Astron. Astrophys. 360, 777-788 (2000)

5. Conclusions

In this section, we summarize the main results obtained in this work.

• In all cases studied, the scattering functions are smooth functions of the scattering angle, having almost no structure at back-scattering directions. Although the shapes of are very similar for all the samples, their steepnesses vary with the size of the particles and with the imaginary part of the refractive index.

• The ratio is dramatically different from 1 over almost the whole scattering angle range. The values of this ratio decrease from almost one at angles close to the forward direction to a minimum at side-scattering angles, and increase again towards back-scattering directions. The lowest values of have been measured for olivine sample L at 442 nm.

• The measured , tends to be larger than at back-scattering angles, which is in agreement with the general trend for scattering by non-spherical particles (Mishchenko et al. 2000).

• The maximum degree of linear polarization as measured for the four olivine samples at a wavelength of 633 nm increases as the effective radius of the particles decreases, as is expected if the smallest particles govern the behavior of . However, at a wavelength of 442 nm only minor differences in the maximum degree of linear polarization are observed, and no clear relationship with the size of the particles can be established. As yet, this behavior is unexplained and calculations for broad size distributions are needed to explain our results in blue light.

• Single scattering by irregular particles may be responsible for the negative branch of the degree of linear polarization observed in cometary comae.

• A high value of the inversion angle of the degree of linear polarization can be produced by irregular compact particles.

• Reported changes in the observed polarization of cometary comae as a function of the distance to the nucleus, indicate that the size distribution and/or color of the particles in the comae change as a function of that distance.

• Computational results are needed to interpret the trends presented by the measured scattering matrix elements as functions of the scattering angle and to investigate the effects of differences of the physical properties of the particles (e.g. size, color and shape) on the scattering behavior systematically. This strategy will also provide an opportunity of using the measured results in an indirect way, by using theory to obtain inter- or extrapolated results at other wavelengths.

© European Southern Observatory (ESO) 2000

Online publication: August 17, 2000
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