We propose that many of the denser dust clouds represent transient stages between the initiation of gravitational collapse of ordinary interstellar clouds by passage through a spiral galactic shock and the appearance of a stellar association. The interstellar gas is not processed at too rapid a rate (1) because the time spent between galactic shocks is appreciably longer than the free-fall collapse time of a typical dust cloud, and (2) because only a small fraction of the total mass of gas of a given collapsing cloud is converted into stars – the rest is presumably redispersed by various dynamical processes which accompany the formation of massive stars. We note that the residue of atomic hydrogen observed to exist in dark dust clouds is compatible with the interpretation that they are transient objects whose lifetimes are not longer than ~ 107 yr.

Measures in the Strömgren four-color and Hβ systems provide an accurate way to determine color excesses of early-type stars. Fourteen areas at high galactic latitude have now been searched for faint A stars which are then measured photoelectrically to obtain the color excesses. Non-main sequence A stars, which are easily detected by means of the four-color photometry, are not included in the analysis. Within 40° of each pole, the reddening is essentially zero, Eb–y = 0.00 north of the galactic plane and Eb–y = 0.01 south of the plane.

A survey of H2CO absorption in dark clouds in IC 1795 (W3) shows that the material in the direction of W3 can be distinctly separated into three groups: one, immediately surrounding W3, has a uniform velocity of about −41 km–1 s; the second, associated with the W3 (OH) source has a velocity of about −49 km–1 s; the third, as a velocity of about −20 km–1 s and may be associated with an interarm spur in the foreground.

The 4830 MHz absorption line of formaldehyde has been observed in the SNR complex W44 and the H ii region complex W3. The results are presented as maps of apparent optical depth. The formaldehyde is concentrated toward the eastern edge of the shell and reaches a maximum apparent optical depth of 1.5 compared with 0.2 for OH; furthermore the centers of the H2CO and OH concentrations are displaced by about 6′. The velocities of the H2CO and OH agree and both molecular lines have the same velocity gradient across the source.

The H2CO absorption profiles for W3 (IC 1795) exhibit at least two narrow components whose average velocity becomes more negative toward the NE. The greatest optical depth occurs where the optical obscurration is high but not at the position of maximum Av. A general agreement between the velocities of other molecules and the H2CO values is indicated.

Observational results of cosmic diffuse X-rays are reviewed with particular emphasis on soft X-rays. The intensity distribution of soft X-rays over the celestial sphere indicates that the diffuse component of soft X-rays consists of an extra-galactic and a galactic component. The absorption of the soft X-rays in the interstellar medium results in heating and ionization of interstellar matter. The ionization rate by X-rays is estimated as about 10–16 s–1 per H atom.

The scattering of X-rays by interstellar dust grains produces a halo of an X-ray source and smears out the pulsation of X-ray emission. The scattering coefficient and the halo size are given for some typical grain models.

The possibility that the dust grains gain relativistic energy is suggested. It is speculated that the relativistic dust grains in metagalactic space may be responsible for cosmic rays of ultrahigh energies and also for the diffuse X-rays by the interactions with cosmic black-body radiation.

Optical constants of olivine – (Mg, Fe)2SiO4 and magnetite – Fe3O4 are presented for the wavelength range from near infrared to far ultraviolet. A feature occurs in the optical constants of olivine at about 1460 Å, but no structure that could give rise to a 2200 Å interstellar feature is found. The most peculiar characteristic of the magnetite results is the large change of optical properties with temperature in the infrared.

The optical properties and collective excitations are discussed for graphite and vitreous carbon. The bulk resonance energy for the π-plasmon is seen to be shifted by 1.4 eV from the crystalline to the amorphous case, indicating that the degree of structural disorder is an important parameter in the optical properties of carbon grains. Data are presented on work function, photoelectric yield, and photoelectron energy distribution for both graphite and vitreous carbon. Under solar irradiation, photoelectrons from graphite are emitted with an average energy of about 1 eV (12000 K).

An experiment has been performed to determine the thermal accommodation coefficients for the gases hydrogen, oxygen, methane, and carbon dioxide on graphite, in the temperature range 273–77 K. The experimental results are compared to those indirectly predicted in a theoretical paper by Hollenbach and Salpeter, and agreement is found to be satisfactory. A sticking coefficient for atomic hydrogen on a graphite grain is derived which would support the conclusion that recombination of hydrogen on grain surfaces could be a significant process. The very high measured thermal accommodation coefficients for the heavier gases supports the popular assumption that retention of such gases on grain surfaces should be considered extremely likely.

The interstellar existence of dipyridyl magnesium tetrabenzporphin (I) is supported by the spectroscopic coincidence of laboratory generated and astronomically observed diffuse interstellar lines. A potential route for the synthesis of I in space from molecules which exist or are expected to exist there will be discussed.

Small particles of silica, carbon, and iron are produced in the laboratory for the eventual purpose of studying their optical properties.

The possibility that many different diffuse interstellar absorption features may be produced by the same type of absorbing atom in different hydrocarbon matrices on interstellar grains has been examined experimentally. The present study shows that absorption bands due to Na atoms in various hydrocarbon matrices can occur within the wavelength range 5400–5800 Å. A study of the molecules generated from the radiation-induced polymerization of C2H2 at 55 K is also reported. It is shown that C6H6 is an abundant product of this polymerization and may therefore be an important constituent of interstellar grains.

Photoelectric emission from interstellar dust grains due to ultraviolet starlight is reinvestigated. Ejection of photoelectrons can make a substantial contribution to the heating of standard interstellar clouds. Both positive and negative grain charges are shown to be possible.

Dirty ice mantles are destroyed efficiently by sputtering of He atoms when clouds encounter shock velocities greater than 13–15 km s–1. Destruction due to grain-grain collisions is found to be about 10–3 times less efficient. Sputtering in the hot intercloud medium should make intercloud grains smaller than cloud grains.

Dust grains expelled from cool stars are injected into interstellar clouds with typical velocities of ~ 103 km s–1. The interaction of such high-speed grains with gas is discussed. Grains are preferentially stopped within clouds of number density nH ≈ 10–30 cm–3. Collisions of grains with gas atoms could lead to heating and ionization of clouds and to the production of displacement defects in grains.

It is suggested that molecular observations of interstellar clouds can be used to determine the stage of cloud evolution. However, before observational data can reliably be used to investigate the evolutionary path leading from a tenuous cloud of gas and interstellar dust to a star cluster, mechanisms of molecular formation, excitation, and destruction must be taken into account. The evidence is growing that interstellar dust grains have an important role in molecular formation and destruction.

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.

The triggering of an explosive chemical reaction in a grain which consists of free radicals frozen in a matrix of photolyzed dirty ice material is seen as a possible source of complex interstellar organic molecules.

Many of the places in which radio astronomers search for molecules are located in exotic regions lying near the galactic center, near H II regions or other continuum sources, or in high density regions. Such regions have wide variations in their physical environment. Here we wish to compare two types of region whose environments are thought to be well-understood. One of these is dust clouds; the other is unshielded (from UV) cold clouds in front of bright radio sources. For the sake of interest, we include without comment results for molecular regions near the Galactic center and near Orion A.

Comparison of dust clouds and unshielded clouds is of interest because the volume density differs by a factor of 104 or more and the extinction differs by 8 mag. or more. Neither type of cloud lies near H II regions or other sources of intense radiation. Even though their properties differ widely, we will see that the abundances of OH and H2CO relative to hydrogen are approximately the same in the two. Although this is understandable for OH, no current theory would predict this situation for H2CO.

Microwave emission from the J = 8 → 7 through the J = 11 → 10 rotational transitions of HC3N has been detected in the directions of Sgr(NH3A), Sgr B2, and Orion A. A map of the J = 8 → 7 emission around Sgr B2 shows that the emitting region extends at least 2′ in galactic longitude and 3′ in galactic latitude. Column densities may be comparable to those of HCN in some sources. This molecule is thus more abundant in the interstellar medium than would be expected from its complexity, and may provide clues to formation processes where it is found.

With a reasonable model for excitation of the rotational levels, we have used the ratio of antenna temperatures of these 2 lines to derive a value for the molecular density in the region where the lines originate. For Sgr B2, this yields NH2 ≈ 106, and for Sgr(NH3A), NH2 ≈ 6 × 104.

Thermal reactions largely determine the chemical evolution of interstellar molecules during various phases associated with the evolution of interstellar clouds. A similar mechanism may be the source of the array of observed polyatomic molecules. If astronomical time scales are comparable to or less than chemical time scales, thermodynamic equilibrium will not be attained. Other factors also affect the validity of thermodynamic calculations of molecular composition. An experimental investigation of thermal reactions in a simplified system is under way. Preliminary results of the HCN–H2 reaction at 1000 K indicate reaction times of several hours. These have been extrapolated to lower temperatures and fitted to models of the primordial solar nebula, proto stars and red giants. In these situations equilibrium would generally not be attained and reaction products would freeze out at temperatures near 700 K or greater.