<jats:title>Abstract</jats:title>
<jats:p>We report on a theoretical study of the geometrical structure of porous dust aggregates formed via ballistic cluster–cluster aggregation (BCCA). We calculated the gyration radius $R_{\rm gyr}$ and the graph-based geodesic radius $R_{\rm geo}$ as a function of the number of constituent particles $N$. We found that $R_{\rm gyr} / r_{0} \sim N^{0.531 \pm 0.011}$ and $R_{\rm geo} / r_{0} \sim N^{0.710 \pm 0.013}$, where $r_{0}$ is the radius of the constituent particles. Furthermore, we defined two constants that characterize the geometrical structure of fractal aggregates: $D_{\rm f}$ and $\alpha$. The definitions of $D_{\rm f}$ and $\alpha$ are $N \sim {( R_{\rm gyr} / r_{0} )}^{D_{\rm f}}$ and ${R_{\rm geo}} / {r_{0}} \sim {\left( {R_{\rm gyr}} / {r_{0}} \right)}^{\alpha}$, respectively. Our study revealed that $D_{\rm f} \simeq 1.88$ and $\alpha \simeq 1.34$ for the clusters of the BCCA.</jats:p>
<jats:p>In addition, we also studied the filling factor dependence of the thermal conductivity of statically compressed fractal aggregates. From this study we reveal that the thermal conductivity of statically compressed aggregates $k$ is given by $k \sim 2 k_{\rm mat} {( r_{\rm c} / r_{0} )} \phi^{(1 + \alpha) / (3 - D_{\rm f})}$, where $k_{\rm mat}$ is the material thermal conductivity, $r_{\rm c}$ is the contact radius of the constituent particles, and $\phi$ is the filling factor of the dust aggregates.</jats:p>
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