Claude Arpigny

Comet P/Halley’s nucleus and its activity

The Halley Multicolour Camera on board ESA’s Giotto spacecraft observed the nucleus of comet P/Halley and its environment and returned more than 2000 images. The observations are summarized, their calibration is described, the status of the analysis and the results are discussed. Topographic features on the nuclear surface and areas of activity are identified. The optical thickness of the dust produced in jet-like features is estimated. The impact and constraints of the observations on cometary nucleus models are discussed.

Large Excess of Heavy Nitrogen in Both Hydrogen Cyanide and Cyanogen from Comet 17P/Holmes

From millimeter and optical observations of the Jupiter-family comet 17P/Holmes performed soon after its huge outburst of 2007 October 24, we derive 14N/15N = 139 ± 26 in HCN and 14N/15N = 165 ± 40 in CN, establishing that HCN has the same nonterrestrial isotopic composition as CN. The same conclusion is obtained for the long-period comet C/1995 O1 (Hale-Bopp) after a reanalysis of previously published measurements. These results are compatible with HCN being the prime parent of CN in cometary atmospheres. The15N excess relative to the Earths atmospheric value indicates that N-bearing volatiles in the solar nebula underwent important N isotopic fractionation at some stage of solar system formation. HCN molecules never isotopically equilibrated with the main nitrogen reservoir in the solar nebula before being incorporated in Oort Cloud and Kuiper Belt comets. The 12C/13C ratios in HCN and CN are measured to be consistent with the terrestrial value.

Hubble Space Telescope Observations of Comet P/Shoemaker-Levy 9 (1993e)

The Hubble Space Telescope observed the fragmented comet P/Shoemaker-Levy 9 (1993e) (P indicates that it is a periodic comet) on 1 July 1993. Approximately 20 individual nuclei and their comae were observed in images taken with the Planetary Camera. After subtraction of the comae light, the 11 brightest nuclei have magnitudes between ∼23.7 and 24.8. Assuming that the geometric albedo is 0.04, these magnitudes imply that the nuclear diameters are in the range ∼2.5 to 4.3 kilometers. If the density of each nucleus is 1 gram per cubic centimeter, the total energy deposited by the impact of these 11 nuclei into Jupiters atmosphere next July will be ∼4 x 1030 ergs (∼108 megatons of TNT). This latter number should be regarded as an upper limit because the nuclear magnitudes probably contain a small residual coma contribution. The Faint Object Spectrograph was used to search for fluorescence from OH, which is usually an excellent indicator of cometary activity. No OH emission was detected, and this can be translated into an upper limit on the water production rate of ∼2 x 1027 molecules per second.

Long-term optical spectrophotometric monitoring of comet C/1995 O1 (Hale-Bopp)

We observed comet C/1995 O1 (Hale-Bopp) at 4.6-2.9 AU pre-perihelion and 2.8-12.8 AU post-perihelion with optical long-slit spectroscopy. Emission bands of CN, C 3 , C 2 and NH 2 have been covered. Emission of C 3 was detected up to 7.0 AU, and CN could be followed up to 9.8 AU post-perihelion. Spatial column density profiles of the radicals have been used to derive effective parent Haser scale lengths for heliocentric distances beyond 3 AU. Production rates were derived based on these Haser scale lengths. The observations of CN are in agreement with HCN as the major parent molecule of this radical at large distances from the Sun (i.e. beyond ∼3 AU). We compare the measured CN production rate to sublimation rates of HCN from a simple nucleus sublimation model. The variation of CN production rates with changing heliocentric distance gives no indication for sublimation from the interior and is consistent with very little thermal lag of the nucleus.

TOWARD A UNIQUE NITROGEN ISOTOPIC RATIO IN COMETARY ICES

Determination of the nitrogen isotopic ratios in different bodies of the solar system provides important information regarding the solar systems origin. We unambiguously identified emission lines in comets due to the 15NH2 radical produced by the photodissociation of 15NH3. Analysis of our data has permitted us to measure the 14N/15N isotopic ratio in comets for a molecule carrying the amine (-NH) functional group. This ratio, within the error, appears similar to that measured in comets in the HCN molecule and the CN radical, and lower than the protosolar value, suggesting that N2 and NH3 result from the separation of nitrogen into two distinct reservoirs in the solar nebula. This ratio also appears similar to that measured in Titans atmospheric N2, supporting the hypothesis that, if the latter is representative of its primordial value in NH3, these bodies were assembled from building blocks sharing a common formation location.

The 16OH/18OH and OD/OH isotope ratios in comet C/2002 T7 (LINEAR)

The 16 OH/ 18 OH and OD/OH isotope ratios are measured in the Oort-Cloud comet C/2002 T7 (LINEAR) through ground-based observations of the OH A 2 Σ + − X 2 Πi ultraviolet bands at 3063 A (0, 0) and 3121 A (1, 1) obtained with the Very Large Telescope (VLT) feeding the Ultraviolet-Visual Echelle Spectrograph (UVES). From the 16 OH/ 18 OH ratio, we find 16 O/ 18 O = 425 ± 55, equal within the uncertainties to the terrestrial value and to the ratio measured in other comets, although marginally smaller. We also estimate OD/OH from which we derive D/H = 2.5 ± 0.7 × 10 −4 in water. This value is compatible with the water D/H ratios evaluated in other

IUE observations of comet P/Halley: evolution of the ultraviolet spectrum between September 1985 and July 1986

The ultraviolet spectrum of comet P/Halley (1982i) was monitored with the International Ultraviolet Explorer (IUE) satellite observatory between 1985 September 12 and 1986 July 8 (r < 2.6 AU pre- and post-perihelion) at regular time intervals except for a two-month period around the time of perihelion. A complete characterization of the UV spectrum of the comet was obtained which allows us to derive coma abundances and to study the light emission mechanisms of the observed species. IUE observations at the time of the Giotto encounter provide a unique opportunity to compare the in situ measurements with remote observations of the principal coma species. The Fine Error Sensor (FES) camera of the IUE was used to photometrically investigate the coma brightness variation on time scales of the order of hours. Spectroscopic observations as well as FES measurements show that the activity of the nucleus was highly variable, particularly at the end of December 1985 and during March and April 1986. The production rates of OH, CS and dust are derived for the entire period of the observations. The total water loss rate for this period is estimated to be 3108 metric tons.

Isotopic abundance of nitrogen and carbon in distant comets

The 12 C 14 N/ 12 C 15 Na nd 12 C 14 N/ 13 C 14 N isotopic ratios have been determined in comets C/1995 O1 (Hale-Bopp), C/2001 Q4 (NEAT) and C/2003 K4 (LINEAR) at heliocentric distances of, respectively, 2.7, 3.7 and 2.6 AU. These ratios have also been measured at r ∼ 1 AU. No significant differences were found between all determinations, nor with the value obtained for other comets. If confirmed, the discrepancy between the nitrogen isotopic ratios from optical and millimeter measurements on CN and HCN would rule out HCN as a major parent of the cometary CN radicals.

The C-12/C-13 ratio in comet Kohoutek /1973f/

Results are presented for photoelectric scans of the C sub 2 1 to 0 band head at 4737 A at resolutions of 0.14 to 0.50 A in the comet Kohoutek. On the basis of medium-resolution scans and of one high-resolution scan two values are obtained for the C-12/C-13 ratio which are consistent within the experimental errors with the terrestrial values of C-12/C-13 equal to 89. The study also demonstrates the limitations imposed by NH2 blends.

High-resolution spectroscopy of the C2 Swan 0-0 band from Comet P/Halley

High-resolution spectra of the C2 Swan systems 0-0 band from Comet P/Halley in March 1986 show that the populations of the upper states rotational levels may be described by two rotational temperatures. The low rotational levels provide a low temperature, Trot = about 600 - 700 K. The higher levels correspond to about 3200 K. If a contribution from the 3200-K molecules is subtracted from the populations of the low-J-prime levels, the latter are characterized by Trot = about 190 K. A comparison with recent predictions for C2 molecules fluorescing in sunlight shows that the observed and predicted level populations are in good agreement for J-prime greater than about 15, but there is a sharp disagreement for the low rotational levels. 26 refs.