Calculations concerning the HCO+/HOC+ abundance ratio in dense interstellar clouds

Calculations have been performed to determine the rate coefficients of several reactions involved in both the formation and depletion of interstellar HCO+ and HOC+. The abundance of HOC+ deduced from these calculations is consistent with the tentative identification of HOC+ in Sgr B2 by Woods et al. The large HCO+/HOC+ abundance ratio observed by Woods et al. is due at least in part to a more rapid formation rate for HCO+ and probably due as well as to a more rapid depletion rate for HOC+.     

A reanalysis of the HCO+/HOC+ abundance ratio in dense interstellar clouds

New theoretical and experimental results have prompted a reinvestigation of the HCO+/HOC+ abundance ratio in dense interstellar clouds. These results pertain principally but not exclusively to the reaction between HOC+ and H2, which was previously calculated by DeFrees, McLean, and Herbst to possess a large activation energy barrier. New calculations, reported here, indicate that this activation energy barrier is quite small and may well be zero. In addition, experimental results at higher energy and temperature indicate strongly that the reaction proceeds efficiently at interstellar temperatures. If HOC+ does indeed react efficiently with H2 in interstellar clouds, the calculated HCO+/HOC+ abundance ratio rises to substantially greater value under standard dense cloud conditions than in deduced via the tentative observation of HOC+ in Sgr B2.     

Diffuse interstellar band formation in dense clouds

Measurements of the strengths of the diffuse interstellar bands at 4430, 5780 and 5797 Å show that the bands tend to be week with respect to extinction in dense interstellar clouds. Data on 10 stars in the ? Ophiuchi cloud complex show further that the diffuse band-producing efficiency of the grains decreases systematically with increasing grain size. It is concluded that the diffuse bands are not formed in the mantles which accrete on the grains in interstellar clouds, but that they could be produced in the cores of grains or in some molecular species.     

The abundance of negative hydrogen in small hot interstellar clouds

The observed intensities of the diffuse interstellar absorption band at 4430 have minimum values, for given stellar distances, that are equivalent to the reddening caused by the small Ambarzumian-type clouds studied by B Strömgren. This indicates that there is no negative hydrogen present between these clouds, which are then identified with the hot H I phase of interstellar matter.The equilibrium density of H^– inside such clouds is calculated from mean densities of neutral hydrogen atoms and free electrons, derived from radio observations for the local region outside the large Orion-arm clouds. The filling factor of the small clouds is taken to be the same as that preserved in the structure of the Gum Nebula from before its ionization. An electron temperature of 3375 K corresponding to the local degree of ionization of the small-cloud hydrogen then leads to a mean density of negative hydrogen equal to 2×10¹³ cm^–2 kpc^–1, in agreement with the observed diffuseband intensities.     

Vibrationally excited molecular hydrogen in circumstellar clouds and the interstellar CH+ abundance

The production of CH⁺ in dense interstellar clouds under intense UV irradiation is discussed. A model applicable to the cloud towards the star 20 Tau is described.     

Far UV radiation transfer and H2CO lifetime in dense interstellar clouds

The UV radiation transfer within spherical interstellar dust clouds is analyzed using the method of successive scatterings. The results are used to determine the lifetime of interstellar H₂CO against photo-destruction. The effectiveness of this process is compared with those of chemical mechanisms.     

Masses of interstellar clouds

Equilibrium masses of slowly rotating interstellar gas clouds have been calculated using an equation of state for the interstellar gas developed by Penston and Brown. The rotation is found to increase the equilibrium masses, but still the cloud masses are not as large as indicated by other considerations.     

Time dependent chemical study of contracting interstellar clouds. III. the charge distribution in interstellar clouds

We have constructed a chemical reaction model in a contracting interstellar cloud including 104 species which are involved in a network of 557 reactions. The chemical kinetic equations were integrated as a function of time by using gear package. The evolution of the system was followed in the density range 10–10⁷ particles cm^-3.The calculated fractional abundances of the charged species are in good agreement with those given by other investigators. The charge density has been followed in diffuse, intermediate and dense regions. The most dominant ionic species are metallic ions, HCO⁺ and H ₃ ⁺ in the shielded regions and atomic ions H⁺, C⁺, Si⁺, He⁺, S⁺ and metal ions in the diffuse and intermediate regions. The abundances of negatively charged ions were found to be negligible. The results of the calculations on the different metallic ions are interpreted.     

Carbon chain molecules in interstellar clouds

A survey of the distribution of long carbon chain molecules in interstellar clouds shows that their abundance is correlated. The various formation schemes for these molecules are discussed. We conclude that the ion-molecule type formation mechanisms are more promising than their competitors. They have also the advantage of allowing predictions which can be tested by observations. Acetylene C₂H₂ and diacetylene HCCCCH, may be very abundant in interstellar clouds.Invited contribution to the Proceedings of a Workshop onThermodynamics and Kinetics of Dust Formation in the Space Medium held at the Lunar and Planetary Institute, Houston, 6–8 September, 1978.     

The turbulent destruction of interstellar clouds

The interaction of an astrophysical shock with a cloud typically occurs at high Reynolds number, and in such cases will be highly turbulent. However, the formation of fully developed turbulence is usually prevented by the artificial viscosity inherent in hydrodynamical simulations. Upstream structures mean that the flow behind the shock is also likely to be turbulent, as it sweeps over such inhomogeneities. We study the nature of adiabatic shock-cloud interactions using a subgrid compressible k–ε turbulence model.     

Composition of grain mantles in interstellar clouds

The question of grain mantle composition in dense clouds is examined in the light of new observational and theoretical results on atomic and molecular concentrations in the gas phase. It is shown that if grain temperatures are less than about 20K these mantles will be primarily solid CO. Methods of identifying CO coated grains are discussed.     

Gravitational instability of interstellar magnetic clouds

The gravitational instability of a nonrotating isothermal gaseous disk permeated by a uniform frozen-in magnetic field is investigated using a fourth-order perturbation technique. From the results it is found that the disk is stable whenn/B ₀ < (4/3³ G)^–1/2, wheren andB are the column density of the disk and unperturbed magnetic field, respectively, andG is the gravitational constant. The disk is gravitationally unstable only whenn/B ₀ > (4/3³ G)^–1/2.     

Helicoidal magneto-electron waves in interstellar molecular clouds

We investigate the evolution of the magnetic flux density in a magnetically supported molecular cloud driven by Hall and Ohmic components of the electric field generated by the flows of thermal electrons. Particular attention is given to the wave transport of the magnetic field in a cloud whose gas dynamics is dominated by electron flows; the mobility of neutrals and ions is regarded as heavily suppressed. It is shown that electromagnetic waves penetrating such a cloud can be converted into helicons – weakly damped, circularly polarized waves in which the densities of the magnetic flux and the electron current undergo coherent oscillations. These waves are interesting in their own right, because for electron magnetohydrodynamics the low-frequency helicoidal waves have the same physical significance as the transverse Alfvén waves do for a single-component magnetohydrodynamics. The latter, as is known, are considered to be responsible for the widths of molecular lines detected in dark, magnetically supported clouds. From our numerical estimates for the group velocity and the rate of dissipation of helicons it follows that a possible contribution of these waves to the broadening of molecular lines is consistent with the conditions typical of dark molecular clouds.     

On water ice formation in interstellar clouds

A model is proposed for the formation of water ice mantles on grains in interstellar clouds. This occurs by direct accretion of monomers from the gas, be they formed by gas or surface reactions. The formation of the first monolayer requires a minimum extinction of interstellar radiation, sufficient to lower the grain temperature to the point where thermal evaporation of monomers is just offset by monomer accretion from the gas. This threshold is mainly determined by the adsorption energy of water molecules on the grain material; for hydrocarbon material, chemical simulation places this energy between 0.5 and 2 kcal mol⁻¹, which sets the (true) visible extinction threshold at a few magnitudes. However, realistic distributions of matter in a cloud will usually add to this an unrelated amount of cloud core extinction, which can explain the large dispersion of observed (apparent) thresholds. Once the threshold is crossed, all available water molecules in the gas are quickly adsorbed, because the grain cools down and the adsorption energy on ice is higher than on bare grain. The relative thickness of the mantle, and, hence, the slope of  τ₃( A _v)  depend only on the available water vapour, which is a small fraction of the oxygen abundance. Chemical simulation was also used to determine the adsorption sites and energies of O and OH on hydrocarbons and study the dynamics of formation of water molecules by surface reactions with gaseous H atoms, as well as their chances to stick in situ.     

Silicon chemistry in dense clouds

The equilibrium chemistry of silicon in dense interstellar clouds is discussed in terms of both gas phase and grain surface reactions. Unless the metal depletion is very large, the gas phase scheme tends to over-produce SiO and/or SiS when compared to the observations of Sgr B2. The scheme also predicts SiC to be an abundant form of silicon. There is a great need for relevant laboratory data on the reactions used here—of the 35 rate coefficients adopted in the scheme, only three have been measured in the laboratory. Reactions between positively charged gas phase ions and small grains can lead to the formation of SiO and SiS. This type of reaction seems to offer a simple explanation for the observed differences between sulphur and silicon chemistry in dense clouds.     

On interstellar reddening law in the Taurus dark clouds

The interstellar reddening law in the Taurus dark clouds is investigated from visual, infrared, and ground based ultraviolet photometric observations of heavily reddened stars HD 29647, HDE 283701, and HDE 283812. The star HD 29647 located in the center of a very dense dark cloud Khavtassi 278 shows a reddening law which is anomalous in the wavelengths shorter than 400 nm. When combined with observations by Snow and Seab from the IUE it can be represented by a single straight line from 1 m to 250 nm. Other two stars situated in less dense parts of the Taurus dark clouds show normal reddening law.     

Possible mechanism of formation of interstellar clouds

A possible mechanism of formation of interstellar clouds in the Galaxy by aggregation from gas “cloudlets” ejected by red giants is considered. A numerical model of the motion of a “cloudlet” in the interstellar medium of the Galaxy is constructed. The resulting clouds are fractal with a fractal dimensionality that coincides with the observed one. The characteristic time of formation of a cloud agrees with observations. Translated from Astrofizika, Vol. 43, No. 2, pp. 229–237, April–June, 2000.