252 research outputs found
Photophoretic Strength on Chondrules. 2. Experiment
Photophoretic motion can transport illuminated particles in protoplanetary
disks. In a previous paper we focused on the modeling of steady state
photophoretic forces based on the compositions derived from tomography and heat
transfer. Here, we present microgravity experiments which deviate significantly
from the steady state calculations of the first paper. The experiments on
average show a significantly smaller force than predicted with a large
variation in absolute photophoretic force and in the direction of motion with
respect to the illumination. Time-dependent modeling of photophoretic forces
for heat-up and rotation show that the variations in strength and direction
observed can be well explained by the particle reorientation in the limited
experiment time of a drop tower experiment. In protoplanetary disks, random
rotation subsides due to gas friction on short timescales and the results of
our earlier paper hold. Rotation has a significant influence in short duration
laboratory studies. Observing particle motion and rotation under the influence
of photophoresis can be considered as a basic laboratory analog experiment to
Yarkovsky and YORP effects
Fourier Transforms of Lorentz Invariant Functions
Fourier transforms of Lorentz invariant functions in Minkowski space, with
support on both the timelike and the spacelike domains are performed by means
of direct integration. The cases of 1+1 and 1+2 dimensions are worked out in
detail, and the results for 1+n dimensions are given.Comment: 15 pages, 1 figur
Light-induced disassembly of dusty bodies in inner protoplanetary discs: implications for the formation of planets
Laboratory experiments show that a solid-state greenhouse effect in
combination with thermophoresis can efficiently erode a dust bed in a
low-pressure gaseous environment. The surface of an illuminated, light
absorbing dusty body is cooler than the dust below the surface (solidstate
greenhouse effect). This temperature gradient leads to a directed momentum
transfer between gas and dust particles and the dust particles are subject to a
force towards the surface(thermophoresis). If the thermophoretic force is
stronger than gravity and cohesion, dust particles are ejected. Applied to
protoplanetary discs, dusty bodies smaller than several kilometres in size
which are closer to a star than about 0.4 au are subject to a rapid and
complete disassembly to submillimetre size dust aggregates by this process.
While an inward-drifting dusty body is destroyed, the generated dust is not
lost for the disc by sublimation or subsequent accretion on to the star but can
be reprocessed by photophoresis or radiation pressure. Planetesimals cannot
originate through aggregation of dust inside the erosion zone. If objects
larger than several kilometres already exist, they prevail and further grow by
collecting dust from disassembled smaller bodies. The pile-up of solids in a
confined inner region of the disc, in general, boosts the formation of planets.
Erosion is possible in even strongly gas-depleted inner regions as observed for
TW Hya. Reprocessing of dust through light-induced erosion offers one possible
explanation for growth of large cores of gas-poor giant planets in a
gas-starved region as recently found around HD 149026b
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The Janus-faced Nature of miR-22 in Hematopoiesis: Is It an Oncogenic Tumor Suppressor or Rather a Tumor-Suppressive Oncogene?
Turbulence induced collisional velocities and density enhancements: large inertial range results from shell models
To understand the earliest stages of planet formation, it is crucial to be
able to predict the rate and the outcome of dust grains collisions, be it
sticking and growth, bouncing, or fragmentation. The outcome of such collisions
depends on the collision speed, so we need a solid understanding of the rate
and velocity distribution of turbulence-induced dust grain collisions. The rate
of the collisions depends both on the speed of the collisions and the degree of
clustering experienced by the dust grains, which is a known outcome of
turbulence. We evolve the motion of dust grains in simulated turbulence, an
approach that allows a large turbulent inertial range making it possible to
investigate the effect of turbulence on meso-scale grains (millimeter and
centimeter). We find three populations of dust grains: one highly clustered,
cold and collisionless; one warm; and the third "hot". Our results can be fit
by a simple formula, and predict both significantly slower typical collisional
velocities for a given turbulent strength than previously considered, and
modest effective clustering of the collisional populations, easing difficulties
associated with bouncing and fragmentation barriers to dust grain growth.
Nonetheless, the rate of high velocity collisions falls off merely
exponentially with relative velocity so some mid- or high-velocity collisions
will still occur, promising some fragmentation.Comment: 14 pages, 8 figures, 4 tables, Accepted, MNRA
Asteroids Were Born Big
How big were the first planetesimals? We attempt to answer this question by
conducting coagulation simulations in which the planetesimals grow by mutual
collisions and form larger bodies and planetary embryos. The size frequency
distribution (SFD) of the initial planetesimals is considered a free parameter
in these simulations, and we search for the one that produces at the end
objects with a SFD that is consistent with asteroid belt constraints. We find
that, if the initial planetesimals were small (e.g. km-sized), the final SFD
fails to fulfill these constraints. In particular, reproducing the bump
observed at diameter D~100km in the current SFD of the asteroids requires that
the minimal size of the initial planetesimals was also ~100km. This supports
the idea that planetesimals formed big, namely that the size of solids in the
proto-planetary disk ``jumped'' from sub-meter scale to multi-kilometer scale,
without passing through intermediate values. Moreover, we find evidence that
the initial planetesimals had to have sizes ranging from 100 to several 100km,
probably even 1,000km, and that their SFD had to have a slope over this
interval that was similar to the one characterizing the current asteroids in
the same size-range. This result sets a new constraint on planetesimal
formation models and opens new perspectives for the investigation of the
collisional evolution in the asteroid and Kuiper belts as well as of the
accretion of the cores of the giant planets.Comment: Icarus (2009) in pres
The Molecular Clockwork of the Fire Ant Solenopsis invicta
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The ribosome assembly factor Nep1 responsible for Bowen–Conradi syndrome is a pseudouridine-N1-specific methyltransferase
Nep1 (Emg1) is a highly conserved nucleolar protein with an essential function in ribosome biogenesis. A mutation in the human Nep1 homolog causes Bowen–Conradi syndrome—a severe developmental disorder. Structures of Nep1 revealed a dimer with a fold similar to the SPOUT-class of RNA-methyltransferases suggesting that Nep1 acts as a methyltransferase in ribosome biogenesis. The target for this putative methyltransferase activity has not been identified yet. We characterized the RNA-binding specificity of Methanocaldococcus jannaschii Nep1 by fluorescence- and NMR-spectroscopy as well as by yeast three-hybrid screening. Nep1 binds with high affinity to short RNA oligonucleotides corresponding to nt 910–921 of M. jannaschii 16S rRNA through a highly conserved basic surface cleft along the dimer interface. Nep1 only methylates RNAs containing a pseudouridine at a position corresponding to a previously identified hypermodified N1-methyl-N3-(3-amino-3-carboxypropyl) pseudouridine (m1acp3-Ψ) in eukaryotic 18S rRNAs. Analysis of the methylated nucleoside by MALDI-mass spectrometry, HPLC and NMR shows that the methyl group is transferred to the N1 of the pseudouridine. Thus, Nep1 is the first identified example of an N1-specific pseudouridine methyltransferase. This enzymatic activity is also conserved in human Nep1 suggesting that Nep1 is the methyltransferase in the biosynthesis of m1acp3-Ψ in eukaryotic 18S rRNAs
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