Comet Halley: an Optical Continuum Study.
Abstract
From an analysis of narrowband CCD images of Comet Halley from 1986 January, March and April, certain dust structures which are redder than the remainder of the dust coma have become apparent. Mie calculations suggest that this reddening is due to an enhancement of particles with sizes comparable to the observing wavelengths. Assuming a grain density of p = 1 g cm^{-3} , this size range corresponds to masses of 5 times 10^{-13} -3 times 10^{ -12} g. The location of the sunward extension of the red envelopes which are evident in the color-maps is consistent with the measurements by Vega 1 and 2 of a depletion of intermediate mass particles (10 ^{-14}-10^{ -11} g). This dip in the mass distribution occurs at the region where particles are most sensitive to solar radiation pressure. Although the mass range derived from the calculations presented here is somewhat uncertain as a result of the limitations of Mie theory, these values are in the expected range of particle sizes which would be both sensitive to radiation pressure and significantly reddened with respect to the solar spectrum at the observing wavelengths. Thus, the red envelopes are plausibly the result of size sorting by solar radiation pressure. The red jets observed on 1986 January 10, March 1 and March 9 can then be explained by the enhanced dust flux at the jet sources, and the subsequent trapping of a relative excess of intermediate mass (i.e. red) particles into the jets which are visible in the continuum images. Analysis of narrowband photometry of the optical continuum of Comet Halley reveals no correlation between the color of the dust and heliocentric distance, phase angle, strength of the continuum or gas-to-dust ratio. The photometric data are thus consistent with a post-ejection sorting mechanism. Chemical inhomogeneities of the nucleus are therefore not necessary to explain the observed structure in the color of the dust in Comet Halley.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1989
- Bibcode:
- 1989PhDT.........2H
- Keywords:
-
- Physics: Astronomy and Astrophysics;
- Color;
- Cometary Atmospheres;
- Continuums;
- Dust;
- Halley'S Comet;
- Interstellar Extinction;
- Solar Spectra;
- Charge Coupled Devices;
- Mass Distribution;
- Mie Scattering;
- Particle Size Distribution;
- Radiation Pressure;
- Solar Radiation;
- Stellar Evolution;
- Stellar Spectra;
- Astrophysics