ALBERT

Description: Ultraviolet (UV) spectra of comet P/Hartley 2 (1991 XV) taken with the Faint Object Spectrograph (FOS) on the Hubble Space Telescope (HST) in 1991 September reveal several bands of the Cameron system of CO (a 3 Pi-X 1 Sigma). These band are most likely due to 'prompt' emission from CO2 and, thus, provide a direct tracer of the CO2 abundance in the nucleus. Photodissociative excitation of CO2 is probably the largest contributor to the Cameron band emission, but significant contributions from electron impact excitation of CO, electron impact dissociation of CO2, and dissociative recombination of CO2(+), are also possible. Using our estimate that photodissociative excitation is responsible for approximately 60% of the total excitation of the Cameron system, we derive Q(sub CO2) approximately 2.6 x 10(exp 27) molecules/s, which implies CO2/H20 approximately 4%. If all of the Cameron band emission is due to photodissociative excitation, then CO2/H2O = 7 +/- 2%. For the largest possible contributions from the other excitation mechanisms considered, the CO2 abundance could be as a small as aproximately 2-3%. We did not detect CO Fourth Positive Group emission in our data and derive an upper limit of CO/H2O less than or equal to 1% (3 sigma) for CO coming directly from the nucleus. Comparison of the relative CO2 and CO abundances in P/Hartley 2 to those in P/Halley (CO2/H2O approximately 3%-4%, CO/H20 approximately 4% for the nucleus source) indicates that selective devolatilization of the nucleus may have occurred for P/Hartley 2. A relatively large CO2/CO ratio (i.e., approximately greater than 1) seems to be a common property of cometary nuclei. Since gas phase chemistry, in either the solar nebula or the interstellar medium (ISM), appears incapable of producing large relative CO2 abundances, the CO2 in cometary nuclei is probably produced either by UV and/or cosmic ray irradiation of ISM grains prior to the formation of the Solar System, or by condensation fractionation in the solar nebula.

Description: A visible lightcurve of Comet Levy obtained with the IUE Fine Error Sensor has revealed short-term coma variability. A production-rate source function is derivable from these data which implies a nucleus exhibiting hemispherically asymmetric activity. The ratio of gas-to-dust-production rates is also noted to exhibit asymmetry. The low dust-outflow velocity derived from observations, at about 200 m/sec, indicates a distribution that is rich in large, 3-10 micron particles.

Description: IUE observations of Comet Wilson from September 1986 to November 1987, through perihelion, allow a comparison to be conducted between this 'new' comet and the highly evolved P/Halley, at comparable heliocentric distances. The temporal decreases of both OH and dust in Comet Wilson near perihelion were monotonic and slow, by contrast to Comet Halley's rapid fluctuations. Despite these differences, relative gas abundances were similar within a factor of about 2 for comparable heliocentric and geocentric distances; this indicates that P/Halley's in situ gas measurements may be typical of comets generally.

Description: IUE data on the UV and visible coma emissions of the comets Bradfield, P/Tempel 2, Wilson, and P/Halley, are presently compared with the visual lightcurves from magnitudes reported in the IAU circulars to consider the temporal evolution of these comets. While the water-production rates obtainable from visual magnitudes on the basis of Newburn's (1984) method are consistent with OH-derived rates to first order, they are sometimes either displaced or unable to exhibit the same pre/postperihelion asymmetry. The best agreement is obtained for the relatively dust-free Comet P/Tempel 2. IUE Fine Error Sensor lightcurves are generally in agreement with curves based on total visual magnitude.

Description: The paper presents an analysis of a set of CCD images of Comet Halley which were taken in the light of the CN radical to search for evidence of a 2.25-day and/or a 7.37-day periodicity in the evolution of the CN jets. It is found that, for a phase sorted set of CCD images, the geometry of the CN jets is more easily understood when a 7.37-day periodicity is assumed. The extent of the CN jets indicates a dominant periodicity that is greater than 2.25 days.