Wesley T. Huntress

The lower ionosphere of Titan

Abstract Ionization of the atmosphere of Titan by galactic cosmic rays is a very significant process throughout the altitude range of 100 to 400 km. An approximate form of the Boltzmann equation for cosmic ray transport has been used to obtain local ionization rates. Models of both ion and neutral chemistry have been employed to compute electron and ion density profiles for three different values of the H 2 /CH 4 abundance ratio. The peak electron density is of the order 10 3 cm −3 . The most abundant positive ions are C 2 H 9 + and C 3 H 9 + , while the predicted densities of the negative ions H − and CH 3 − are very small ( −4 that of the positive ions). It is suggested that inclusion of the ion chemistry is important in the computation of the H and CH 3 density profiles in the lower ionosphere.

Chemistry of chlorine in dense interstellar clouds

Laboratory experiments and theoretical modeling show that the chemistry of chlorine is fairly simple in dense interstellar clouds, with Cl and HCl as the only species whose fractional abundances are significant. The estimated fraction of gas-phase chlorine present as HCl lies between 25-65 percent, in good agreement with the recent observations of the ground state HCl transition by Blake, Keene, and Philips (1985). These results, combined with the observational limits on HCl, indicate that chlorine is not severely depleted in dense interstellar clouds.

The chemistry of phosphorus in dense interstellar clouds

Laboratory experiments show that the ion-molecule chemistry of phosphorus is significantly different from that of nitrogen in dense interstellar clouds. The PH/sub 3/ molecule is not readily formed by gas-phase, ion-molecule reactions in these regions. Laboratory results used in a simple kinetic model indicate that the most abundant moleule containing phosphorus in dense clouds is PO.

Chemistry in dynamically evolving clouds

We present a unified model of chemistry and dynamical evolution of isolated, initially diffuse and quiescent interstellar clouds. This model uses a semiempirically derived dependence of the observed cloud temperatures on the visual extinction and density. Even low-mass, low-density, diffuse clouds can collapse in this model, because the inward pressure gradient force assists gravitational contraction. In contrast, previous isothermal collapse models required the low-mass diffuse clouds to be unrealistically cold before gravitational contraction could start. Theoretically predicted dependences of the column densities of various atoms and molecules, such as C and CO, on visual extinction in diffuse clouds are in accord with observations. Similarly, our prediction dependences of the fracitonal abundances of various chemical species (e.g., CO, H/sub 2/CO, CN, HCN, HCO/sup +/) on the total hydrogen density in the core of the dense clouds also agree with observations reported to date in the literature. Compared with previous models of interstellar chemistry, the present model has the potential to explain the wide spectrum of chemical and physical properties of both diffuse and dense clouds with a common formalism employing only a few simple initial conditions. Subject headings: interstellar: molecules: molecular process

An ICR study of ion—molecule reactions of PHn+ ions

Abstract The reaction of PH n + ions ( n = 0–3) were examined with a number of neutrals using ion cyclotron resonance techniques. The reactions examined have significance for the distribution of phosphorus in interstellar molecules. The results indicate that interstellar molecules containing the Pue5f8O bond are likely to be more abundant than those containing the Pue5f8H bond.

A low pressure study of the reaction CH+3 + HCN → CH3 · HCN+: A case for radiative association

Abstract An ion cyclotron resonance investigation of the association reaction between CH + 3 and HCN from 1 × 10 −6 to 2 × 10 −2 Torr is reported. Results are interpreted in terms of an unstable excited intermediate (CH 3 · HCN + ) ∗ for which there are two stabilization channels Rate coefficients at 300 K for low pressure (radiative) and high pressure (collisional) association rates are 2 × 10 −10 cm 3 molecule −1 s −1 and ≈ 5 × 10 −25 cm 6 molecule −2 s −1 , respectively

Magnetic resonance studies of lunar samples.

Electron spin resonance searches at 9.5 gigahertz on several fines samples and portions of several rocks have yielded signals whose lineshapes and temperature dependences show that the samples are principally ferromagnetic in nature. Proton magnetic resonance searches at 60 megahertz of these samples have not revealed any signals ascribable to water or any other types of hydrogen in concentrations greater than 0.0001 percent by weight contained in narrow lines (5 oersteds wide or less) and 0.01 percent by weight in wide lines (as wide as 100 oersteds).

Chemical evolution of molecular clouds

Chemical evolution of molecular clouds seems to be governed by both chemistry and dynamics. We briefly describe the basics of the coupled chemical-dynamical evolution. Current problems involving the simplest species, such as C, CO, O2 , H2O, in quiescent clouds are then discussed. We emphasize the simplest molecules, because their chemistry is much less uncertain than the chemistry of the larger, more complex molecules. This allows the role of dynamics to be elucidated with a little bit more confidence. A particularly interesting issue in this area is the possibility that a molecule or a suite of molecules may serve as signpost of quiescent dynamical evolution. It is recognized that complex molecules may be more useful for this purpose, once their chemistry is at least fairly understood. Energetic dynamical phenomenon, such as shocks, produce dramatic chemical signatures, such as the dramatically enhanced CH+ abundance, due to the opening of otherwise forbidden chemical reaction paths. As a corollary, chemistry may have the potential to improve our understanding of the impulsive dynamical processes in a significant manner. Equally impressive are the footprints of star formation on the chemical evolution of the parent dense cloud. This has been discussed with respect to the star forming region in the Orion. It seems possible that chemistry might act as a tracer of embedded protostellar or stellar objects which manage to elude direct observations.

Dependence of interstellar depletion on hydrogen column density - Possibilities and implications

A reexamination of the observed column densities of various elements in diffuse clouds suggests that almost all elements including oxygen, nitrogen, sulfur, and argon may be depleted with respect to hydrogen in interstellar clouds with large hydrogen column density. The amount of depletion varies from element to element and increases with increasing column density of hydrogen nuclei. This result is in qualitative agreement with the depletion of oxygen and sulfur independently inferred from the gas phase chemistry of sulfur and dense clouds. The rate of increase of depletion with hydrogen column density implied by the present study is large. It is possible that observational selection effects may have amplified the real dependence on N(H). A broad spectrum of C/O ratios ranging from values greater than unity of values less than unity appears possible for interstellar clouds, which would have the effect of a large variation in chemical composition from cloud to cloud.

The constituents of cometary nuclei

Close to the edge of our Solar System is a cloud containing many tiny objects only a few kilometers in diameter. Even though their number has been estimated to be as large as ~1013, their total mass is still negligible compared to that of the major planets. Their existence would go unnoticed were it not that, from time to time, some of them are gravitational by pertured and move inward, towards the Sun, and can then be observed as comets. Several space missions have been aimed uniquely at comets, among them the first European deep space mission, Giotto. Rosetta, a cornerstone mission of the Horizon 2000 program of the European Space Agency (ESA), is one of several comet missions being planned for the first decade of the twenty-first century.