9,155 research outputs found
From Dust To Planetesimal: The Snowball Phase ?
The standard model of planet formation considers an initial phase in which
planetesimals form from a dust disk, followed by a phase of mutual
planetesimal-planetesimal collisions, leading eventually to the formation of
planetary embryos. However, there is a potential transition phase (which we
call the "snowball phase"), between the formation of the first planetesimals
and the onset of mutual collisions amongst them, which has often been either
ignored or underestimated in previous studies. In this snowball phase, isolated
planetesimals move on Keplerian orbits and grow solely via the direct accretion
of sub-cm sized dust entrained with the gas in the protoplanetary disk. Using a
simplified model in which planetesimals are progressively produced from the
dust, we consider the expected sizes to which the planetesimals can grow before
mutual collisions commence and derive the dependence of this size on a number
of critical parameters, including the degree of disk turbulence, the
planetesimal size at birth and the rate of planetesimal creation. For systems
in which turbulence is weak and the planetesimals are created at a low rate and
with relatively small birth size, we show that the snowball growth phase can be
very important, allowing planetesimals to grow by a factor of 10^6 in mass
before mutual collisions take over. In such cases, the snowball growth phase
can be the dominant mode to transfer mass from the dust to planetesimals.
Moreover, such growth can take place within the typical lifetime of a
protoplanetary gas disk. A noteworthy result is that ... ...(see the paper).
For the specific case of close binaries such as Alpha Centauri ... ... (see the
paper). From a more general perspective, these preliminary results suggest that
an efficient snowball growth phase provides a large amount of "room at the
bottom" for theories of planet formation.Comment: Accepted for publication in the Astrophysical Journal. 15 pages, 4
figures, 1 tabl
Molecular Dynamics Simulation of Macromolecules Using Graphics Processing Unit
Molecular dynamics (MD) simulation is a powerful computational tool to study
the behavior of macromolecular systems. But many simulations of this field are
limited in spatial or temporal scale by the available computational resource.
In recent years, graphics processing unit (GPU) provides unprecedented
computational power for scientific applications. Many MD algorithms suit with
the multithread nature of GPU. In this paper, MD algorithms for macromolecular
systems that run entirely on GPU are presented. Compared to the MD simulation
with free software GROMACS on a single CPU core, our codes achieve about 10
times speed-up on a single GPU. For validation, we have performed MD
simulations of polymer crystallization on GPU, and the results observed
perfectly agree with computations on CPU. Therefore, our single GPU codes have
already provided an inexpensive alternative for macromolecular simulations on
traditional CPU clusters and they can also be used as a basis to develop
parallel GPU programs to further speedup the computations.Comment: 21 pages, 16 figure
Valley Carrier Dynamics in Monolayer Molybdenum Disulphide from Helicity Resolved Ultrafast Pump-probe Spectroscopy
We investigate the valley related carrier dynamics in monolayer MoS2 using
helicity resolved non-degenerate ultrafast pump-probe spectroscopy at the
vicinity of the high-symmetry K point under the temperature down to 78 K.
Monolayer MoS2 shows remarkable transient reflection signals, in stark contrast
to bilayer and bulk MoS2 due to the enhancement of many-body effect at reduced
dimensionality. The helicity resolved ultrafast time-resolved result shows that
the valley polarization is preserved for only several ps before scattering
process makes it undistinguishable. We suggest that the dynamical degradation
of valley polarization is attributable primarily to the exciton trapping by
defect states in the exfoliated MoS2 samples. Our experiment and a
tight-binding model analysis also show that the perfect valley CD selectivity
is fairly robust against disorder at the K point, but quickly decays from the
high-symmetry point in the momentum space in the presence of disorder.Comment: 15 pages,Accepted by ACS Nan
Soil, wheat, cabbage and drinking water iodine in relation to human iodine status and iodine deficiency disorders in Xinjiang Province, China
Iodine is an essential trace element for humans and animals. A lack in the diet can
lead to iodine deficiency disorders (IDD) the most common manifestation being
goitre, an enlargement of the thyroid gland in the neck. Infants born to severely Ideficient
mothers may suffer cretinism and I-deficiency is the world's most common
cause of preventable mental retardation. In many countries this problem has been
tackled successfully using medical interventions such as the iodination of table salt.
Xinjiang Province in northwest China is a remote desert region where goitre and
cretinism have been reported for many years. People in this region do not like the
taste of iodised salt and prefer to use local rock-salt with very low concentrations of
iodine. As an alternative treatment, previous investigators added potassium-iodate to
irrigation waters in an attempt to increase the I-concentrations of crops and animals in
the food supply and the I-status of the population. Initial successes were reported but
the long-term effectiveness of the method had not been tested. The present study aims
to assess environmental controls on iodine uptake into the food chain and in Xinjiang
had the opportunity to study three contrasting area
List Decodability at Small Radii
, the smallest for which every binary error-correcting code
of length and minimum distance is decodable with a list of size
up to radius , is determined for all . As a result,
is determined for all , except for 42 values of .Comment: to appear in Designs, Codes, and Cryptography (accepted October 2010
Fundamental Performance of a Dispersed Fixed Delay Interferometer In Searching For Planets Around M Dwarfs
We present a new method to calculate fundamental Doppler measurement limits
with a dispersed fixed-delay interferometer (DFDI) in the near infrared
wavelength region for searching for exoplanets around M dwarfs in the coming
decade. It is based on calculating the Q factor, a measure of flux-normalized
Doppler sensitivity in the fringing spectra created with DFDI. We calculate the
Q factor as a function of spectral resolution R, stellar projected rotational
velocity V sini, stellar effective temperature T_eff and optical path
difference (OPD) of the interferometer. We also compare the DFDI Q factor to
that for the popular cross-dispersed echelle spectrograph method (the direct
echelle (DE) method). Given the IR Doppler measurement is likely to be
detector-limited for a while, we introduce new merit functions, which is
directly related to photon-limited RV uncertainty, to evaluate Doppler
performance with the DFDI and DE methods. We find that DFDI has strength in
wavelength coverage and multi-object capability over the DE for a limited
detector resource. We simulate the performance of the InfraRed Exoplanet
Tracker (IRET) based on the DFDI design, being considered for the next
generation IR Doppler measurements. The predicted photon-limited RV uncertainty
suggests that IRET is capable of detecting Earth-like exoplanets in habitable
zone around nearby bright M dwarfs if they exist. A new method is developed to
quantitatively estimate the influence of telluric lines on RV uncertainty. Our
study shows that photon-limited RV uncertainty can be reached if 99% of the
strength of telluric lines can be removed from the measured stellar spectra. At
low to moderate levels of telluric line strength removal (50% to 90%), the
optimal RV uncertainty is typically a factor of 2-3 times larger than
photon-limited RV uncertainty.Comment: 43 pages, 20 figures, 6 tables. Accepted by Ap
Observation of electric current induced by optically injected spin current
A normally incident light of linear polarization injects a pure spin current
in a strip of 2-dimensional electron gas with spin-orbit coupling. We report
observation of an electric current with a butterfly-like pattern induced by
such a light shed on the vicinity of a crossbar shaped InGaAs/InAlAs quantum
well. Its light polarization dependence is the same as that of the spin
current. We attribute the observed electric current to be converted from the
optically injected spin current caused by scatterings near the crossing. Our
observation provides a realistic technique to detect spin currents, and opens a
new route to study the spin-related science and engineering in semiconductors.Comment: 15 pages, 4 figure
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