Showing papers by "Alan Fitzsimmons published in 1997"

Neutral Sodium from Comet Hale-Bopp: A Third Type of Tail

Abstract: We report on the discovery and analysis of a striking neutral sodium gas tail associated with comet C/1995 O1 Hale-Bopp. Sodium D line emission has been observed at heliocentric distance r≤1.4 AU in some long-period comets, and the presence of neutral sodium in the tailward direction of a few bright comets has been noted, but the extent, and in particular the source, has never been clear. Here we describe the first observations and analysis of a neutral sodium gas tail in comet Hale-Bopp, which is entirely different from the previously known ion and dust tails. We show that the observed characteristics of this third type of tail are consistent with it being produced by radiation pressure due to resonance fluorescence of sodium atoms and that the lifetime for photoionization is consistent with recent theoretical calculations.

Neutral sodium from comet Hale-Bopp: a third type of tail

Abstract: We report on the discovery and analysis of a striking neutral sodium gas tail associated with comet C/1995 O1 Hale-Bopp. Sodium D-line emission has been observed at heliocentric distance r<1.4 AU in some long-period comets and the presence of neutral sodium in the tailward direction of a few bright comets has been noted, but the extent, and in particular the source, has never been clear. Here we describe the first observations and analysis of a neutral sodium gas tail in comet Hale-Bopp, entirely different from the previously known ion and dust tails. We show that the observed characteristics of this third type of tail are consistent with itbeing produced by radiation pressure due to resonance fluorescence of sodium atoms and that the lifetime for photoionization is consistent with recent theoretical calculation.

Surface reflectance properties of distant Solar system bodies

Abstract: The recent discoveries of over 40 new objects with orbits beyond 30 au represent the first sampling of a reservoir of objects lying beyond Neptune, known as the Kuiper Belt (or Edgeworth-Kuiper Belt), which may be a source of short-period comets and Centaurs (objects whose orbits cross those of the giant planets). There are very few observations from which to derive physical properties of these Kuiper Belt objects (KBOs) due to their faintness and the concentration on discovery rather than follow-up, although optical and near-IR photometry of the few brightest KBOs and Centaurs shows a diversity from neutral to extremely red colours. We present new BVRI photometry of five KBOs (1994JQ(1), 1995DC(2), 1994JR(1), 1995DA(2) and one undesignated new KBO) and the Centaur object 1995DW(2). With the current small sample, we find no compelling evidence for a correlation of colours with orbital zones, and consequently no clear mechanism to explain this diversity in terms of the irradiation mantle model.

A study of Hilda asteroids II. Compositional implications from optical spectroscopy

Abstract: We obtained 49 reflectance spectra of 23 Hilda asteroids in the wavelength ranges 3850-7650 and 3850-10150 Angstrom during two observing runs in 1992. The primitive D-types dominate and comprise 34% of the numbered Hilda asteroids, while 28% and 2% are P

The European Hale-Bopp Team Observations of Comet Hale-Bopp

Abstract: The poster reports some preliminary results obtained by the European HB Team during an observational campaign, carried out with asubstantial number of telescopes on the Canary Islands within a programme supported by the European Union. The observations were devoted to a variety of spectroscopic and photometric topics and havealready provided important results. A highlight of this work was the discovery of a third tail composed of neutral sodium and the detection of substantial amount of sodium in the dust tail. Potassiumwas found in the nuclear region andit was possible to obtain good definition of the rotation state of the comet nucleus. Other results are expected from the analysis of high resolution spectra, for instance the carbon isotopic ratio, the relative abundance of some ions in the coma, and of the very large collection of images from different telescopes with which the complex jet structure of the near nucleus region was monitored.

II. Compositional implications from optical spectroscopy

Abstract: Weobtained49reflectancespectraof23Hildaaster- oids in the wavelength ranges 3850-7650 and 3850-10150 u A during two observing runs in 1992. The primitive D-types dom- inateandcomprise34 %ofthenumberedHildaasteroids,while 28 % and 2 % are P and C-types, respectively. This is in con- trast to the results of earlier investigations by Tedesco & Gradie (1982) and Gradie et al. (1989) who found fewer D-type aster- oids in the Hilda group. We confirm the result by Dahlgren & Lagerkvist (1995) that there is a spectral slope - asteroid size re- lationamongtheHildaasteroids,implyingasizedependentsur- face composition. No evidence of absorption features are seen in the spectra of the P and D-types, in particular no features due tophyllosilicatesarefound,implyingthatnoaqueousalteration of the surfaces has occurred. The absence of C-types within the HildagroupsuggeststhattheCpopulationisconfinedtosmaller heliocentric distances than thought before, with very few C as- teroids further out than the Cybele group. The phase reddening effect is absent for the P and D-types observed. Though it is not a Hilda member one spectrum of 279 Thule was obtained and is also presented.