimates of the interstellar spectra can also be checked against observations of the effects that cosmic rays have on the chemistry of the interstellar medium and on the production of the diffuse galactic gamma-ray background. I review the present understanding of the local galactic cosmic-ray spectra, emphasizing the constraints set by observations and the uncertainties that remain.     

Isotopic Signatures of Presolar Materials in Interplanetary Dust

Interplanetary dust particles collected in the stratosphere frequently exhibit enrichments in deuterium (D) and ¹⁵N relative to terrestrial materials. These effects are most likely due to the preservation of presolar interstellar materials. While the elevated D/H ratios probably resulted from mass fractionation during chemical reactions at very low < 100 K temperatures, the origin of the N isotopic anomalies remains unresolved. The bulk of the N-bearing material may have obtained its isotopic signatures from low temperature chemistry, but a nucleosynthetic origin is also possible. This revised version was published online in August 2006 with corrections to the Cover Date.     

Filtration of Interstellar Atoms through the Heliospheric Interface

Interstellar atoms penetrate deep into the heliosphere after passing through the heliospheric interface—the region of the interaction of the solar wind with the interstellar medium. The heliospheric interface serves as a filter for the interstellar atoms of hydrogen and oxygen, and, to a lesser extent, nitrogen, due to their coupling with interstellar and heliospheric plasmas by charge exchange and electron impact ionization. The filtration has great importance for the determination of local interstellar abundances of these elements, which becomes now possible due to measurements of interstellar pickup by Ulysses and ACE, and anomalous cosmic rays by Voyagers, Ulysses, ACE, SAMPEX and Wind. The filtration of the different elements depends on the level of their coupling with the plasma in the interaction region. The recent studies of the filtration of the interstellar atoms in the heliospheric interface region is reviewed in this paper. The dependence of the filtration on the local interstellar proton and H atom number densities is discussed and the roles of the charge exchange and electron impact ionization on the filtration are evaluated. The influence of electron temperature in the inner heliosheath on the filtration process is discussed as well. Using the filtration coefficients obtained from the modeling and SWICS/Ulysses pickup ion measurements, the local interstellar abundances of the considered elements are determined.     

Deuterium in Molecules of the Interstellar Medium

Deuterated molecules are detected both in interstellar translucent clouds and in cold dark clouds, as well as in star forming regions. We review the recent observational studies of deuterated molecules ranging from the VUV to the millimeter wavelength range. We outline some sources of uncertainties on the deuterium fractionation and on the subsequent derivation of the elemental deuterium to hydrogen ratio. Steady state versus time dependent models are discussed and the role of initial conditions is emphasized. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

Cosmic-ray acceleration and transport,and diffuse galactic γ-ray emission

Cosmic-ray acceleration and transport is considered from the point of view of application to diffuse galactic -ray sources. As an introduction we review several source models, in particular supernovae exploding inside or near large interstellar clouds. The complex problem of cosmic ray transport in random electromagnetic fields is reduced to three cases which should be sufficient for practical purposes. As far as diffusive acceleration is concerned, apart from reviewing the basic physical principles, we point out the relation between shock acceleration and 2nd order Fermi acceleration, and the relative importance of the two processes around interstellar shock waves. For -ray source models the interaction of cosmic rays with dense clouds assumes great importance. Past discussions had been confined to static interactions of clouds with the ambient medium in the sense that no large scale mass motions in the ambient interstellar medium were considered. The well-known result then is that down to some tens of MeV or less, cosmic-ray nucleons should freely penetrate molecular clouds of typical masses and sizes. The self-exclusion of very low energy nucleons however may affect electron transport with consequences for the Bremsstrahlung -luminosity of such clouds.In this paper we consider also the dynamical interaction of dense clouds with a surrounding hot interstellar medium. Through cloud evaporation and accretion there exist mass flows in the cloud surroundings. We argue that in the case of (small) cloud evaporation the galactic cosmic rays will be essentially excluded from the clouds. The dynamic effects of cosmic rays on the flow should be minor in this case. For the opposite case of gas accretion onto (large) clouds, cosmic-ray effects on the flow will in general be large, limiting the cosmic-ray compression inside the cloud to dynamic pressure equilibrium. This should have a number of interesting and new consequences for -ray astronomy. A first, qualitative discussion is given in the last section.Proceedings of the XVIII General Assembly of the IAU: Galactic Astrophysics and Gamma-Ray Astronomy, held at Patras, Greece, 19 August 1982.     

Local clouds: Distribution, density and kinematics through ground-based and HST spectroscopy

The distribution, kinematics and physical properties of the interstellar matter surrounding the Sun can be inferred from ground-based and UV spectroscopic observations. On a 200 pc scale the local interstellar matter appears inhomogeneous and asymmetric. Although it generally flows towards the lower density region, it is composed of numerous small components a few parsecs in size with slightly different velocities. On a smaller scale the extent and the nature of the Local Cloud which flows over the Sun are discussed based on HST-GHRS observations of nearby stars.     

Stellar winds of massive stars in M31

We have obtained the first UV high resolution spectra of hot luminous stars in M31 with the FOS onHubble Space Telescope. The spectra, combined with optical spectroscopic and photometric observations, enable us to study their stellar winds and photospheric parameters. We derive mass-loss rates and velocity laws from the wind line profiles, with the SEI method, as well as information on abundances. The wind lines and photospheric spectra are compared with galactic stars of the same spectral type.The spectra analyzed so far indicate that the stars have mass-loss rates comparable or slightly lower than galactic stars of the same spectral type, but possibly different velocity laws in their winds. The spectra of two stars are discussed here.