Tentative Explanation For The Observed Trends In Colour And Polarization

From Figures 1 and 2 one can see that all the observed comets show a correlation between colour and polarization in the innermost coma. The correlation could be due to the following changes in the dust particles:

1. Grain sizes decrease as a result of evaporation or particle break-up. It is well known that particles much smaller than the wavelength (Rayleigh particles) have blue colour and show maximum polarization of P = 100% at 90° phase angle. As the particle dimensions grow further and no longer are negligible in comparison to the wavelength, the polarization decreases. Also, the colour starts to increase and becomes 'more red'. Thus, a consequence of a gradually changing particle size is an anti-correlation between colour and polarization.

2. The composition changes as volatile materials evaporate or are otherwise destroyed. Cometary dust composition changes, most likely, due to the sublimation of ice or the evaporation/destruction of some organics. Absorption increases with increasing wavelength for water ices and decreases for organics [6]. That translates in changing colour toward red as ice sublimates and toward blue during decomposition of organics. At the same time, polarization increases as water ice evaporates and decreases as organics disintegrates because the change in polarization is related to the change in absorption [6].

As a result, a positive correlation between colour and polarization should be observed.

3. The porosity increases as embedded volatiles evaporate. Using microwave analog measurements we studied light-scattering by aggregates of varying packing [4]. No significant influence of compactness on either colour or polarization in the range of packing factor (the ratio of the volume occupied by the material to the total volume) 10-50 % was found.

Thus, the observed correlation between colour and polarization can be only explained by changing particle composition due to sublimation of volatiles.

4. CORE-MANTLE MODEL OF COMETARY GRAINS

For a quantitative analysis we simulate cometary dust as aggregates of core-mantle particles packed so loosely that interactions between constituent grains can be neglected. This is reasonable since the expected packing factor should be near 10% (see [2], [3]). We suppose that there is some size distribution of the cores of the dust grains, which has maximum at a specific grain radius (we consider ro=0.05, 0.1, 0.2, 0.5 and 1 fim) and is normally distributed around this maximum with some deviation a (we consider <7=0.1, 0.5 and 1). To specify the mass ratio of organic to silicate material, we use the estimate based on the cosmic abundances [3] and consider that the mass ratio of organic to silicate can be M0/s=0.5, 1 and 2. Input parameters include also the complex refractive index m = n + in for silicates and for organics. We consider a range of refractive indices: from 1.5+i0 to 1.5+i0.5 for the silicate core and from 1.3+i0 to 1.7+il for the mantle, covering materials from ice to carbon for the mantle and silicates of different absorption for the core. Output parameters of the calculations are colour, C, polarization, P, and polarimetric colour, PC, defined at the phase angle 90° and at the blue (443 nm) and red (642 nm) filters. We calculated the initial mantle thickness based on the size distribution of the cores and on the organics to silicate mass ratio. We then obtained the presublimation values of C, P, and PC from the core-mantle light scattering calculations [9]. The calculations were repeated assuming a given mantle recession rate. We thus calculated the thickness of all mantles and new light-scattering properties of the core-mantle particles at each time step and obtained the change in colour, polarization and polarimetric colour. Table 1 presents the input parameters that provide the observed changes in the output parameters.

5. CONCLUSIONS

The trends in colour, polarization and polarimetric colour in the comae of comets Hale-Bopp, Hyakutake and Tabur have been studied. The main result is a correlation between colour and polarization in the innermost coma. The correlation can be due to changing the grain material during the sublimation of volatile constituents of the dust. Aggregated core-mantle particles whose mantles get thinner as the grains recede from the nucleus is a possible model of cometary grains. The observed correlations can be obtained from calculations of the light scattering by core-mantle particles at:

• the size of the cores peaks around 0.05-0.2 fim;

• the mantle material is absorptive and can be a porous organic-carbon material (more porous for comets Tabur and Hyakutake than for comet Hale-Bopp) with the

Table 1

Characteristics of near-nucleus dust particles, results of core-mantle calculations Size distribution Refractive index Mass ratio, Correlation between colour, C,

Table 1

Characteristics of near-nucleus dust particles, results of core-mantle calculations Size distribution Refractive index Mass ratio, Correlation between colour, C,

r0 cr

core

mantle

K/s

and Polarimetrie colour, PC

Comets C/1996 B2 Hyakutake and C/1996 Q1 Tabur

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