52 research outputs found
Dust coagulation and fragmentation in molecular clouds. I. How collisions between dust aggregates alter the dust size distribution
In dense molecular clouds collisions between dust grains alter the ISM-dust
size distribution. We study this process by inserting the results from detailed
numerical simulations of two colliding dust aggregates into a coagulation model
that computes the dust size distribution with time. All collisional outcomes --
sticking, fragmentation (shattering, breakage, and erosion) -- are included and
the effects on the internal structure of the aggregates are also tabulated. The
dust aggregate evolution model is applied to an homogeneous and static cloud of
temperature 10 K and gas densities between 10^3 and 10^7 cm^-3. The coagulation
is followed locally on timescales of ~10^7 yr. We find that the growth can be
divided into two stages: a growth dominated phase and a fragmentation dominated
phase. Initially, the mass distribution is relatively narrow and shifts to
larger sizes with time. At a certain point, dependent on the material
properties of the grains as well as on the gas density, collision velocities
will become sufficiently energetic to fragment particles, halting the growth
and replenishing particles of lower mass. Eventually, a steady state is
reached, where the mass distribution is characterized by a mass spectrum of
approximately equal amount of mass per logarithmic size bin. The amount of
growth that is achieved depends on the cloud's lifetime. If clouds exist on
free-fall timescales the effects of coagulation on the dust size distribution
are very minor. On the other hand, if clouds have long-term support mechanisms,
the impact of coagulation is important, resulting in a significant decrease of
the opacity on timescales longer than the initial collision timescale between
big grains.Comment: 25 pages, 20 figures, accepted for publication in Astronomy &
Astrophysic
Numerical determination of the material properties of porous dust cakes
The formation of planetesimals requires the growth of dust particles through
collisions. Micron-sized particles must grow by many orders of magnitude in
mass. In order to understand and model the processes during this growth, the
mechanical properties, and the interaction cross sections of aggregates with
surrounding gas must be well understood. Recent advances in experimental
(laboratory) studies now provide the background for pushing numerical aggregate
models onto a new level. We present the calibration of a previously tested
model of aggregate dynamics. We use plastic deformation of surface asperities
as the physical model to bring critical velocities for sticking into accordance
with experimental results. The modified code is then used to compute
compression strength and the velocity of sound in the aggregate at different
densities. We compare these predictions with experimental results and conclude
that the new code is capable of studying the properties of small aggregates.Comment: Accepted for publication in A&
The influence of grain rotation on the structure of dust aggregates
We study the effect of rotation during the collision between dust aggregates,
in order to address a mismatch between previous model calculations of Brownian
motion driven aggregation and experiments. We show that rotation during the
collision does influence the shape and internal structure of the aggregates
formed. The effect is limited in the ballistic regime when aggregates can be
considered to move on straight lines during a collision. However, if the
stopping length of an aggregate becomes smaller than its physical size,
extremely elongated aggregates can be produced. We show that this effect may
have played a role in the inner regions of the solar nebula where densities
were high.Comment: 15 pages, 6 figures, accepted for publication in Icarus, typos
correcte
Coagulation and Fragmentation in molecular clouds. II. The opacity of the dust aggregate size distribution
The dust size distribution in molecular clouds can be strongly affected by
ice-mantle formation and (subsequent) grain coagulation. Following previous
work where the dust size distribution has been calculated from a state-of-the
art collision model for dust aggregates that involves both coagulation and
fragmentation (Paper I), the corresponding opacities are presented in this
study. The opacities are calculated by applying the effective medium theory
assuming that the dust aggregates are a mix of 0.1{\mu}m silicate and graphite
grains and vacuum. In particular, we explore how the coagulation affects the
near-IR opacities and the opacity in the 9.7{\mu}m silicate feature. We find
that as dust aggregates grow to {\mu}m-sizes both the near-IR color excess and
the opacity in the 9.7 {\mu}m feature increases. Despite their coagulation,
porous aggregates help to prolong the presence of the 9.7{\mu}m feature. We
find that the ratio between the opacity in the silicate feature and the near-IR
color excess becomes lower with respect to the ISM, in accordance with many
observations of dark clouds. However, this trend is primarily a result of ice
mantle formation and the mixed material composition of the aggregates, rather
than being driven by coagulation. With stronger growth, when most of the dust
mass resides in particles of size 10{\mu}m or larger, both the near-IR color
excess and the 9.7{\mu}m silicate feature significantly diminish. Observations
at additional wavelengths, in particular in the sub-mm range, are essential to
provide quantitative constraints on the dust size distribution within dense
cores. Our results indicate that the sub-mm index {\beta} will increase
appreciably, if aggregates grow to ~100{\mu}m in size.Comment: 10 pages, accepted for publication in A&
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