The production of helium within supermassive stars, supermassive disks, and ‘little bangs’ is discussed. The conclusions are summarized at the end of the paper.

The aim of the present report is to emphasize the role of the helium problem as the key one for different aspects of cosmological speculations. This is not surprising, because the processes which produce the helium and other elements in primordial matter take place during the first stages of the cosmological expansion of the Universe.

The theory gives three critical values of the He4 abundance in pre-stellar matter, depending on assumptions about the behaviour of the Universe near the singularity. The three critical values are:

(1) practically no He4 at all;

(2) about 25% of He4 by mass;

(3) practically entirely He4.

Although intermediate values of He4 are in principle possible, the cosmological models permitting such values are highly improbable. It is obvious that the helium abundance is less than 100%, and that even rough estimates of whether the stellar matter consists almost entirely of hydrogen, or if it has a significant part of He4, are of tremendous importance. The much-more-difficult determination of traces of primeval He3, D, Li6, would be of great help in making definite cosmological conclusions.

It is my task to summarize for you what we have learned today about the helium problem. Before attempting this, let me first very briefly outline the problem as it appears to us in 1970. It amounts to the questions: What is the fractional abundance of helium in different regions in the universe, is it the same in all places, and how did it originate?

The simple answers to these questions which appealed to many and seemed to have a reasonable basis in fact a few years ago were:

The He/H ratio is of ~ 25-30% by mass; the ratio is the same everywhere, and the helium was most probably synthesized in the early stages of a primordial fireball in which the universe began. However, for some time it has been clear that the situation is more complicated than this, as I pointed out in an article published last year in Comm. Astrophys. Space Sci.

Considerable attention has recently been given to the observation of interstellar molecules by radio methods. Interstellar molecules have also been given a boost because of excitation by the 3K black body radiation field (to use a convenient description). Nevertheless optical studies of interstellar lines have reached a very low ebb and, with brighter moments, have remained so since the work of Adams (1949).

The years 1937-1942 were the exciting period for optical studies of interstellar molecular lines. It is interesting to note that it was not molecular lines as such that led to the suspicion that molecules might exist in interstellar space but it was the discovery of the diffuse features at 5780, 5797, 6284 and 6614 by Merrill (1934) that led Russell (1935) to speculate on the possibility of their existence. It was Swings and Rosenfeld (1937) who stressed that likely interstellar molecules would be CH, OH, NH, CN and C2. Dunham, Adams and McKellar did much of the work of identifying interstellar lines in the period 1937-1940 and indeed McKellar (1941) had derived an excitation temperature of 2-3 K for CN - a value subsequently rediscovered.

It has been common practice to separate the study of interstellar matter from that of stellar evolution. However, infrared astronomy deals mainly with observations of stars forming and stars dying. Interstellar matter represents a phase intermediate between these two stages, part of a cyclic process (Figure 1).

We find molecules in interstellar space and want to know how they came to be there. Molecules form most easily at high densities and moderately high temperatures. These conditions prevail both in envelopes around forming stars and also around evolved red giants. However, the matter going into star formation is mainly leaving interstellar space, while that from the red giants is going into space. Therefore the evolved red giants are potentially the source of interstellar molecules.

This paper will propose first that the chief interstellar solid molecules were formed in the atmospheres of red giant stars. Volatile solids that might condense in space do not seem to be a major constituent of the grains. Secondly it will be demonstrated that a major part of interstellar matter has probably come from the outer layers of red giant stars. Since the solids formed in these envelopes, it appears possible that less complicated gaseous molecules have also been ejected into space. Whether they are in fact the same molecules that are now observed is a question that cannot yet be answered.

Some time ago, optical astronomy was successful in detecting the first molecular species in interstellar space. However, these have been limited to the three diatomic molecules CH, CH+, and CN. Consideration of this list made it rather natural, after the development of microwave astronomy, for some attention to be given to a search for OH, which has a rather strong microwave transition. This molecule was first detected, after a considerable search, by Weinreb et al.. The analogous Λ-doublet transitions in SH and CH have since been searched for assiduously; failure to detect them does not necessarily mean these radicals are excessively rare in interstellar space, since various unfavorable factors make the search difficult.

Any consideration of polyatomic molecules in interstellar space immediately raises the question as to whether and where such molecules are likely to exist. Of course, they can occur in stellar atmospheres, where they have already been found to a limited extent. But in free interstellar space they are less likely to exist because of slow rates of formation and a rather short lifetime, the latter being typically limited to a few hundred years because of dissociation by ultraviolet radiation from stars. In dust clouds, molecules are more protected from ultraviolet radiation and their formation may be aided by catalysis on the dust grain surfaces.

This paper deals with the interpretation of molecular line emission from class I OH/H2O emission centers associated with compact HII regions and with the OH 18 cm emission from dark clouds in T-Tauri star associations. Observational evidence is presented, that class IOH/H2O emission centers represent a particular stage in the evolution of a massive star (or a group of massive stars) whereas protostars of lower mass apparently do not go through such a stage.

It appears that in associations the low-mass stars are formed first and the massive O-stars are formed last. T-Tauri star associations may represent an early evolutionary stage of a star association where low-mass stars are formed. Evidence is presented that the physical conditions in some parts of the Taurus complex of dust clouds and T-Tauri stars are appropriate for the formation of single stars of about a solar mass.

OH is at present the most ubiquitous molecule known in the interstellar medium. It is primarily associated with galactic continuum sources, both thermal and nonthermal. The fraction of all sources showing OH decreases monotonically with galactic longitude away from the Galactic Center region. However, neither the projected density of OH nor the abundance relative to hydrogen or other molecules seems to depend on location within the galactic plane. OH also occurs in many dark dust clouds, where its density is typically 10−5—10−4 cm−3 and its abundance relative to hydrogen is very high. OH is not found in the heavily reddened stellar clusters of Reddish, in Wolf Rayet stars, in planetary nebulae, or in globules. Recent surveys have indicated that OH both in emission and in absorption, is highly correlated with H2CO in direction and in velocity. OH emission, but not absorption, is also highly correlated with anomalous H2O emission. These relationships are important in deciding between various processes for interstellar molecule formation. On present information, it appears that more than one such process is operative in different regions of the interstellar medium.

I will briefly summarize some recent observational results on Ʌ doublet maser radiation from OH excited rotational states. Combination of these results with interferometric observations should enable us to distinguish between the variety of pumping models that have been proposed for the ground state OH masers. I will not discuss pumping models since the time is limited and you would not believe me anyway.

Formaldehyde (H2CO) was found to be present in the interstellar medium less than 1½ yr ago. This discovery raised a number of new astrophysical problems specific to H2CO such as how is the molecule formed, how is it destroyed, and how do the anomalous energy level populations leading to absorption of the isotropic microwave background arise. In addition, H2CO has provided a new means of studying several problems of long standing interest in astronomy. These include large scale galactic structure, distribution and motions of local gas and dust, and isotopic abundance ratios. A surprisingly large amount of work has been done on these problems in the rather short time since the initial discovery. I will confine myself to a brief summary of the observational aspects, omitting many of the topics that will be discussed in more detail by others later today, and then describe in detail some of our more recent work.

I have attempted to obtain an answer to the question “In which gas (neutral hydrogen) clouds do we find molecules?” By limiting the investigation to those clouds which might be considered normal (i.e. specifically excluding the class of clouds which emit by maser action) it is possible to obtain a definitive answer to the question namely, those clouds which have the highest gas concentration. Further data on the correlation between the distribution of neutral hydrogen and of different molecules can be found.

The clouds which are seen in absorption against background radio sources are most ideally suited for this purpose and I have used the absorption spectra of neutral hydrogen (21 cm wavelength), of the OH radical (18 cm, the 1667 MHz line) and the formaldehyde molecule (6 cm). Only the absorption spectra of the three strongest non-thermal sources (Cas A, Cyg A, and Tau A) were used because only these have unambiguous neutral hydrogen absorption spectra and also because they, unlike the HII regions, do not have 1667 MHz OH spectra which may be confused by anomalous emission. In the absorption spectra of these three sources individual clouds at specific velocities can be recognized on the H, OH and H2CO spectra. We can then investigate the OH and H2CO content of each cloud relative to its neutral hydrogen content.

In 1968 NH3 molecules were discovered in gas clouds near the center of our Galaxy. The NH3 molecules showed emission lines in transitions arising from metastable J = K rotational states. NH3 has many transitions at about 1.25 cm; hence, a great deal can be learned about the gas clouds. In particular, 1-1, 2-2, 3-3, 4-4 have excitation temperatures of 24K, 65K, 125K, 203K respectively. Thus, one can determine temperatures very accurately by comparing line strengths to statistical theory. NH3 is particularly useful as an interstellar thermometer since all of these lines can be detected by a single radio telescope with essentially the same beam size. Generally, a telescope measures the antenna temperature TA of an object which is related to the brightness temperature TB of an object by TA=Ωs/ΩA TBj; Ωs/ΩA≤1. TA = TB when Ωs≥ΩA where ΩA is the solid angle of the antenna pattern and Ωs, is the solid angle of the source as seen from the antenna. If measurements can be made with the same telescope, TB for different spectral lines will be simply related to the TA for the lines and one does not have the complication of source size and geometry.

Radio emission spectra from the J = 1−0 transition of H12C14N and H13C14N, two molecular isotopes of hydrogen cyanide, have been detected at 88.6 and 86.3 GHz, respectively. In addition, an emission signal from a new and unidentified interstellar molecule was detected at 89.2 GHz. The new molecule temporarily has been named ‘X-ogen’ until its chemical identity can be determined. X-ogen has been detected in several regions and may be as common as hydrogen cyanide in the Galaxy.

During the last twenty years many studies have been carried out in our laboratory which were aimed at establishing the exact positions of possible sharp interstellar lines of various diatomic and polyatomic molecules. In addition we have been trying to see whether the diffuse interstellar lines cannot be explained as produced by some molecule or molecular ion in the interstellar gas.

The present report is a summary of our recent work in this field.

The present report is a summary of a study, made in collaboration with P. Solomon, on the formation of interstellar molecules by radiative association and chemical reactions. The model to be discussed is limited practically to gas phase reactions in an HI region, where the principal constituents are atomic. It therefore is limited in scope and applicability. The role of the interstellar dust in this model is passive. It shields the region from radiation. Since a number of molecular species are dissociated by radiation and the extent of carbon ionization is determined by radiation, the radiation density is important in determining compositional equilibria.

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.

Assuming interstellar dust particles being composed at least partly of materials with catalytic activity or of refractory metals and being exposed to the radiation field of the Sun or other stars they can favour the formation of molecules as combinations of the most abundant gases H, C, N and O mainly for three reasons:

(1) By the adsorption at the surface of dust particles the collision probability for two or more atoms is greatly enhanced with respect to the gas phase.

(2) Because of the adsorption bond the excitation and dissociation energies are shifted to lower values.

(3) For highly exothermic reactions dust particles can absorb the excess energy and thus lead to the formation of molecules which in the gas phase can be formed only by three body collisions.

Problems relating to the formation of interstellar molecules have assumed an enhanced importance in recent years mainly because of the detection of a variety of molecular species in interstellar space. The interstellar molecules CH, CH+, CN were discovered several decades ago by their characteristic bands in optical stellar spectra, while OH, H2O, H2CO, HCN, HC3N, NH3 and CO were detected during the past few years by their spectral features in the radio waveband. The optical detection of interstellar H2 was reported only several weeks ago.