The Most Completely Observed Characteristics of Interstellar Grains are their ability to dim and polarize the light of distant stars in our own galaxy. The principal limitation in previous theoretical models of interstellar dust clouds has been the fact that the extinction and polarization have been calculated for different particles, the connection between these types of particles being semiquantitative at best. A knowledge of the electromagnetic scattering properties of the various proposed grains is a fundamental ingredient of any theory of extinction and polarization. All the types of grains which have been suggested, except those of reference 1, scatter electromagnetic radiation by a classical process, i.e., a process which is characterized by a classical application of Maxwell's equations and for which the particles are defined by a size, shape, and index of refraction. In spite of the fact that the methods of application of the classical theory are completely understood, numerical results for the particular range of application which appears to be needed in the interstellar dust problem have been somewhat limited.

Since the closing date for material included in the FK4 many new positions of fundamental stars have been published, most of them observed differentially. I think in these few minutes it is neither desirable nor necessary to present alist of all these catalogues. We are mainly interested in recent observations which allow us to improve the fundamental system of positions and even more the system of proper motions.

Among the First Requirements for a Grain Model is that it should explain the observed interstellar extinction law or laws as they are now understood; a desirable feature of such a model is that as few ad hoc assumptions be made as possible.

A criticism of the classical ice grain theory is that a very narrow range of grain sizes (or a size distribution with a size parameter specified to within a few percent) must be postulated in order to obtain a fit with the mean extinction law. (See refs. 1 and 2.) While it is true that regional variations in the extinction law have recently been detected (refs. 3 and 4), the best available evidence indicates that the extinction law is quite uniform when averaged over individual large regions which are widely distributed in the galaxy. (See ref. 5 and paper by Nandy in the present compilation.) The restriction of particle size to a radius within a few percent of an arbitrarily specified value (r ≈ 3 X 10-5 cm) demanded on the basis of pure ice absorption is therefore considered quite unsatis-factory, particularly in view of the fact that no characteristic size parameter emerges from the Oort-van de Hulst theory for the formation and destruction of grains. (See ref. 6.)

Thanks to W. Dieckvoss’s great obligingness who communicated to us all his results, we could apply the overlap method to the A.G.K. 2 -A.G.K. 3.

Some studies are given in (1) and (2) on the use of overlapping in A.G.K. 2 – A.G.K. 3; here only qualities of the results are studied from the pole to +40° of declination. Moreover some results will be improved by new measures. The details of the method will be published later.

The Past Several Years there has been a growing interestin the possibility of observing infrared radiation at wavelengths greater than 10 μ, from various astronomical objects. For example, Gould in references 1 and 2 has suggested trying to observe several lines in the infrared with emphasis on the 12.8-μ line of Ne+ and the 28-μ line of molecular hydrogen. The possibility of being able to observe the emission from interstellar grains in the wavelength region between 10 μ and 100 μ was suggested in a discussion presented in references 3 and 4. The intensity expected from thermal emission by the grains has also been investigated by F. D. Kahn at Manchester.

The present paper indicates information about interstellar grains that could be gained by measuring the intensity as a function of wavelength and the polarization of radiation emitted by the grains in the far infrared.

Some years ago at Bergedorf we studied the field around Alpha Persei (Heckmann, O. et alt., 1956). In an area of 5° x 5° more than 100 cluster members were found from an analysis of proper motions (p.m.) of 1453 stars. Meanwhile the field was studied photo metric ally (e.g. Mitchell, R. I., 1960). The age of the group seems to be comparable to that of the Pleiades (cp. e.g. Dachs, J., 1970), of the order of magnitude 108 years.

The First Detailed Studies to determine the processes by which interstellar grains may form were made by a group of Dutch astronomers in the 1940's. (See refs. 1 to 5.) Since that time very little systematic work on this problem has been done until very recently when Hoyle and Wickramasinghe (ref. 6) investigated graphite formation in cool stellar atmospheres. Van de Hulst's paper in 1949 (ref. 5) represents the culmination of an intensive attack which had considerable influence on astronomical thought about interstellar grains.

Somewhat ironically, beginning about 1949 many significant advances in physics and chemistry having a direct bearing on this problem were made. In 1949, Frank in reference 7 presented a theory which explained how real crystals tend to grow, and much work, both theoretical and experimental, has been done since then. (See ref. 8.) Recent extensive research in chemical reactions at low temperatures both in solids and on surfaces is reported in reference 9.

At the University of South Florida Observatory, a program is under way to determine the proper motions of the star-like objects which are possibly or probably associated with X-Ray sources. The objects under study are Sco X-l, and objects believed to be identical with Cen X-2, Cyg X-2, and Co D -32° 1057. Since it has been speculated that these might be neutron stars, an effort was made to study not only their proper motions but also those of other stars in the neighborhood to ascertain membership of any moving cluster with known age and distance, whereby their own ages and distances might become known. These efforts were successful so far in the case of ScoX-1 (Sofia, Eichhorn and Gatewood, 1969), whose proper motion is virtually identical to that which a member of theScorpio-Centaurus Association would have at this position, so that the membership of Sco X-l in this association becomes highly probable. On the basis of this and other information, Gatewood and Sofia (1969) conclude that Sco X-l is quite likely a neutron star. The reduction was described in detail by Gatewood (1968). No definitive results are available yet for the other three objects.

In Spite of Many Years of Research on Interstellar Grains, their composition is still unknown. Evidently it can be found only through the interaction of the grains with radiation, and one is led to the spectroscopic study of the solids composing the grains. If discrete lines, bands, or other spectral features can be discovered, they may be of great importance in the chemical analysis of the grains. So far, data of this type include the diffuse interstellar lines, the optical lines of CH, CH +, and CN (almost certainly once part of the grains), the radio lines of OH, and the distinct “kink” or curvature in the optical extinction curve (see paper by Nandy in present compilation), which may reflect spectral variations in refractive index and hence indicate a specific material.

The advent of ultraviolet and infrared techniques suggests that consideration be given to new features to be expected in those regions.

Photography of the second epoch of the Lick Proper Motion Program (Wright 1950) was started in 1968. The original plan (Vasilevskis 1954) was modified in several respects. It was decided to carry out a pilot program first, before the start of a full coverage of the sky accessible from Lick. It was also decided to make use of the new lens corrected for yellow light (Vasilevskis 1964), in addition to the blue lens used for the first epoch. The aims of the pilot program were formulated: (1) carry out the necessary error investigations, (2) develop methods and procedures to a stage of routine, (3) derive proper motions of stars measured, and (4) attempt to arrive at some astronomical conclusions based on these motions. The program was limited to 97 fields nearly uniformly distributed over the sky north of -23° all these fields were photographed, measured and reduced, 78 of them contain a sufficient number of galaxies for proper motion reference. This report will be limited to presenting a few results obtained from the first crude analysis of proper motions that became available only a week ago. These results, therefore may change significantly in the process of further analysis.

The Interactions of Grains With Radiation show up either as modifications of the radiation field (scattering, extinction) by grains or as modifications of grains by the radiation. The principal effects on grains are to control their temperature and to exert forces (radiation pressure). This paper presents some radiation pressure calculations which may be pertinent to the problem of propulsion of grains outward from very hot, bright stars or more generally OB complexes. The radiation force is equal to the rate of transfer of momentum to the grain, which is equal, in turn, to the net rate of loss of momentum in the incident beam of electromagnetic radiation.

This program is the southern counterpart of the well-known Lick proper motion program. These programs are almost identical in design and execution, their chief feature being the use of faint extragalactic nebulae and QSO’s as stationary reference objects.

The Lick program has been described by S. Vasilevskis on various occasions; it serves as an example to the Yale-Columbia work.

There are two methods for connecting the proper motions to the galaxies. The one used under the direction of A. N. Deutsch at Pulkovo, and the other method at the Lick Observatory.

Only the method used by the Lick Observatory allows the use of overlapping plates. It is the only one which will be dealt with in this first part.

The Problem of Chemical Identification of the interstellar dust grains has vexed astronomers for over three decades. The earliest evidence for the mere presence of interstellar dust was deduced from the observation of stars (via star counts) in the Milky Way. The best presently available clues for possible chemical identification are spectroscope data, particularly the diffuse (hitherto unidentified) interstellar absorption lines. This paper is concerned primarily with a discussion of new theoretical interpretations of the spectroscopic data.

Much related information on the dust grains was obtained from observations of space reddening, its wavelength dependence, and from interstellar polarization. Furthermore, correlation of space reddening and the strength of some of the diffuse lines is important to the discussion of possible chemical identification of the dust.

The theoretical computations suggested in section 5 of the preceeding paper have now been done.

It is necessary to do many approximations in order that the successive values of ∑β2 give the limit. We have done as many as 8. After that the computation becomes too difficult.

The Last 20 Years much work has been done, both theoretical and experimental, that has improved the understanding of the nature and characteristics of interstellar grains; most of it, however, has not been primarily involved in this particular field. Unfortunately, the problem is extremelv complex. As recently pointed out (ref. 1), an analysis of the nature and growth of interstellar grains calls for consideration of: (1) nucleation of condensing vapors; (2) mechanisms of crystal growth; (3) chemical reactions at low temperature including surface reactions of free radicals; and (4) radiation chemistry of solids. Simultaneous analysis of all these factors seems to be, at the present time, a hopeless endeavor. With the exception of radiation effects, separate analyses of each one have extensively been made. So far, however, it seems that combined effects cannot be overlooked. Partial aspects of the problem are discussed in this paper. The treatment of radiation is essentially exploratory, and its relative success ought not to blind us to the uncertainty of the data and to the speculative character of some of the assumptions.

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.

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.

Here Seems to be Agreement that laboratory studies are necessary in the pursuit of a better understanding of the interstellar medium. Herein is described some experiments performed on the effects of far ultraviolet radiation on molecules containing the astronomically abundant atoms H, O, C, and N. In particular, the attempts that have been made to prepare, isolate, and identify various free radicals and other reactive species, some of which may play a role in the processes occurring in the interstellar medium, are discussed.

In one of the studies, a frozen film of pure carbon dioxide at 77° K was irradiated with the light from a low pressure xenon discharge lamp equipped with a LiF window. This lamp is activated by radio-frequency radiation, and the emitted light is rich in the 1470-Å resonance line of xenon.