scholarly journals Molecule Formation on Grain Surfaces

1971 ◽  
Vol 2 ◽  
pp. 429-431
Author(s):  
E. E. Salpeter
Keyword(s):  
Interstellar Dust ◽  
Adsorption Site ◽  
Sticking Coefficient ◽  
Interstellar Gas ◽  
Molecule Formation ◽  
Recombination Efficiency ◽  
Two Factors ◽  
Time Required ◽  
Dust Grain ◽  
Grain Surface

I will discuss the formation of molecules AB from two atoms or radicals A and B only for cases where the reaction A + B → AB is exothermic. The efficiency of this reaction on the surface of an interstellar dust grain is the product of two factors: (i) the ‘sticking coefficient’ or probability that the first radical hitting the grain surface from the interstellar gas becomes thermalized and sticks to an adsorption site; (ii) the recombination efficiency or probability that the first adsorbed radical will remain adsorbed, rather than evaporating, during the time required for the second radical to hit the grain, be adsorbed and find its partner.

scholarly journals Formation of Molecular Hydrogen in Interstellar Space

1965 ◽  
Vol 7 ◽  
pp. 253-257
Author(s):  
H. F. P. Knaap ◽  
C. J. N. Van Den Meijdenberg ◽  
J. J. M. Beenakker ◽  
H. C. Van De Hulst
Keyword(s):  
Interstellar Dust ◽  
Interstellar Space ◽  
Interstellar Gas ◽  
Interstellar Clouds ◽  
Dark Clouds ◽  
Hydrogen Molecules ◽  
Large Factor ◽  
Dust Grain ◽  
Indirect Argument ◽  
Made In

Although Several Attempts at observing the interstellar hydrogen molecules in the ultraviolet or infrared are in preparation (ref. 1), these molecules are still undetected. They may form the most abundant unobserved constituent of the interstellar gas. The strongest indirect argument for the presence of these molecules lies in the fact that the density of atomic hydrogen observed by the 21-cm line goes down in some dark clouds, where the dust density and, presumably, the total gas density goes up by a large factor.Inasmuch as the density in the interstellar clouds is of the order of 10 atoms/cm3 and the temperature is only of the order of 100° K, any formation of molecules by atom-atom collisions is too slow to be of importance. The most eligible process for H2 formation is recombination on the surface of an interstellar dust grain. Rate estimates of this process have been made in various degrees of detail, as reported in references 2 to 4.

scholarly journals Interstellar Dust and the H2 Molecule

1965 ◽  
Vol 7 ◽  
pp. 259-264
Author(s):  
K. H. Schmidt
Keyword(s):  
Hydrogen Atom ◽  
Molecular Hydrogen ◽  
Interstellar Dust ◽  
Unit Volume ◽  
Formation Rate ◽  
Dust Particles ◽  
Interstellar Gas ◽  
The Past ◽  
Grain Surface ◽  
H2 Molecule

Nearly 20 Years Ago van de Hulst stated that the formation of molecular hydrogen occurs on the surfaces of the interstellar grains. (See ref. 1.) In the last years several authors discussed the problem of the interstellar abundance of the H2 molecule. (See refs. 2 to 9.) They all found that the percentage of the molecular hydrogen in the interstellar gas probably is much larger than had been thought in the past and that the essential mechanism of H2 formation is the formation on the particle surfaces. Therefore, the formation rate of interstellar H2 is a function of the area of the grain surface per unit volume, which is dependent on the average radius of the grains ā, on the number of dust particles per unit volume N(ā), and on the distribution function of the particle radii. The formation rate is determined by the density of the atomic hydrogen nH and the temperature of the interstellar gas Tgas. Finally, the formation rate of H2 depends on the probability π that an impinging hydrogen atom on a grain joins with another hydrogen atom to form a molecule.

scholarly journals Peeling the astronomical onion

2016 ◽  
Vol 18 (46) ◽  
pp. 31930-31935 ◽  
Author(s):  
Alexander Rosu-Finsen ◽  
Demian Marchione ◽  
Tara L. Salter ◽  
James W. Stubbing ◽  
Wendy A. Brown ◽  
...  
Keyword(s):  
Interstellar Dust ◽  
Water Mobility ◽  
Interstellar Gas ◽  
Dust Grain

This work presents a study of water mobility on interstellar dust grain analogues at temperatures as low as 18 K. The work indicates that water forms pure domains rather than covering the entire grain, thereby leaving bare dust grain surfaces available on which other molecules can adsorb as well as themselves providing surfaces for further adsorption from the interstellar gas.

scholarly journals Abundances of Interstellar Molecules and Their Formation on Grain Surfaces

1973 ◽  
Vol 52 ◽  
pp. 363-367
Author(s):  
E. E. Salpeter ◽  
W. D. Watson
Keyword(s):  
Carbon Monoxide ◽  
Molecular Species ◽  
Interstellar Dust ◽  
Dust Grains ◽  
Surface Phenomena ◽  
Interstellar Gas ◽  
Interstellar Molecules ◽  
Gas Density ◽  
Molecule Formation ◽  
Interstellar Dust Grains

Surface phenomena on interstellar dust grains, which are relevant for molecule formation, are summarized. For various molecular species in the interstellar gas, the dependence of abundance on gas density and the degree of shielding of starlight is predicted. These predictions seem to fit with recent observations on carbon-monoxide, but there seems to be a discrepancy for formaldehyde.

scholarly journals Chemical Kinetics Simulations of Ice Chemistry on Porous Versus Non-Porous Dust Grains

2021 ◽  
Vol 8 ◽  
Author(s):  
Drew A. Christianson ◽  
Robin T. Garrod
Keyword(s):  
Chemical Kinetics ◽  
Interstellar Dust ◽  
Three Dimensional ◽  
Binding Energies ◽  
Inert Gases ◽  
Chemical Effect ◽  
Dust Grains ◽  
Interstellar Clouds ◽  
Dust Grain ◽  
Grain Surface

The degree of porosity in interstellar dust-grain material is poorly defined, although recent work has suggested that the grains could be highly porous. Aside from influencing the optical properties of the dust, porosity has the potential to affect the chemistry occurring on dust-grain surfaces, via increased surface area, enhanced local binding energies, and the possibility of trapping of molecules within the pores as ice mantles build up on the grains. Through computational kinetics simulations, we investigate how interstellar grain-surface chemistry and ice composition are affected by the porosity of the underlying dust-grain material. Using a simple routine, idealized three-dimensional dust-grains are constructed, atom by atom, with varying degrees of porosity. Diffusive chemistry is then simulated on these surfaces using the off-lattice microscopic Monte Carlo chemical kinetics model, MIMICK, assuming physical conditions appropriate to dark interstellar clouds. On the porous grain surface, the build-up of ice mantles, mostly composed of water, leads to the covering over of the pores, leaving empty pockets. Once the pores are completely covered, the chemical and structural behavior is similar to non-porous grains of the same size. The most prominent chemical effect of the presence of grain porosity is the trapping of molecular hydrogen, formed on the grain surfaces, within the ices and voids inside the grain pores. Trapping of H2 in this way may indicate that other volatiles, such as inert gases not included in these models, could be trapped within dust-grain porous structures when ices begin to form.

scholarly journals Connecting the Interstellar Gas and Dust Properties in Distant Galaxies Using Quasar Absorption Systems

2015 ◽  
Vol 11 (S315) ◽  
Author(s):  
Monique C. Aller ◽  
Varsha P. Kulkarni ◽  
Donald G. York ◽  
Daniel E. Welty ◽  
Giovanni Vladilo ◽  
...  
Keyword(s):  
Star Formation ◽  
Galaxy Evolution ◽  
Absorption Feature ◽  
Dust Grains ◽  
Interstellar Gas ◽  
Chemical Enrichment ◽  
Normal Galaxies ◽  
History Of ◽  
Dust Grain ◽  
Grain Properties

AbstractGas and dust grains are fundamental components of the interstellar medium and significantly impact many of the physical processes driving galaxy evolution, such as star-formation, and the heating, cooling, and ionization of the interstellar material. Quasar absorption systems (QASs), which trace intervening galaxies along the sightlines to luminous quasars, provide a valuable tool to directly study the properties of the interstellar gas and dust in distant, normal galaxies. We have established the presence of silicate dust grains in at least some gas-rich QASs, and find that they exist at higher optical depths than expected for diffuse gas in the Milky Way. Differences in the absorption feature shapes additionally suggest variations in the silicate dust grain properties, such as in the level of grain crystallinity, from system-to-system. We present results from a study of the gas and dust properties of QASs with adequate archival IR data to probe the silicate dust grain properties. We discuss our measurements of the strengths of the 10 and 18 μm silicate dust absorption features in the QASs, and constraints on the grain properties (e.g., composition, shape, crystallinity) based on fitted silicate profile templates. We investigate correlations between silicate dust abundance, reddening, and gas metallicity, which will yield valuable insights into the history of star formation and chemical enrichment in galaxies.

scholarly journals Catalytic Role of Refractory Interstellar Grain Analogs on H2 Formation

2021 ◽  
Vol 8 ◽  
Author(s):  
Tushar Suhasaria ◽  
Vito Mennella
Keyword(s):  
Molecular Hydrogen ◽  
Laboratory Experiments ◽  
Dust Grains ◽  
Interstellar Gas ◽  
Complex Molecules ◽  
Catalytic Role ◽  
H2 Formation ◽  
Dust Grain

Refractory dust grains have an important role to play in the chemistry of star and planet-forming regions. Their surfaces interact with interstellar gas and act as a catalyst for the formation of simple and complex molecules in space. Several mechanisms have been invoked to explain how molecular hydrogen is formed in reactions on dust grain surfaces in different regions of space. In this article, we give an overview of our understanding of the laboratory experiments, conducted over the last 20 years, that deal with H2 formation on interstellar grain analogs in space simulated conditions.

scholarly journals Reproduction of the Interstellar Ice Band by Grain Mantle Analogs

1980 ◽  
Vol 87 ◽  
pp. 387-388
Author(s):  
W. Hagen ◽  
A.G.G.M. Tielens ◽  
J. M. Greenberg
Keyword(s):  
Low Temperature ◽  
Interstellar Dust ◽  
Near Infrared ◽  
Absorption Feature ◽  
Band Shape ◽  
Interstellar Ice ◽  
Infrared Spectrometer ◽  
Simple Molecules ◽  
Many Sources ◽  
Dust Grain

The near-infrared spectrum of many sources associated with molecular clouds shows a broad absorption feature at 3.08 μm (e.g. Merrill et al., 1976; Harris et al., 1978). This feature has usually been attributed to absorption by H2O ice frozen on grains, but it has been impossible to satisfactorily reproduce the observed band shape (Merrill et al., 1976; Mukai et al., 1978). We have been able to obtain a complete fit of this absorption feature in the laboratory using very low temperature mixtures of H2O with other polar molecules. The preparation of these interstellar dust grain-mantle analogs has been described elsewhere (Greenberg, 1979; Hagen et al., 1979). They are prepared by allowing a gas mixture of simple molecules (e.g. CO, H2O, NH3, CH4 etc.) to condense on a low temperature (10 K) substrate. This frozen mixture can be heated and recooled. The samples are analyzed with an infrared spectrometer.

scholarly journals Astrochemical relevance of VUV ionization of large PAH cations

2020 ◽  
Vol 641 ◽  
pp. A98 ◽  
Author(s):  
G. Wenzel ◽  
C. Joblin ◽  
A. Giuliani ◽  
S. Rodriguez Castillo ◽  
G. Mulas ◽  
...  
Keyword(s):  
Cross Sections ◽  
Density Functional ◽  
Interstellar Dust ◽  
Vacuum Ultraviolet ◽  
Branching Ratio ◽  
Molecular Size ◽  
Photoelectric Effect ◽  
Interstellar Gas ◽  
Ionization Cross Sections ◽  
The Impact

Context. As part of interstellar dust, polycyclic aromatic hydrocarbons (PAHs) are processed by an interaction with vacuum ultraviolet (VUV) photons emitted by hot stars. This interaction leads to the emission of not only the well-known aromatic infrared bands, but also electrons, which can significantly contribute to the heating of the interstellar gas. Aims. Our aim is to investigate the impact of molecular size on the photoionization properties of cationic PAHs. Methods. Trapped PAH cations of sizes between 30 and 48 carbon atoms were submitted to VUV photons in the range of 9–20 eV from the DESIRS beamline at the synchrotron SOLEIL. All resulting photoproducts including dications and fragment cations were mass-analyzed and recorded as a function of photon energy. Results. Photoionization is found to be predominant over dissociation at all energies, which differs from the conclusions of an earlier study on smaller PAHs. The photoionization branching ratio reaches 0.98 at 20 eV for the largest studied PAH. The photoionization threshold is observed to be between 9.1 and 10.2 eV, in agreement with the evolution of the ionization potential with size. Ionization cross sections were indirectly obtained and photoionization yields extracted from their ratio with theoretical photoabsorption cross sections, which were calculated using time-dependent density functional theory. An analytical function was derived to calculate this yield for a given molecular size. Conclusions. Large PAH cations could be efficiently ionized in H I regions and contribute to the heating of the gas by the photoelectric effect. Also, at the border of or in H II regions, PAHs could be exposed to photons of energy higher than 13.6 eV. Our work provides recipes to be used in astronomical models to quantify these points.

scholarly journals The role of wind driving in OB star bow nebulae

2020 ◽  
Vol 494 (2) ◽  
pp. 1838-1847
Author(s):  
Curtis Struck
Keyword(s):  
Interstellar Dust ◽  
Gas Flow ◽  
Ionization Front ◽  
Emission Region ◽  
Interstellar Gas ◽  
Weibel Instability ◽  
Peculiar Velocities ◽  
Interstellar Dust Grains ◽  
Energy And Momentum ◽  
Ir Emission

ABSTRACT Bow-shaped mid-infrared (mid-IR) emission regions have been discovered in satellite observations of numerous late-type O and early-type B stars with moderate velocities relative to the ambient interstellar medium. Previously, hydrodynamical bow shock models have been used to study this emission. It appears that such models are incomplete in that they neglect kinetic effects associated with long mean free paths of stellar wind particles, and the complexity of Weibel instability fronts. Wind ions are scattered in the Weibel instability and mix with the interstellar gas. However, they do not lose their momentum and most ultimately diffuse further into the ambient gas like cosmic rays, and share their energy and momentum. Lacking other coolants, the heated gas transfers energy into interstellar dust grains, which radiate it. This process, in addition to grain photoheating, provides the energy for the emission. A weak R-type ionization front, formed well outside the IR emission region, generally moderates the interstellar gas flow into the emission region. The theory suggests that the IR emission process is limited to cases of moderate stellar peculiar velocities, evidently in accord with the observations.

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