Engineering topics

Jewitt, D.

Publications and source records attributed to Jewitt, D..

6 records

NASA NTRS1998

Chemistry in cometary comae

Significant gas-phase chemistry occurs in the comae of bright comets, as is demonstrated here for the case of Comet Hale-Bopp. The abundance ratio of the two isomers, hydrogen cyanide and hydrogen isocyanide, is shown to vary with heliocentric distance in a way that is consistent with production of HNC by ion-molecule chemistry initiated by the photoionization of water. Likewise, the first maps of emission from HCO+ show an abundance and an extended distribution that are consistent with the same chemical model.

Non-NASA Center↗

NASA NTRS1997

Interstellar and cometary ices: Molecular emission from comet Hale-Bopp

Observations of rotational translations of neutral molecules, radicals and ions in the comet Hale-Bopp are reported on. Sample spectra and maps of the emission for the J = 1 to 0 transition of HCN, the J = 2 to 1 transition of CS and the J = 1 to 0 transition of HCO+ are presented. While the emission from HCN is typically centered on the position of the nucleus and is symmetric, the emission from HCO+ exhibits multiple peaks, together with evidence for acceleration away from the nucleus.

Irvine, W. M.↗

NASA NTRS1997

The HNC/HCN ratio in comets

The abundance ratio of the isomers HCN and HNC has been investigated in comet Hale-Bopp (C/1995 O1) through observations of the J = 4-3 rotational transitions of both species for heliocentric distances 0.93 < r < 3 AU, both pre- and post-perihelion. After correcting for the optical depth of the stronger HCN line, we find that the column density ratio of HNC/HCN in our telescope beam increases significantly as the comet approaches the Sun. We compare this behavior to that predicted from an ion-molecule chemical model and conclude that the HNC is produced in significant measure by chemical processes in the coma; i.e., for comet Hale-Bopp, HNC is not a parent molecule sublimating from the nucleus.

NASA Discipline Exobiology↗

NASA NTRS1991

Cometary photometry

The study of comets using modern optical photometric techniques is reviewed. Particular emphasis is given to the physical constraints imposed on the nature of comets by photometric data. The photometric study of bare nuclei and of active comets is discussed.

Jewitt, D.↗

NASA NTRS1988

The absence of a color-distance trend in comets

New near-IR J-H and H-K observations of 23 comets have been obtained in the heliocentric distance range of 1 to 6 AU. No evidence is found for a color-distance trend in the present sample. Significant differences between the grain populations in different comets are pointed out. The results indicate that the color distributions are unimodal, and that, unlike asteroids, the comets cannot be classified into distinct color-defined groups.

Jewitt, D.↗

NASA NTRS1986

Early photometry of comet p/Halley - Development of the coma

The observation of comets over a large range of heliocentric distances can provide important information, taking into account, for instance, the appearance of true cometary characteristics. Photometric evidence is provided regarding coma formation at approximately 5.9 AU, giving attention to aspects of water-ice sublimation. New photometry of p/Halley was obtained when the comet was at 5.9 and 5.1 AU, during 1984 October and 1985 January, respectively. The new observations are compared with data obtained at larger heliocentric distances. It is argued that sustained mass loss from the nucleus began at about 5.9 AU, at a rate which is consistent with production by sublimation from a predominantly water-ice nucleus.

Meech, K. J.↗