Photometry and polarimetry of the cometary heads still constitute one of the most important sources of information about the physical processes in comets. For instance, most of the present estimates of molecular lifetimes are based on the observed distribution of molecules in the cometary head and the assumption of a particular kinematical behaviour of the matter in the cometary atmospheres.

The study of kinematics and dynamics of cometary heads and tails has been based upon the analysis of the forms and apparent motions of well-defined envelopes, halos, knots in tails and streams. Direct inspection of a large number of photographs (or drawings from the last century) of several bright comets demonstrates that the cometary head is generally a complicated object. The heads consist of nearly circular diffuse patterns with superposition of different features, particularly of curved streams. This is illustrated by the Atlas of the Cometary Forms compiled by Rahe, Donn and Wurm (1970).

Comet Kohoutek (1973f) has been observed with the 50 cm (f/15) reflecting telescope of the European Southern Observatory, La Silla, Chile, on fourteen nights between January 16 and 30, when the heliocentric and geocentric distances of the comet were r=0 66 - 1 00 A. U and Δ =0 81 - 0.96 A. U. , respectively The 40″ and 80″ diaphragms were used for the photometry of the cometary head (EMI 6256 A photomultiplier) in the UBV system and with six interference filters: CN 3884 Å, CO+ 4267 Å, C2 4737 Å, C2 5172 Å, cont. 5300 Å and Na 5893 Å.

The atmospheric conditions were good but the accuracy of our observations was lower than usual due to large extinction (air mass 25 to 43) and twilight. The mean error of one measurement of log F in all but Na 5893 Å filters can be estimated at ±0.02, whereas the accuracy through the Na filter was substantially lower

Photometric observations of emission line features of the coma of comet Kohoutek (1973f) were made with narrow band interference filters. The emission features observed were CN(3879 A°), C3(4057 A°), and C2(4732, 5165,5634 A°). The radial dependence of the emission was investigated by employing six diaphragms set concentrically about the brightest part of the comet's head.

The photometric observations were made at the Optical Research Facility and Observatory of NASA/Goddard Space Flight Center (GSFC) in Greenbelt, Maryland. The observatory is located 13 miles Northeast of Washington, D.C. (longitude = 5h07m18s22, latitude = +39° 01′ 11.″48, altitude = 49 meters) and is operated by the Laboratory for Optical Astronomy of Goddard (GSFC).

With few exceptions, measurements of cometary brightness and polarization have been restricted to regions in or near the coma and therefore to a relatively small range of phase angles. Photoelectric techniques are required for detailed wavelength coverage, whereas large-field photographic techniques are better suited for mapping the large regions of sky spanned by a comet tail. Observations with a small field of view provide high spatial resolution but generally restrict multicolor measurements of brightness and polarization to a small region of the comet. Observations with a large field of view (diameter larger than 1 or 2 deg) provide adequate color and spatial coverage but can result in the loss of detail. A compromise is afforded by Fabry photometry, using a modest telescope of small aperture and relatively large field of view.

Narrow-band filler photographs of comet TSK were taken in the light of C2, CN and C3 by Rahe McCracken and Donn, have been analysed in terms of Haser's model of the coma. The isophotes obtained from these photographs were corrected for sky background. The isophotes were circularly symmetric. Radial intensity profiles were obtained along the sunward, antisun and the two perpendicular directions. In each case, these profiles were the same within the experimental errors in intensity (±5%).

Theoretical curves based on Haser's model were computed for different combinations of the parameters β0 and β1 where l/β0 is the scale length for the decay of the unobserved parent molecule and 1/β1 the corresponding quantity for the observed daughter molecule. A comparison of the theoretical and observed intensity profiles, yielded our best estimates for β0 and β1 for the different molecules. These are listed in Table 1. In some cases, we were able to obtain lower limits only to the ratio β0/β1 because the isophotes do not go out far enough, a limitation caused by the sky background. It is to be noted that the same values of the parameters were obtained from observations on different dates indicating no unusual activity in the comet in the intervening period. We believe ours is the first result ever obtained for C3.

In Table 2, we summarize all the results for C2 and CN in different comets. It appears that the scale lengths differ between comets, a result which cannot be attributed to different helio-centric distances alone.

This paper consisted of a running commentary by Ernest Hildner on the movie as it was shown. The data from the movie has not yet been fully analyzed and will be published elsewhere. Discussion immediately followed the movie. (ed.)

The question of the present day usefulness of the great wealth of observational material on comets buried in the archives of the older observatories is an important problem that should be evaluated soon if any action is to be taken before much of it is lost or destroyed. The beginning step has been taken with the publication of the isophotometric atlas and the Atlas of Cometary Forms which is a direct result of Recommendation No. 1 of Commission 15 of the IAU meeting at Prague in 1967. At that time a repository for cometary photographs and spectrograms was also discussed. However, in the light of observational advances and the present capability of quantitative correlation of structural changes with condensations or annomalies of the solar plasma, it is desirable to establish again that this old observational material may be useful.

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.

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 %.

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.

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.

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.

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.

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.

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.