The gas phase production of molecules in dust clouds in interstellar space may occur largely as H2 formation on the interstellar grains, with relatively minor formation in the gas phase. The formation of more complex molecules, however, may be augmented by gas phase processes including (1) negative ions formed by radiative attachment of electrons, (2) direct radiative association of molecules and atoms, and (3) reaction of atoms and molecules with H3+ molecules.

Irradiation of adsorbed simple gases leads to the formation of rather complex molecules. On the basis of the wavelength dependence and the cross sections for formation and desorption an average molecule density can be estimated. The wavelength dependence further shows that molecules can be formed in denser regions where they are protected from dissociating radiation. A simple model is proposed which shows the molecule distribution as a function of dust density.

The analysis of the abundance of interstellar molecules in compact H II regions suggests that the molecules are formed in stellar atmospheres (possibly of protostars or of late type stars), transported to the location of the neutral clouds in the cpmpact H II regions, and shielded from decomposition due to stellar radiation by the dust in the clouds. Cometary nuclei and interstellar dust are argued from the astrochemical point of view to be dirty ice of the second kind (or a sort of frozen interstellar molecules). The chemical structure of the primordial solar nebula is discussed under the assumption that long-period comets consist of the most primordial substances of the solar system.

An experiment to measure the photodesorption of physisorbed gases from a cold fused quartz substrate by near UV photons is described. The observed yield (mol photon–1) is as large as 10–5, a higher yield is likely in the vacuum UV. Included is a discussion of the photodesorption process and some applications to the interstellar medium. The observed yield is enough to maintain molecular abundances in moderate density interstellar clouds in equilibrium.

An interferometer with a 24 km baseline has been used to map components of OH sources to a precision of a few hundredths of a second of arc. The W3 OH source which is associated with an H II region appears to be rotating slowly. There is evidence for Zeeman splitting in the components of this source. The IR/OH sources NML Cygnus and VY CMa are found to be expanding objects with a large rate of mass outflow. Rotation is also evident in both these objects.

From the star counts on the Palomar Sky Survey prints, dust and star distributions in the Orion Association are studied. The Hydrogen distribution in the same region is obtained from an Hα photograph. It is found that a considerable part of the interstellar absorption on the line of sight is due to the circumstellar dust grains in the Orion Association.

The observed far infrared flux of H II regions is interpreted as evidence for the presence of internal dust absorbing a fraction of Lyman continuum photons. The effect of this dust on the required number of ionizing photons and on the helium to hydrogen line intensity ratios are discussed and analytic formulae describing these effects are presented for simplified models.

Airborne observations of the Trapezium region of the Orion Nebula in the 60–300 μ range have been made from 13.7 km altitude using a Michelson interferometer with the Rice University 12-in. Flying Infrared Telescope. Fourier analysis of five interferometer scans provided spectra with resolution ranging from 7 cm–1 to 20 cm–1. These spectra were compared with lunar spectra taken with the same instrument at the same altitude to correct for instrumental and atmospheric effects. A weighted combination of these scans provides a low resolution spectrum. The radiation per unit frequency interval at 190 μ was found to be at least 40% of that measured between 75 and 90 μ. Neglecting possible features in the spectrum, its general shape is consistent with a blackbody near 70 K. The resolution is inadequate to resolve spectral lines.

Photometry and mapping in the wavelength range 1.65–20 μ of the H II regions W3 and NGC 7538 have led to the discovery of a dozen new infrared emission sources. The sources have flux densities ranging up to 10–23 Wm–2 Hz–1 at 20 μ and diameters from < 3 to 40″. Some are associated with, and have similar brightness distributions to compact H II condensations observed at radio wavelengths; they have, however, 20-μ flux densities much greater than, and 2-μ flux densities much less than are predicted from the radio free-free spectra. It is concluded that in many cases there exist close spatial relationships between the hot dust and the ionized gas, and between infrared and OH/H2O emission sources.

The wellknown ‘Horsehead’ nebula is photographed with narrow interference filters centered respectively on Hα and [N II] (λ = 6584 Å). The [N II]/Hα ratio is deduced and its variation towards the Horsehead bright rim is studied. It is shown that the maximum value of the temperature is not reached on the rim itself but in a narrow zone located just behind it and ahead of the dark matter.

A brief review of extinction effects is followed by a discussion of new Cerro Tololo Schmidt plates. They were exposed on parts of the LMC with filters which reveal the presence or absence of a continuum, such as one scattered on dust, in H ii regions. The results are presented in comparison with the Orion nebula which was observed in the same way. The continuum is always relatively weak in those LMC H ii regions which are significantly larger than Orion, but the continuum averages strong in the Orion-class and point-image nebulae.

Two arguments are presented in support of the now well-established fact of the tight mixing of neutral gas and dust with the ionized gas of H II regions.

Infrared astronomy has in the past decade emerged from being a part-time occupation of a few astronomers. Three major subdivisions of research have become apparent, solar system, galactic and extragalactic studies. In each of these fields infrared studies have made unique contributions. Planets emit the bulk of their radiation in the infrared, and infrared studies are essential to study planetary thermal problems. Many extragalactic objects have been found to emit astonishingly large fluxes in the infrared.

In galactic astronomy the current major contribution of infrared studies has been to act as a bridge between two separate disciplines, stellar astronomy, and studies of the interstellar medium. Infrared studies have proved invaluable for studying star birth and star death. Both of these phases had previously seemed mysterious and invisible. And indeed they were not visible, because they occurred shrouded in dust that blocked transmission of visible rays. However, the dust that is merely opaque in the visible, is self-luminous in the infrared, and so in the midst of this optical darkness there has appeared a great infrared light.

At this time, we have progressed further with the study of star death than of star birth. The ejected matter from dying stars carries the dust shroud with it into space, and so the gas and dust become part of the interstellar medium. This process is clearly significant for understanding the composition and origin of interstellar dust.

Because star death and birth are embedded in dust, there has developed a separate interest in explaining the physical processes at work in these dust clouds. This study explains processes of optical circumstellar absorption lines, intrinsic polarization of cool star light, and stellar molecular masers.

Perhaps what these two paragraphs have just said is that our conceptual scheme of separating stellar astronomy and interstellar astronomy still acts as such a division that the infrared astronomer needs to present different aspects of this one topic, circumstellar infrared emission to different audiences. Such an opportunity has been given to the author in that he has been asked to give within a few weeks two talks. The first of these reviews is being presented at IAU Symposium # 52 on Interstellar Dust and Related Topics. The second is being given at the summer meeting of the Astronomical Society of the Pacific which has a symposium on Circumstellar Dust.

The two reviews have been made complementary. The first of these is primarily an observational study. It shows the infrared observations of stellar and interstellar dust, and in a qualitative way shows that one gives rise to the other. The second review is theoretical and attempts to place the first study in its theoretical context. It deals almost exclusively with the stellar and circumstellar parts of the topic. Together they present one man's view of Circumstellar Infrared Emission.

The literature relevant to this topic is voluminous. There have been false leads, dead ends, and irrelevant detail. This review has attempted to follow a thread through this detail, and to expose the skeleton of a scheme for understanding the processes at work. Such a review is intrinsically more dangerous, more likely to become obsolete than a comprehensive one. However, by carrying the seeds of its own destruction it seems to offer a greater opportunity for the growth of astronomy.

Radiative transfer methods are used to show how and to what extent the usual techniques relating color excess to optical depth through circumstellar material are not applicable.

It is shown that the absorption due to a circumstellar shell containing solid silicon carbide particles can very nicely explain the observed strong violet opacity in stars in which the carbon to oxygen ratio is > 1. It has been shown by Friedemann and Gilman that solid SiC particles can form in the cooler outer layers of such stars. Thermal re-emission from SiC particles is predicted to be in the 10–13 μ region and recent infrared observations by Hackwell show an emission band in this region, thereby strongly supporting the SiC suggestion. It is also shown that the opacity due to C3 pseudo-continuum is not adequate to explain the observed violet opacity.

It is suggested that the vibrational bands of C3 and SiC2 molecules should be among the major opacity sources in the infrared spectra of the late N-type carbon stars and some of the observed bands may be, at least in part, due to these molecules. The frequencies of their isotopic species have been calculated and attempts should be made to observe these bands.

About 80 stars of the association Cyg OB2 have been observed in J, H, K and L. These measurements have been made in search for infrared circumstellar emission in this highly reddened association.

We have formulated and studied the properties of a mathematical model intended to span between the hydrodynamical and discontinuous regimes. Model parameters and predictions permit matching the observed mass spectrum of those interstellar clouds with masses between 103 and 106M⊙. The central element of the theory is the specification of P (m, m′, μ, μ′, … μ″), the conditional probability that once a collision has occurred between clouds of mass m, m′ the result is clouds of mass μ, μ′, … μ″. For stable solutions to exist we require that the total mass in the system be conserved. Such simple models as total coalescence, geometric overlap and partition statistics were considered for P as well as several probabilities based on possible physical conditions that might prevail in cloud-cloud collisions. One immediate numerical result of these models is that nearly total coalescence must obtain in actual cloud-cloud collisions before one can build up a non-infinitesimal concentration of large mass couds. Field and coworkers (1965, 1968) indicate that total coalescence would produce a minimum in the mass spectrum and that as the position of the minimum went to infinity the spectrum itself more closely approximates a power law curve with index − 3/2. In none of our calculations was anything but strict monotonic decrease observed and under the same physical assumptions as Field the curve we obtain (analytically) has no minimum, independent of the largest mass in the system. Both of these spectra are flatter than recent observational evidence indicate. True equilibrium solutions exist in our formulation with an e-folding time ≃ 107 yr for objects such as these. We are continuing work on the detailed time evolution of interstellar clouds in particular and formulating other astronomically interesting applications of the general theory.

It is argued that carbonaceous chondrites, particularly of type I, are probably collections of interstellar grains which have been mildly transformed through exposure to higher than normal temperatures, resulting in a loss of volatile materials.

The view that dust is essential to star formation is challenged on the ground that other interstellar constituents can provide more rapid cooling. From the evidence of stellar minimum masses it is suggested that self absorption of the radiation emitted by the coolant H2 is the dominant mechanism leading to the heating of a collapsing fragment. It is however shown that extensive dust and molecule formation may take place during star formation and that a natural explanation for the 4 terrestrial planets in the neighbourhood of the Sun is then provided.

Measurements of the diffuse background radiation in the 85–115 μ band are presented, as observed from altitudes close to 190 km with a rocket-borne, liquid helium cooled telescope. Evidence is given for detection of the galactic background due to thermal grain emission at galactic latitudes of 5–35°, and at a galactic longitude of ~ 163°. At small latitudes, the background intensity is measured to be ~ 9 × 10–11 W cm–2 sr–1; the average number density of grains derived is consistent with the optically determined measure. The 100 μ data is compared with 20 μ data taken on the same flight, in order to draw some conclusions about the grain emissivity and temperature.