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     

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.     

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.     

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.     

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.     

Spatial pattern formation of interstellar medium

Population dynamics of multi-phased interstellar medium (ISM) is investigated by using the lattice model in position-fixed reaction. Interactions between three distinct phases of gas, cold clouds, warm gas, and hot gas give rise to cyclic phase changes in ISM. Such local phase changes are propagated in space, and stochastic steady-state spatial pattern is finally achieved. We obtain the following two characteristic patterns:

1. When the sweeping rate of a warm gas into a cold component is relatively high, cold clouds associated with warm gas form small-scale clumps and are dispersively distributed, whereas hot gas covers large fraction of space.
2. When the sweeping rate is relatively low, in contrast, warm gas and cold clouds are diffusively and equally distributed, while hot gas component is substantially localized.

     

Preface: Stars and interstellar medium

This issue presents proceedings of the "Stars and Interstellar Medium" section of the AllRussian Astronomical Conference VAK-2017. Sixteen papers(selected from about 70 talks) cover different problems related to stars, pulsars, interstellar gas and dust, and star formation. The preface briefly reviews these papers.     

Temperature of interstellar warm ionized medium

This investigation on the temperature of the interstellar warm ionized medium (WIM) is characterized by the number and energy balance of the constituents of the WIM complex plasma viz. H plasma (electrons/ions/neutral atoms) and graphite dust, having a size distribution, characterized by the MRN (Mathis, Rumpl and Nordsieck) power law. Ionization of neutral atoms, electron–ion recombination, photoemission of electrons from and accretion on the dust and cooling through electron collisional excitation, followed by radiative decay of atoms has been included in the analysis. An appropriate expression for the rate of emission and mean energy of photoelectrons emitted from the surface of positively charged dust particles has been used which takes into account the dependence of absorption efficiency on wavelength of the radiation, radius of the particle and spectral irradiance distribution. The results of the parametric analysis have been displayed graphically. It is seen that the consensus values of temperature, surface potential on the dust particles and electron/ion/neutral atom densities, characteristic of interstellar warm ionized medium can be explained on the basis of plausible combinations of the dust particle density n _d and the parameter f _ex α _ex , where f _ex is the fraction of the energy of the neutral gas atoms which gets irradiated, α _ex n _e n _n is the number of the neutral atoms, which get excited per unit volume per unit time and n _e (n _n ) correspond to the density of electrons (neutral atoms).     

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.     

Bremsstrahlung gamma radiation and interstellar electron spectrum in the local interstellar medium

We present a detailed study of the bremsstrahlung gamma-ray emissivity of the galactic disk. We show that there are large uncertainties in the production spectrum of photons in the medium energy range (10–100 MeV) due to our lack of knowledge of the interstellar electron spectrum below a few hundred MeV. In fact, gamma-ray observations can be of great help in determining this spectrum. At present, the spectral shape of the local gamma-ray emissivity above 30 MeV is available, thanks to the SAS-II and the COS-B satellites. Comparing it to our calculations, we determine the local interstellar electron flux in the 50–500 MeV range; the corresponding integrated gamma-ray emissivity above 100 MeV is equal to 2.4×10^–25 photons s^–1 (H-atom)^–1, 60% higher than previously accepted values.     

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.     

Electromagnetic instability of a homogeneous interstellar medium

The electromagnetic instability of an interstellar medium with an arbitrary velocity distribution is examined over the large scale lengths typical of gas-dust clouds without a significant magnetic field. It is shown that over a moderate time scale (months and years) these instabilities can develop and that the requirement of stability is satisfied by a narrow class of distributions that are close to spherical. __________ Translated from Astrofizika, Vol. 49, No. 2, pp. 289–297 (May 2006).     

Star formation in a multi-phase interstellar medium

This contribution reports on our first efforts to simulate a multiphase interstellar medium on a kiloparsec scale in three dimensions with the stars and gas modeled self-consistently. Starting from inhomogenous initial conditions, our closed box simulations follow the gas as it cools and collapses under its own self-gravity to form stars which eventually return material and energy back through supernovae explosions and winds. This revised version was published online in August 2006 with corrections to the Cover Date.     

The global structure of the interstellar medium

In the context of a review of work on the global structure of the interstellar medium, supernova remnant evolution, flows in multiphase media, cosmic ray moderation of flows, theories of the Galactic halo gas, and the nature of the local superbubble are considered. Speculations about the nature of a one parameter fully self-consistent model of the interstellar medium-supernova-radiation and cosmic ray background system are offered.     

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.