Those investigators who have been active in cometary studies for a long time must by now have detected in their midst a number of interlopers from outside the solar system. As one of these newcomers from a far-off field of astronomy, who cannot possibly claim any expertise on comets, I feel compelled to account for the presence of stellar and interstellar astronomers in this arena. The reason is that some of us believe that the comets represent a sample of relatively unprocessed material from the early solar system, from which we may be able to learn something not only of the young sun and planets, but also something about the material of the parent interstellar cloud plus any fresher interstellar material that the cometary nuclei have been able to accrete over the past 4.6 billion years.

Many years ago K. Wurm (1) suggested that the photodissociation of stable molecules such as H2O, HCN, CH4, NH3, etc., could account for the observed cometary radicals. This postulate can be represented schematically by the following photochemical reaction,

A relatively new technique called Laser Induced Photoluminescence Spectroscopy has been applied to laboratory studies of cometary radicals. This technique can be used to measure properties of radicals, to determine photodissociation processes in parent molecules, and to investigate reactions of radicals in specific vibration-rotation levels. Thus far, the LIPS method has been applied to the CN radical to determine: (1) the radiative lifetime and quenching constants for the B2Σ+ state and (2) the photodissociative formation of CN from several parent molecules.

Between January 5 and January 15, 1974, nine coudé spectrograms of Comet Kohoutek (1973f) were obtained with the ESO 152-cm telescope in La Silla, Chile. The emulsion is Kodak IIa-0 (3 plates) and Kodak 103a-F (6 plates), the dispersion is 20.2 Å/mm. The useful spectral range extends from about 3500 Å to about 5000 Å (Kodak IIa-0 plates) and from about 4500 Å to about 6700 Å (Kodak 103a-F plates). The original scale was 4.55 arc sec/mm on the slit, and the full length of the slit was about 3 arc minutes. 1 mm on the plates corresponds to 66.5 arc sec, or about 3.9-4 4x104 km at the Comet projected on the plane of the sky. The slit was always centered on the image of the Comet, and except for plate No. 1436, was oriented along the radius vector. A field rotator was used which dimished the stellar light by about 30 %.

Most records of the first half billion years of the solar system have been wiped out from the planets and from their satellites by their evolution and their morphological differentiation. However, two sources of information seem still to be available on this early period: some meteorites give us clues on the non-volatile fraction that condensed from the primeval nebula, whereas the clues on a more volatile fraction, possibly condensed at a colder temperature, may come from the comets.

Systematic spectroscopic observations of comet Kohoutek (1973f) were scheduled at the Asiago Astrophysical Observatory, starting from the end of October. The nebular spectrograph for the newtonian focus of the 122 cm reflector was selected as main instrument for this research. This spectrograph, described by Bertola (1970), is followed by a WL-30677 image tube and it is particularly designed for extended objects of low surface brightness. Two gratings were used giving a dispersion in the first order of 125 Å mm-1 and 240 Å mm-1 respectively. The scale normal to the dispersion is 127 arcsec mm-1 and the full length of the slit is 8 arcmin.

I would just like to make a brief summary of some of the major points in my review two days ago. The most important results from the cometary UV observations are: (1) We know now that the gas production rates of a medium bright comet is on the order of 1030 molecules s-1 based on hydrogen Lα observations and interpretations. (2) The large amounts of OH indicate that water is an abundant molecule. I do not know of any other probable parent molecule of OH. The case for water as a parent molecule is strengthened by the parallel decrease of OH and H in comet Bennett and by arguments given by Delsemme. (3) Disregarding the observed amounts of OH and O, one would not necessarily conclude that the hydrogen is a dissociation product of water. The uncertainty is due to the deduced outflow velocities of about 8 km s-1. I feel this value for the velocity is pretty mysterious and not yet explained. We have yet to connect this velocity with the dissociation — or formation — producing the hydrogen atoms. The possibility that an appreciable amount of hydrogen does not stem from water cannot be excluded. (4) The UV observations of comet Kohoutek seem to indicate that water is not predominate over other molecules by two orders of magnitudes. The production of carbon atoms (and, therefore, of parent molecules containing carbon) seems to be nearly as great as that of water. The uncertainties of the numbers for the production rates are at least factors of two. I, personally, think it is too early to draw definite conclusions on the various abundance ratios. H, O, OH, and C may well be produced in the same order of magnitude. But a C deficiency by an order of magnitude (or even more) is not rules out either.

Well, I just said that I disagree with nearly all that was said in the review paper. Now I have to illustrate this point a little, so I will first write something on the board.

Herzberg and Douglas (1973) first predicted that the H20+ ion might be observable in comet tail spectra. Subsequently Lew and Heiber (1973) discovered the laboratory spectrum of H20+. In October and November 1973, Herbig (1973), Benvenuti and Wurm (1974), and Wehinger and Wyckoff (1974) reported unidentified features in the tail spectrum of comet Kohoutek. On the basis of these observations, Herzberg and Lew (1974) made a tentative identification by pointing out coincidences with several strong low-lying lines in the H20+ spectrum. Further observations were obtained in November 1973 and January 1974 at the Wise Observatory and by Herbig at the Lick Observatory. Six vibronic bands (5-0 through 10-0), including a total of approximately 50 rotational lines were identified with laboratory H20+ spectrum (Wehinger et al. 1974). The predicted spin splitting of those lines with 2 to 4 Å separation was observed in the coude spectra of Herbig.

Present theories for the formation of molecular species observed in comets predict the sublimation of parent molecules such as H2O, CH4, CO2, and NH3, from the surface of the nucleus and their subsequent photodissociation and ionization to form the observed species (Delsemme 1973). It can be shown (Oppenheimer 19 75) that gas phase chemical reactions occur between these fragments which have characteristic timescales which are short compared to the timescale for significant variation in the solar flux incident on the comet. Hence, a steady-state approximation may be used for determining the densities of many species. It can also be shown (Oppenheimer 1975) that the rate of formation of many species is faster by gas phase reactions than by photoprocess. For instance, the formation of OH+ from H2O+ by the reaction

H2O+ + O + → OH+ + OH

Twenty pre- and post-perihelion calibrated image-tube spectrograms (3400-9000A; 75 and 150 A/mm) were obtained of Comet Kohoutek covering a range in heliocentric distances of 0.9 to 0.4 a. u. (30 November 1973 - 14 February 1974). Total band intensities have been measured for the CN, C2 and NH2 bands in the coma spectrum, and for the CO+ and H2O+ bands in the tail spectrum. Changes in the total band intensities as a function of heliocentric distance are discussed, as are the pre- and post-perihelion characteristics of the spectra. The line intensity distributions within individual bands of the heteronuclear molecules (with large dipole moments) indicate very low rotational “temperatures” as expected. The measured band intensity ratios of the A2π-X2Σ (red) CN bands and the B2Σ-A2π (violet) CN bands are compared with predicted band intensity ratios calculated by T. Danks and C. Arpigny (1973 Astron. and Astrophys. 29, 347). Finally column densities are determined for CN and C2 in the coma and CO+ in the tail of Comet Kohoutek at various heliocentric distances.

The spectral analysis of the coma and type I tails of a comet gives a clue to clarify the composition of the volatiles in its nucleus. The ultraviolet observation of strong H, O, and OH emissions in comets 1969 g and i by OAO II (Code and Savage, 1972) and the recent identification of H2O+ in the spectrum of 1973f (Wehinger et al., 1974) have suggested that the main constituent of cometary coma is H.O. It was also suggested from the computation of the ionization processes (Jackson and Donn, 1968) that CO and N. may be the second most abundant gases (some ten% by number) in comets. Other gases detected in the optical (C2, CN, C3…), ultraviolet (NH, CN) and radio (CH, CHg, CN) regions appears to be minor constituents of cometary atmospheres.

The recent observations of atomic oxygen and carbon in the far ultraviolet spectrum of Comet Kohoutek (1973f) (Feldman et al. 1974; Opal et al. 1974) have demonstrated the existence of these atomic species in the cometary coma. However, in order to identify the source of their origin, it is necessary to relate the observed ultraviolet flux to the atomic production rate. Assuming the only excitation mechanisms allowed are those produced by resonance scattering and fluorescence of solar ultraviolet radiation, the problem reduces to finding the emission rate factors (g-factors) as a function of the heliocentric comet velocity. Since the widths of the solar emission lines are smaller than the maximum heliocentric Doppler shift, given by

where q is the perihelion distance in A.U., it is necessary to consider the detailed multiplet structure of the transition, the solar line shape and the relaxation of excited fine structure levels.

Between 1973 December 1 and 1974 February 2, optical emission lines from the gas cloud surrounding comet Kohoutek were observed using a double Fabry-Perot etalon spectrometer at Kitt Peak National Observatory. The spectrometer had a resolving power of 40,000, corresponding to a velocity resolution of about 7.5 km sec-1. With this resolution it was possible to use the cometearth relative velocity to resolve faint cometary Hα λ6563, [01] λ6300 and other emission lines from geocoronal and airglow emissions and to study the cometary line profiles in order to obtain information about the composition, effective temperatures, outflow velocities, and production rates of atoms and ions in the cometary envelope.

This report will be devoted to a discussion of the theoretical interpretation of the spectra of comets and in this discussion we shall consider successively a number of molecules that have been studied recently: CN, CH, C2, C3, OH, CH+. The first two of this list, CN and CH, have been analysed in greatest detail until now, so that we shall be rather brief on the other molecules. We shall try to indicate what kind of information can be derived from these studies concerning the conditions prevailing in cometary atmospheres and to show which fundamental data are needed in the calculation of the theoretical spectra.

A total of eleven scans of the head of comet Kohoutek (1973f) were obtained during the pre-perihelion period with a photoelectric spectrum scanner (Babu, 1971), mounted at the Nasmyth focus (f/13.1) of the 52-cm telescope, on 7, 10, 11, 13, 14 and 15 December 1973 at the rate of two scans per day, except on 7 December when only one scan was taken. The observational procedure was similar to that for comet Bennett (1969 i) (Babu and Saxena, 1972), excepting that a diaphragm of 3 mm diameter equivalent of 93″ of the sky was used at the entrance of the monochromator. The width of the exit slit was equivalent to a spectral window of 3.5 nm. The scans covered the spectral range from 370 to 640 nm. The basic data of the comet during the period of observations is given in Table I.

Up to now a few hundred of spectra of several tens of comets have been obtained at different observatories over the whole world. On the one hand, there are high dispersion spectra photographed with slit spectrographs attached to large telescopes, and on the other, small dispersion spectra obtained by means of objective prisms. These objective prism spectra, showing the principal emission bands only, are known for a considerable number of comets, and its appearance is usually described verbally.

The various radio searches for cometary molecules finally came to fruition with the radio detections of CH, OH, HCN and CH3CN and several unidentified species in Comet Kohoutek (1973f) and the detection of H2O emission from Comet Bradfield (1974b) As a result of these detections we have learned something about why past radio searches of other comets for molecules were less than successful. Also, it is now possible to identify and discuss some of the similarities and differences between cometary molecules and interstellar molecules, particularly with regard to the excitation and chemistry Finally, given the observed projected densities and resulting gas production rates, the feasibility of future radio molecular observations of comets can be discussed.