O VI in the local interstellar medium

We report preliminary results of a search for O VI absorption in the spectra of ～100 hot DA white dwarfs observed by the FUSE satellite. We have carried out a detailed analysis of the radial velocities of interstellar and (where present) stellar absorption lines for the entire sample of stars. In many cases, the velocity differences between the interstellar and photospheric components are below the resolution of the FUSE spectrographs. However, in a significant number of cases the interstellar and photospheric contributions can be separated. In the majority of stars where we find O VI absorption lines, the material is clearly associated with the stellar photosphere and not the interstellar medium. There are a small number of lines-of-sight where the gas is interstellar in nature but the stars are located beyond the boundaries of the local cavity.     

A local interstellar spectrum for galactic electrons

A heliopause spectrum at 122 AU from the Sun is presented for galactic electrons over an energy range from 1 MeV to 50 GeV that can be considered the lowest possible local interstellar spectrum (LIS). The focus of this work is on the spectral shape of the LIS below ∼1.0 GeV. The study is done by using a comprehensive numerical model for solar modulation in comparison with Voyager 1 observations at ∼112 AU from the Sun and PAMELA data at Earth. Below ∼1.0 GeV, this LIS exhibits a power law with E^{−(1.55 ± 0.05)}, where E is the kinetic energy of these electrons. However, reproducing the PAMELA electron spectrum averaged for 2009, requires a LIS with a different power law of the form E^{−(3.15 ± 0.05)} above ∼5 GeV. Combining the two power laws with a smooth transition from low to high energies yields a LIS over the full energy range that is relevant and applicable to the modulation of cosmic ray electrons in the heliosphere. The break occurs between ∼800 MeV and ∼2 GeV as a characteristic feature of this LIS. The power-law form below ∼1 GeV produces a challenge to the origin of these low energy galactic electrons. On the other hand, the results of this study can be used as a gauge for astrophysical modeling of the local interstellar spectrum for electrons.     

On the state of ionization of the local interstellar medium

The abundance ratio of neutral hydrogen to helium, as deduced from interplanetary observations of Lyman-alpha and He 584 Å radiation by Mariner 10, is significantly lower than the cosmic abundance ratio of these elements, thus showing that the local interstellar medium (LISM) is partly ionized. The effect of various sources of ionization — especially electron impact and EUV photon flux — on hydrogen and helium is discussed. It is shown that the observational data on the temperature of the LISM, on the diffuse EUV flux and on the neutral and electron densities in the nearby interstellar medium (NISM) are not all compatible. However, if the diffuse EUV flux below 912 Å as deduced from the preliminary analysis of Voyager observations is not representative, then it is easy to reconcile all observations. In this case an important source of ionization of the LISM would be electron impact, yielding an ionization degree of about 50% for the hydrogen component.     

Production spectrum of gamma rays in interstellar space through neutral pion decay

We have obtained a simple representation to the observed invariant cross-section for the production of neutral pions in proton-proton collisions. Making use of this representation, we have calculated the differential and integral production spectra of gamma rays in the Galaxy from interactions of cosmic ray nuclei with interstellar gas. It is shown that the uncertainties in deducing interstellar proton spectrum by demodulating the observed spectrum do not affect very much the gamma ray spectrum. We have also determined the gamma ray production spectrum through bremsstrahlung process for a typical interstellar electron spectrum derived from the radio spectrum in the Galaxy. From these, the total gamma ray production spectrum resulting from the interaction of cosmic rays with interstellar matter is compared with the observed gamma ray spectrum in the Galaxy and some inferences have been obtained. We also point out the possible uncertainty in the present calculation and suggest the improvements needed.     

The local interstellar medium: A test-bed for the galactic ISM

We outline a method to explore the column density of the Local Interstellar Medium (LISM) using absorptions in the resonance H and K lines of Mgii. The intrinsic strengths of these lines in the temperature and density conditions prevailing in warm clouds (T _eff<10⁴ K) in the LISM allows them to be used to explore many lines of sight where lines such a NaD and Caii H and K are too weak, but where L is saturated. The number of measurable lines-of-sight is greatly enhanced by using cool stars as the background emitters, but this implies reliable separation of the LISM components from stellar chromospheric selfabsorptions. We explain how to do this, and how to use a combination of column density and radial velocity data to measure the spatial extent and the physical parameters of the single cloud in which the Sun is embedded. This proves to be an oblate spheroid, of characteristic diameter 8 pc, withT _eff 10⁴ K,n(Hi) of 0.1 cm^–3 and a mass <5M , streaming in the LSR from a point 1=4°,B=+16° with velocity equal to 16 km s^–1, and is surrounded by the much hotter lower density ionized gas of the local supernova bubble.     

Abundances in the interstellar medium

Summary Recent developments in the theory of element production and the chemical evolution of the galaxy are presented. Following this, observational data and their interpretation are given. A case by case analysis of results for D, He, Li and CNO isotope data in the disk and center of our galaxy is presented; previous results for element gradients are also summarized.The primordial abundances of D and He cannot be directly obtained from observations; corrections for stellar processing are discussed. From these data and the Li abundances, it appears that the abundance of the light elements is consistent with the standard big bang. In agreement with previous results, the range of, the baryon to photon ratio, is 5–8 10^–10. If the amount of non-baryonic matter is small, these results indicate an open universe, in the standard big bang model.New data show a gradient in the (¹²C/¹³C) and (¹⁶O/¹⁸O) ratios with galactocentric distance, D_GC. The presence of a gradient in the (¹⁴N/¹⁵N) ratio is less clear and there is no measurable gradient in the (³²S/³⁴S) ratio. In the interstellar medium near the sun, the carbon isotope ratio is –20 percent lower than the solar system ratio. This indicates that there has been only a moderate amount of enrichment of the nearby interstellar medium since the formation of the solar system. These results and previously determined galactic element gradients are interpreted in the framework of chemical evolution models. Delayed recycling of nucleosynthesis products is essential for the correct interpretation of the results. Comparisons of data with galactic evolution models are discussed.This article was processed by the author using the Springer-Verlag T_EX AAR macro package 1991     

Radio sources flickering and local interstellar medium (LISM) structure

A possibility of obtaining information on small-scale inhomogeneities of the electron component of the local interstellar medium (LISM) was investigated using interstellar scintillations of extragalactic radio sources. We analysed Culgoora array observational variability data of 190 extragalactic radio sources, covering most of the sky, at 80 and 160 MHz. The variability at time scales from 1 month to 15 years is interpreted as refractive interstellar scintillations in fast-moving nearby (less than 150 pc) hot gas, near shock waves in the LISM. An all-sky map of scintillation indices, m, averaged over three–five sources closest to one another, shows several m maxima. Two of the three most pronounced maxima are probably connected with Loop I; the third one coincides with the soft X-ray (0.1–0.3 keV) background maximum near the South Galactic Pole. Other, less certain, m maxima probably correspond to the Orion star-formation region and to a soft X-ray maximum near the North Galactic Pole. The ‘free-of-gas’ tunnel in the direction l=240° corresponds to low values of m. The estimated time scale of interstellar scintillations on the above-mentioned LISM structures is in agreement with that of the observed radio-source variations.     

Velocity of the local interstellar medium relative to the sun from interstellar helium flux measurements onboard the Ulysses and IBEX spacecraft

The question of determining the relative velocity of the local interstellar medium (LISM) based on direct interstellar helium flux measurements in the Solar system is considered. Such measurements were made onboard the Ulysses spacecraft in 1990–2007 at a distance of 2–5 AU from the Sun and have been made from 2009 to the present day onboard the Interstellar Boundary Explorer (IBEX) spacecraft at the Earth’s orbit. Recent works on analyzing the IBEX measurements have shown that the LISM velocity relative to the Sun determined from the IBEX data differs in magnitude (by ≈3 km s^?1) and direction (by ≈4°) from the LISM velocity obtained previously by Witte based on Ulysses measurements. We have modeled the Ulysses data (including the 2007 data that have not been considered previously by anybody) by taking into account various LISM velocity vectors and compare our numerical simulations with experimental data. The LISM velocity vector derived from the IBEX data is shown to contradict the Ulysses data in the position of the measured interstellar helium flux maximum on the sky map. In addition, the position of the flux maximum is shown to be determined exclusively by the LISM velocity vector and to be independent of other model parameters (the LISM temperature and ionization rate). This means that the Ulysses data (including the 2007 data obtained only two years before the IBEX measurements) cannot be explained in terms of the existing models with the LISM velocity vector from the IBEX data. Possible reasons for the detected contradictions are discussed.     

Hypochlorous acid in the interstellar medium

Special Astrophysical Observatory, Russian Academy of Sciences. Translated fromAstrofizika, Vol. 35, No. 1, pp. 151–161, July–August, 1991.     

D/O and D/H ratios in the local interstellar medium from FUSE observations

Since HI, OI, and DI have nearly the same ionization potential, the deuterium-to-oxygen ratio (D/O) is an important tracer of the D/H ratio and of its putative spatial variations. D/O is indeed very sensitive to astration, both because of deuterium destruction and oxygen production. Moreover, D/O measurements are less subject to systematic errors than D/H.A survey of D/O in the interstellar medium is performed by the Deuterium Working Group of the FUSE PI Team. The first results show that D/O is a better D/H proxy than D/N, and that D/O is homogeneous in the local interstellar medium, at least within about 100pc. The stability of D/O appears as a strong argument which supports both D/H and O/H spatial stability in the local interstellar medium.     

Carbon monoxide frosts in the interstellar medium

Summary The presence of solid carbon monoxide (CO) on interstellar grains was confirmed observationally in 1984 with the detection of infrared absorption at 4.67m wavelength in several molecular clouds. Subsequent observations suggest that solid CO is ubiquitous in the quiescent molecular cloud environment. In some lines of sight, the degree of frosting on to grains is sufficient to reduce appreciably the abundance of CO remaining in the gas, a result of considerable astrophysical significance: in addition to its importance as a tracer of molecular material, CO is vital to the production of many polyatomic molecules by gas phase reaction schemes, and its depletion could have a dramatic effect on the abundances of more complex carbon-bearing molecules. The infrared spectrum of solid CO provides an important diagnostic of the chemical composition and thermal evolution of grain mantles, leading to the prediction that CO₂ is also present in solid form.As it is now some six years since observations of interstellar solid CO were first reported, this is an appropriate time to review the topic and to suggest some directions for future research. The introduction (Sect. 1) attempts to place the subject in its broader astrophysical context. The infrared observations and their implications are discussed in detail in Sect. 2. The question of the degree of CO depletion implied by the observations of both solid state and gas phase CO is re-examined in Sect. 3. We assess the possibility of CO detection by means of solid state absorption or luminescence in the ultraviolet in Sect. 4. Future prospects are summarised in the final section.This article was processed by the author using the Springer-Verlag TEX Theaar macro package 1988.     

The electron temperature and density in interstellar medium by using carbon radio recombination line observations

The intensities of carbon radio recombination lines (RRL’s) are definedallowing for the effect of dielectronic-like recombination. The rate ofdielectronic-like recombination is calculated as functions of line number,electron density and temperature accurate to 0.05. Following from the balanceequation solutions for populations, the RRL intensities are analytically foundby the method of successive approximations to an accuracy of 0.15. Theobservations of carbon RRL’s are analyzed toward Cassiopeia A. The averageelectron temperature, density, expanded CII region lengths and inaccuraciesare found with the experimental values of RRL widths and intensities.     

Deuterium enrichment of the interstellar medium

Despite the low elemental abundance of atomic deuterium in the interstellar medium (ISM), observational evidence suggests that several species, both in the gas phase and in ices, could be heavily fractionated. We explore various aspects of deuterium enrichment by constructing a chemical evolution model in both gaseous and granular phases. Depending on various physical parameters, gases and grains are allowed to interact with each other through the exchange of their chemical species. It is known that HCO⁺ and N₂H⁺ are two abundant gas phase ions in the ISM and, their deuterium fractionation is generally used to predict the degree of ionization in the various regions of a molecular cloud. For a more accurate estimation, we consider the density profile of a collapsing cloud. The radial distributions of important interstellar molecules, along with their deuterated isotopomers, are presented. Quantum chemical simulations are computed to study the effects of isotopic substitution on the spectral properties of these interstellar species. We calculate the vibrational (harmonic) frequencies of the most important deuterated species (neutral and ions). The rotational and distortional constants of these molecules are also computed in order to predict the rotational transitions of these species. We compare vibrational (harmonic) and rotational transitions as computed by us with existing experimental and theoretical results. It is hope that our results will assist observers in detecting several hitherto unobserved deuterated species.     

The colours of the interstellar medium

On the basis ofI-I plots, we find that the ISM radiates preferentially at two pairs of far-infrared frequencies which correspond to (scattered) black-body temperatures of (23 ± 1, 187 ± 5) K and (39 ± 1, 104 ± 5) K. The first pair is emitted by the cold matrix, the second pair byHii regions and supernova shells.     

Self-consistent Modelling of the interstellar medium

The dynamical evolution of hot optically thin plasmas in the ISMcrucially depends on the heating and cooling processes. It isessential to realize that all physical processes that contributeoperate on different time scales. In particular detailedbalancing is often violated since the statistically inverseprocess of e.g. collisional ionization is recombination of an ionwith two electrons, which as a three-body collision is usuallydominated by radiative recombination, causing a departure fromcollisional ionization equilibrium. On top of these differences inatomic time scales, hot plasmas are often in a dynamical state,thereby introducing another time scale, which canbe the shortest one.The non-equilibrium effects will be illustrated and discussed inthe case of galactic outflows. It will be shown, that spectralanalyses of X-ray data of edge-on galaxies show a clear signaturein the form of ‘multi-temperature’ halos, which can mostnaturally be explained by the ‘freezing-in’ of highly ionizedspecies in the outflow, which contribute to the overall spectrumby delayed recombination. This naturally leads to anon-equilibrium cooling function, which modifies the dynamics,which in turn changes the plasma densities and thermal energybudget, thus feeding back on the ionization structure. Thereforeself-consistent modelling is needed.     

Extraterrestrial microspherules and iron needles in the interstellar medium

Microspherules in Paleozoic-Mesozoic bedded chert, presumed to be of extraterrestrial origin and comprised of clusters of iron needles,may represent an early stage in the aggregation of the inner planets. A fraction of the constituent iron needles will be expected to be carried along with expanding gas to the periphery of the solar nebula where the outer planets and comets condensed. A substantial fraction of iron needles would escape into interstellar space and add to the population of interstellar grains. Iron needles could contribute substantially to interstellar polarization and to the infrared opacity of the interstellar medium.     

Wave motions in magnetic interstellar medium

This communication considers the continuum approach modelling of large-scale dynamics of a nonconducting interstellar medium capable of sustaining long-ranged filamentary agglomeration of tiny superparamagnetic grains suspended in a dense molecular cloud. The filamentary ordering of permanently magnetized grains, oriented in the direction of the regular galactic field threading the cloud, is thought of as an effect of soft magnetic solidification of a nonconducting gas-dust substance imparting to the interstellar material the mechanical features of single-axis magnetoelastic insulators. With this physical picture in mind, we set up macroscopic equations to study the dissipative-free motions of superparamagnetic gas-dust nonionized matter in terms of continuum mechanics magnetoelastic materials. Particular attention is given to oscillatory behavior in the regime of strong magnetization-flow coupling. The most remarkable inference of this model is that nonconducting magnetically polarized interstellar medium can transmit perturbations by transverse waves of magnetization which can be regarded as a counterpart of Alfvén waves generic to cosmic dusty plasma. Published in Astrofizika, Vol. 43. No. 3, pp. 405-410, July–September, 2000.