Study of grain compositions in comet Levy 1990XX

In the present work, the non-spherical dust grain characteristics of comet Levy 1990XX with different silicate grain compositions (both pyroxene and olivine) are studied using the T-matrix method. Considering amorphous pyroxene grain (MgxFe1−xSiO3) with x (= 0.4, 0.5, 0.6, 0.7, 0.8, 0.95, 1) and amorphous olivine grain (Mg2yFe2−2ySiO4) with y (= 0.4, 0.5) (Dorschner et al. 1995), the observed polarization data of comet Levy1990XX are analyzed using the T-matrix code at λ = 0.485 μm. The best-fit spherical volume equivalent values of effective radius (reff ), effective variance (veff ) and shape parameter (E) obtained for the observed polarization data of comet Levy 1990 XX at λ = 0.485μm for amorphous pyroxene grains (MgxFe1−xSiO3) with x = 0.4, 0.5, 0.6, 0.7, 0.8, 0.95 and 1 are (0.213, 0.0015, 0.486), (0.215, 0.0022, 0.486), (0.215, 0.0019, 0.486), (0.217, 0.0034, 0.486), (0.219, 0.0057, 0.486), (0.222, 0.0083, 0.486) and (0.230, 0.0092, 0.486) respectively. However, the amorphous olivine grains show a bad fit to the observed polarization data. Considering crystalline olivine grain in comet Levy 1990XX, Das & Sen (2006) (our earlier work) already studied non-spherical grain characteristics of that comet using the T-matrix method and found crystalline olivine prolate grains show satisfactory results. It is found that the shape parameter (E = 0.486) coming out from the present analysis for pyroxene grains is same with that of crystalline olivine grains. Thus the analysis of polarimetric data of comet Levy 1990XX at λ = 0.485 μm shows the presence of both olivine and pyroxene grains in that comet.