85 research outputs found
Thermal conductivity of porous aggregates
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. In this study, we investigate the
thermal conductivity of fluffy dust aggregates with filling factors of less
than . We determine the temperature structure and
heat flux of the porous dust aggregates calculated by -body simulations of
static compression in the periodic boundary condition. We
derive an empirical formula for the thermal conductivity through the solid
network as a function of the filling factor of dust aggregates
. The results reveal that is approximately proportional to
, 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
Static compression of porous dust aggregates
Context: In protoplanetary disks, dust grains coagulate with each other and
grow to form aggregates. As these aggregates grow by coagulation, their filling
factor \phi decreases down to \phi << 1. However, comets, the remnants of these
early planetesimals, have \phi ~ 0.1. Thus, static compression of porous dust
aggregates is important in planetesimal formation. However, the static
compression strength has been investigated only for relatively high density
aggregates (\phi > 0.1). Aims: We investigate and find the compression strength
of highly porous aggregates (\phi << 1). Methods: We perform three dimensional
N-body simulations of aggregate compression with a particle-particle
interaction model. We introduce a new method of static compression: the
periodic boundary condition is adopted and the boundaries move with low speed
to get closer. The dust aggregate is compressed uniformly and isotropically by
themselves over the periodic boundaries. Results: We empirically derive a
formula of the compression strength of highly porous aggregates (\phi << 1). We
check the validity of the compression strength formula for wide ranges of
numerical parameters, such as the size of initial aggregates, the boundary
speed, the normal damping force, and material. We also compare our results to
the previous studies of static compression in the relatively high density
region (\phi > 0.1) and confirm that our results consistently connect to those
in the high density region. The compression strength formula is also derived
analytically.Comment: 12 pages, 14 figures, accepted for publication in A&
Opacity of fluffy dust aggregates
Context. Dust grains coagulate to form dust aggregates in protoplanetary
disks. Their porosity can be extremely high in the disks. Although disk
emission may come from fluffy dust aggregates, the emission has been modeled
with compact grains. Aims. We aim to reveal the mass opacity of fluffy
aggregates from infrared to millimeter wavelengths with the filling factor
ranging from 1 down to . Methods. We use Mie calculations with an
effective medium theory. The monomers are assumed to be 0.1 sized
grains, which is much shorter than the wavelengths that we focus on. Results.
We find that the absorption mass opacity of fluffy aggregates are characterized
by the product , where is the dust radius and is the filling
factor, except for the interference structure. The scattering mass opacity is
also characterized by at short wavelengths while it is higher in more
fluffy aggregates at long wavelengths. We also derive the analytic formula of
the mass opacity and find that it reproduces the Mie calculations. We also
calculate the expected difference of the emission between compact and fluffy
aggregates in protoplanetary disks with a simple dust growth and drift model.
We find that compact grains and fluffy aggregates can be distinguished by the
radial distribution of the opacity index . The previous observation of
the radial distribution of is consistent with the fluffy case, but more
observations are required to distinguish between fluffy or compact. In
addition, we find that the scattered light would be another way to distinguish
between compact grains and fluffy aggregates.Comment: 16 pages, 17 figures, published in A&A, 568, A4
Constraints on the dust size distributions in the HD 163296 disk from the difference of the apparent dust ring widths between two ALMA Bands
The dust size in protoplanetary disks is a crucial parameter for
understanding planet formation, while the observational constraints on dust
size distribution have large uncertainties. In this study, we present a new
method to constrain the dust size distribution from the dust spatial
distribution, utilizing the fact that larger dust grains are more spatially
localized. We analyze the ALMA Band 6 (1.25 mm) and Band 4 (2.14 mm)
high-resolution images and constrain the dust size distribution in the two
rings of the HD 163296 disk. We find that the outer ring at 100 au appears
narrower at the longer wavelengths, while the inner ring at 67 au appears to
have similar widths across the two wavelengths. We model dust rings trapped at
gas pressure maxima, where the dust grains follow a power-law size
distribution, and the dust grains of a specific size follow a Gaussian spatial
distribution with the width depending on the grain size. By comparing the
observations with the models, we constrain the maximum dust size
and the exponent of the dust size distribution . We
constrain that and
and in the outer ring. The larger maximum dust size in the
outer ring implies a spatial dependency in dust growth, potentially influencing
the formation location of the planetesimals. We further discuss the turbulence
strength derived from the constrained dust spatial distribution,
assuming equilibrium between turbulent diffusion and accumulation of dust
grains.Comment: 29 pages, 16 figures. Accepted for publication in Ap
Effect of dust size and structure on scattered light images of protoplanetary discs
We study scattered light properties of protoplanetary discs at near-infrared
wavelengths for various dust size and structure by performing radiative
transfer simulations. We show that different dust structures might be probed by
measuring disk polarisation fraction as long as the dust radius is larger than
the wavelength. When the radius is larger than the wavelength, disc scattered
light will be highly polarised for highly porous dust aggregates, whereas more
compact dust structure tends to show low polarisation fraction. Next, roles of
monomer radius and fractal dimension for scattered light colours are studied.
We find that, outside the Rayleigh regime, as fractal dimension or monomer
radius increases, colours of the effective albedo at near-infrared wavelengths
vary from blue to red. Our results imply that discs showing grey or slightly
blue colours and high polarisation fraction in near-infrared wavelengths might
be explained by the presence of large porous aggregates containing sub-microns
sized monomers.Comment: Accepted for publication in MNRAS, 18 pages, 19 figure
- …