43 research outputs found

### Root mean squares of distance and geodesic between two constituent particles within fractal aggregates prepared by BCCA, DLA, and GSAW procedures

Understanding the geodesic properties of fractal aggregates is essential, as
their thermal and mechanical properties are characterized by their geodesics.
In this study, we investigate the root mean square (RMS) of the geodesic
between two constituent particles within fractal aggregates prepared by
ballistic cluster-cluster aggregation (BCCA), diffusion-limited aggregation
(DLA), and growing self-avoiding walk (GSAW) processes in two- and
three-dimensional spaces. We find that the dependence of the RMS of the
geodesic on the number of constituent particles is given by the following
equation: $N \approx k_{\rm g} {D_{\rm RMS}}^{d_{\rm g}}$, where $N$ is the
number of constituent particles and $D_{\rm RMS}$ is the RMS of the geodesic.
We numerically obtain the prefactor $k_{\rm g}$ and exponent $d_{\rm g}$ for
these fractal aggregates. We name the exponent ``the geodesic dimension'', and
it is compared with the fractal dimension. Our findings show that the
difference between fractal and geodesic dimensions varies significantly
depending on the preparation procedure for fractals.Comment: 8 pages, 15 figures. Accepted for publication in JPS

### Thermal conductivity of porous aggregates

$\mathit{Context.}$ The thermal conductivity of highly porous dust aggregates
is a key parameter for many subjects in planetary science; however, it is not
yet fully understood. $\mathit{Aims.}$ In this study, we investigate the
thermal conductivity of fluffy dust aggregates with filling factors of less
than $10^{-1}$. $\mathit{Methods.}$ We determine the temperature structure and
heat flux of the porous dust aggregates calculated by $N$-body simulations of
static compression in the periodic boundary condition. $\mathit{Results.}$ We
derive an empirical formula for the thermal conductivity through the solid
network $k_{\rm sol}$ as a function of the filling factor of dust aggregates
$\phi$. The results reveal that $k_{\rm sol}$ is approximately proportional to
${\phi}^{2}$, and the thermal conductivity through the solid network is
significantly lower than previously assumed. In light of these findings, we
must reconsider the thermal histories of small planetary bodies.Comment: 4 pages, 4 figures. Accepted for publication in Astronomy &
Astrophysic

### Survivability of Amorphous Ice in Comets Depends on the Latent Heat of Crystallization of Impure Water Ice

Comets would have amorphous ice rather than crystalline one at the epoch of
their accretion. Cometary ice contains some impurities that govern the latent
heat of ice crystallization, $L_{\rm cry}$. However, it is still controversial
whether the crystallization process is exothermic or endothermic. In this
study, we perform one-dimensional simulations of the thermal evolution of
km-sized comets and investigate the effect of the latent heat. We find that the
depth where amorphous ice can survive significantly depends on the latent heat
of ice crystallization. Assuming the cometary radius of 2 km, the depth of the
amorphous ice mantle is approximately 100 m when the latent heat is positive
(i.e., the exothermic case with $L_{\rm cry} = + 9 \times 10^{4}$ J/kg). In
contrast, when we consider the impure ice representing the endothermic case
with $L_{\rm cry} = - 9 \times 10^{4}$ J/kg, the depth of the amorphous ice
mantle could exceed 1 km. Although our numerical results indicate that these
depths depend on the size and the accretion age of comets, the depth in a comet
with the negative latent heat is a few to several times larger than the
positive case for a given comet size. This work suggests that the spatial
distribution of the ice crystallinity in a comet nucleus depends on the latent
heat, which can be different from the previous estimates assuming pure water
ice.Comment: 15 pages, 10 figures. Accepted for publication in PAS

### On the Number of Stars in the Sun's Birth Cluster

The Sun is thought to be formed within a star cluster. The coexistence of
$^{26}{\rm Al}$-rich and $^{26}{\rm Al}$-poor calcium--aluminum-rich inclusions
indicates that a direct injection of $^{26}{\rm Al}$-rich materials from a
nearby core-collapse supernova should occur in the first $10^5$ years of the
solar system. Therefore, at least one core-collapse supernova should occur
within the duration of star formation in the Sun's birth cluster. Here we
revisit the number of stars in the Sun's birth cluster from the point of view
of the probability for acquiring at least one core-collapse supernova within
the finite duration of star formation in the birth cluster. We find that the
number of stars in the birth cluster can be significantly larger than that
previously considered, depending on the duration of star formation.Comment: 8 pages, 6 figures. Accepted for publication in A&