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V CometaryDust

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Comet Dust: The view after Hale-Bopp M.S. Hannera aJet Propulsion Laboratory, California Institute of Technology, Pasadena CA, 91109, USA

Extensive observations of comet Hale-Bopp from the ground and space have considerably expanded our knowledge of cometary dust. There is a clear correlation of stronger polarization, higher albedo, more prominent infrared silicate features, and higher infrared color temperature. The composition of the silicates includes both olivine and pyroxenes, in both crystalline and glassy or amorphous form, similar to the chondritic aggregate interplanetary dust particles.

1. INTRODUCTION

The apparition of comet Hale-Bopp (C/1995 01) was a fortunate convergence of a bright comet observable for more than two years over a wide range in heliocentric distance and improved observational capabilities, enabling detailed measurements with high spatial resolution and broad wavelength coverage. ESA's Infrared Space Observatory (ISO) was in operation throughout the apparition, significantly extending the infrared observations in time and spectral range. As a result, considerable progress has been made in elucidating the properties of cometary dust.

Comet Hale-Bopp already displayed an extensive coma when discovered in July 1995 at heliocentric distance r = 7.2 AU. It soon became evident that the activity was driven by strong CO outgassing; the observed CO production rate at r = 6.6 AU was ~ 2 x 1028 mol/sec [1,2]. A strong infrared signal from warm dust was already detected at 4.9 AU [3]. The comet released a prodigious amount of dust; the dust area x albedo product near perihelion was 100 times that of comet Halley [4]. Complex patterns of jets and other structures were evident in the coma throughout the comet's apparition. Perihelion occurred on 1 April 1997 at 0.91 AU from the Sun. The comet was well observed during June to October 1996 (4.2-2.7 AU) and from February to May 1997 (1.25-0.91 AU).

This paper will highlight some of the new results from observations of the dust coma of Hale-Bopp. In particular, spatially resolved measurements of polarization and thermal emission permit us to correlate variations in several optical properties of the dust, while mid-infrared spectra have allowed us to resolve long-standing questions about the mineralogy of the cometary silicates.

2. POLARIZATION 2.1. Observations

Polarization carries important information about the properties of the scattering particles. The polarization P(d) as a function of phase angle e has been established by combining measurements from a number of comets. While the polarization from different comets is similar at small phase angles, the comets tend to divide into two classes at larger 6, having Pmax -15% and -25% respectively near 6 =90°-100° ([5,6] and references therein). Negative polarization of order -2% is observed in comets at 0 < 20°; both classes of comets appear to have similar negative polarization at small 0.

Comet Hale-Bopp was observed at phase angles from 7° to 47° between June 1996 and May 1997 [7,8,9,10,11,12,13]. A phase curve for the average coma at X~ 0.65 nm is shown in Figure 1. The polarization at all phase angles is distinctly higher than that measured for previous comets, even those in the high Pinax class, including P/Halley. At 0 >30° there is a clear trend towards higher polarization at longer wavelength; the higher the polarization, the stronger the wavelength dependence. A similar, but weaker, wavelength dependence was seen in comet P/Halley (e.g., [14]).

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