9 research outputs found
Numerical Modeling of the Coagulation and Porosity Evolution of Dust Aggregates
Porosity evolution of dust aggregates is crucial in understanding dust
evolution in protoplanetary disks. In this study, we present useful tools to
study the coagulation and porosity evolution of dust aggregates. First, we
present a new numerical method for simulating dust coagulation and porosity
evolution as an extension of the conventional Smoluchowski equation. This
method follows the evolution of the mean porosity for each aggregate mass
simultaneously with the evolution of the mass distribution function. This
method reproduces the results of previous Monte Carlo simulations with much
less computational expense. Second, we propose a new collision model for porous
dust aggregates on the basis of our N-body experiments on aggregate collisions.
We first obtain empirical data on porosity changes between the classical limits
of ballistic cluster-cluster and particle-cluster aggregation. Using the data,
we construct a recipe for the porosity change due to general hit-and-stick
collisions as well as formulae for the aerodynamical and collisional cross
sections. Simple coagulation simulations using the extended Smoluchowski method
show that our collision model explains the fractal dimensions of porous
aggregates observed in a full N-body simulation and a laboratory experiment.
Besides, we discover that aggregates at the high-mass end of the distribution
can have a considerably small aerodynamical cross section per unit mass
compared with aggregates of lower masses. We point out an important implication
of this discovery for dust growth in protoplanetary disks.Comment: 17 pages, 15 figures; v2: version to appear in ApJ (typos corrected
Electrostatic Barrier against Dust Growth in Protoplanetary Disks. I. Classifying the Evolution of Size Distribution
Collisional growth of submicron-sized dust grains into macroscopic aggregates
is the first step of planet formation in protoplanetary disks. These grains are
expected to carry nonzero negative charges in the weakly ionized disks, but its
effect on their collisional growth has not been fully understood so far. In
this paper, we investigate how the charging affects the evolution of the dust
size distribution properly taking into account the charging mechanism in a
weakly ionized gas as well as porosity evolution through low-energy collisions.
To clarify the role of the size distribution, we divide our analysis into two
steps. First, we analyze the collisional growth of charged aggregates assuming
a monodisperse (i.e., narrow) size distribution. We show that the monodisperse
growth stalls due to the electrostatic repulsion when a certain condition is
met, as is already expected in the previous work. Second, we numerically
simulate dust coagulation using Smoluchowski's method to see how the outcome
changes when the size distribution is allowed to freely evolve. We find that,
under certain conditions, the dust undergoes bimodal growth where only a
limited number of aggregates continue to grow carrying the major part of the
dust mass in the system. This occurs because remaining small aggregates
efficiently sweep up free electrons to prevent the larger aggregates from being
strongly charged. We obtain a set of simple criteria that allows us to predict
how the size distribution evolves for a given condition. In Paper II
(arXiv:1009.3101), we apply these criteria to dust growth in protoplanetary
disks.Comment: 20 pages, 22 figures, accepted for publication in Ap
Electrostatic Barrier against Dust Growth in Protoplanetary Disks. II. Measuring the Size of the "Frozen" Zone
Coagulation of submicron-sized dust grains into porous aggregates is the
initial step of dust evolution in protoplanetary disks. Recently, it has been
pointed out that negative charging of dust in the weakly ionized disks could
significantly slow down the coagulation process. In this paper, we apply the
growth criteria obtained in Paper I to finding out a location ("frozen" zone)
where the charging stalls dust growth at the fractal growth stage. For
low-turbulence disks, we find that the frozen zone can cover the major part of
the disks at a few to 100 AU from the central star. The maximum mass of the
aggregates is approximately 10^{-7} g at 1 AU and as small as a few monomer
masses at 100 AU. Strong turbulence can significantly reduce the size of the
frozen zone, but such turbulence will cause the fragmentation of macroscopic
aggregates at later stages. We examine a possibility that complete freezeout of
dust evolution in low-turbulence disks could be prevented by global transport
of dust in the disks. Our simple estimation shows that global dust transport
can lead to the supply of macroscopic aggregates and the removal of frozen
aggregates on a timescale of 10^6 yr. This overturns the usual understanding
that tiny dust particles get depleted on much shorter timescales unless
collisional fragmentation is effective. The frozen zone together with global
dust transport might explain "slow" (\sim 10^6 yr) dust evolution suggested by
infrared observation of T Tauri stars and by radioactive dating of chondrites.Comment: 14 pages, 13 figures, accepted for publication in Ap
Prolyl Isomerase Pin1 Suppresses Thermogenic Programs in Adipocytes by Promoting Degradation of Transcriptional Co-activator PRDM16
Summary: Non-shivering thermogenesis in adipocytes provides defense against low temperatures and obesity development, but the underlying regulatory mechanism remains to be fully clarified. Based on both markedly increased Pin1 expression in states of excess nutrition and resistance to obesity development in Pin1 null mice, we speculated that adipocyte Pin1 may play a role in thermogenic programs. Adipose-specific Pin1 knockout (adPin1 KO) mice showed enhanced transcription of thermogenic genes and tolerance to hypothermia when exposed to cold. In addition, adPin1 KO mice were resistant to high-fat diet-induced obesity and glucose intolerance. A series of experiments revealed that Pin1 binds to PRDM16 and thereby promotes its degradation through the ubiquitin-proteasome system. Consistent with these results, Pin1 deletion in differentiated adipocytes showed enhancement of thermogenic programs in response to the β3 agonist CL316243 through the upregulation of PRDM16 proteins. These observations indicate that Pin1 is a negative regulator of non-shivering thermogenesis. : Adipose Pin1 expression increases in obese mice. Pin1 associates with PRDM16 and promotes its degradation, resulting in the downregulation of UCP-1. Pin1 KO mice are resistant to obesity development and cold exposure-induced hypothermia. Thus, Pin1 is a negative regulator of thermogenesis and could be a target of obesity. Keywords: Pin1, PRDM16, UCP-1, thermogenesis, obesit