Sensitivity of Polarization to Grain Shape: II. Aggregates
arxiv(2024)
摘要
A previous study (Paper I) investigated the polarization properties of a
variety of simple convex grain shapes, some of which were found to be
consistent with the observed polarization properties of interstellar dust from
far-ultraviolet to far-infrared. Here we study the optical properties of 45
non-convex shapes, all aggregates of N equal-sized spheres. We consider
N=2, N=3, and N=256 random aggregates obtained from 3 different
aggregation schemes. We also consider "trimmed" N=256 aggregates obtained by
systematically trimming initially random aggregates to increase either
flattening or elongation. The "macroporosities" of the studied aggregates range
from P_ macro=0.18 (for the N=2 bisphere) to P_
macro≈ 0.85 (for the N=256 "BA" aggregates). The only aggregates
consistent with observations of starlight polarization and polarized thermal
emission are shapes that have been trimmed to increase their asymmetry. If
interstellar grains are high-porosity aggregates, there must be processes
causing extreme elongation or flattening; if not, interstellar grains must be
dominated by fairly compact structures, with at most moderate porosities. The
ratio of polarization in the 10μm silicate feature to starlight
polarization in the optical is shown to be insensitive to porosity and shape.
X-ray scattering may be the best tool to determine the porosity of interstellar
grains. We propose that modest porosities of interstellar grains could be the
result of "photolytic densification". High polarization fractions observed in
some Class-0 cores require processes to reduce porosities and/or increase
asymmetries of aggregates in dense regions.
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