11,184 research outputs found
Investigating the rotational evolution of young, low mass stars using Monte Carlo simulations
We investigate the rotational evolution of young stars through Monte Carlo
simulations. We simulate 280,000 stars, each of which is assigned a mass, a
rotational period, and a mass accretion rate. The mass accretion rate depends
on mass and time, following power-laws indices 1.4 and -1.5, respectively. A
mass-dependent accretion threshold is defined below which a star is considered
as diskless, which results in a distribution of disk lifetimes that matches
observations. Stars are evolved at constant angular spin rate while accreting
and at constant angular momentum when they become diskless. We recover the
bimodal period distribution seen in several young clusters. The short period
peak consists mostly of diskless stars and the long period one is mainly
populated by accreting stars. Both distributions present a long tail towards
long periods and a population of slowly rotating diskless stars is observed at
all ages. We reproduce the observed correlations between disk fraction and spin
rate, as well as between IR excess and rotational period. The period-mass
relation we derive from the simulations exhibits the same global trend as
observed in young clusters only if we release the disk locking assumption for
the lowest mass stars. We find that the time evolution of median specific
angular momentum follows a power law index of -0.65 for accreting stars and of
-0.53 for diskless stars, a shallower slope that results from a wide
distribution of disk lifetimes. Using observationally-documented distributions
of disk lifetimes, mass accretion rates, and initial rotation periods, and
evolving an initial population from 1 to 12 Myr, we reproduce the main
characteristics of pre-main sequence angular momentum evolution, which supports
the disk locking hypothesis. (abridged)Comment: 11 pages, 14 figures, accepted for publication in A&
Vortex motion around a circular cylinder above a plane
The study of vortex flows around solid obstacles is of considerable interest
from both a theoretical and practical perspective. One geometry that has
attracted renewed attention recently is that of vortex flows past a circular
cylinder placed above a plane wall, where a stationary recirculating eddy can
form in front of the cylinder, in contradistinction to the usual case (without
the plane boundary) for which a vortex pair appears behind the cylinder. Here
we analyze the problem of vortex flows past a cylinder near a wall through the
lenses of the point-vortex model. By conformally mapping the fluid domain onto
an annular region in an auxiliary complex plane, we compute the vortex
Hamiltonian analytically in terms of certain special functions related to
elliptic theta functions. A detailed analysis of the equilibria of the model is
then presented. The location of the equilibrium in front of the cylinder is
shown to be in qualitative agreement with the experimental findings. We also
show that a topological transition occurs in phase space as the parameters of
the systems are variedComment: 17 pages, 8 figure
Doubly-periodic array of bubbles in a Hele-Shaw cell
Exact solutions are presented for a doubly-periodic array of steadily moving
bubbles in a Hele-Shaw cell when surface tension is neglected. It is assumed
that the bubbles either are symmetrical with respect to the channel centreline
or have fore-and-aft symmetry, or both, so that the relevant flow domain can be
reduced to a simply connected region. By using conformal mapping techniques, a
general solution with any number of bubbles per unit cell is obtained in
integral form. Several examples are given, including solutions for multi-file
arrays of bubbles in the channel geometry and doubly-periodic solutions in an
unbounded cell.Comment: 15 pages, 12 figure
Multi-strange particle production in relativistic heavy ion collisions at GeV
We present preliminary STAR results on measurements of multi-strange
particles , and their anti-particles from Au+Au and Cu+Cu at
GeV collisions. In order to better understand the role of
strangeness enhancement in nucleus-nucleus collisions and its scaling
properties with system size, we compare the results from Au+Au and Cu+Cu
reactions for different event centrality classes. Strangeness enhancement is
discussed in the context of multi-strange to pion ratios. Finally,
ratio is shown for different systems and energies for a
systematic study
Meat yield of Bolinus brandaris (Gastropoda: Muricidae): comparative assessment of the influence of sex, size and reproductive status
The present study assessed the influence of sex, size and reproductive status on the meat yield (soft tissues proportion) of the purple dye murex (Bolinus brandaris) from the Ria Formosa lagoon (southern Portugal). During one year of monthly sampling (October 2008-September 2009), average meat yield of B. brandaris was 40.5 +/- 6.1% (range: 25.8-56.1% wet weight), with no significant differences between sexes. Relationships established between specimen size and soft parts weight indicated that both shell length and total weight are excellent indicators of meat yield. Significant differences in meat yield between size classes further reinforced the trend of increasing meat yield during ontogeny. Meat yield exhibited significant monthly variation and a similar temporal trend in both sexes, which were directly related to the reproductive status. Meat yield of B. brandaris was compared with that of other muricid species and the marked influence of the reproductive status on meat yield prompted a comparative assessment of the spawning season and peak of three sympatric muricids (B. brandaris, Hexaplex trunculus and Stramonita haemastoma). Overall, these findings have implications at diverse levels, including the management, regulation and inspection of this fishing/ harvesting activity and the commercialization and consumption of this seafood product.postdoctoral grant [SFRH/BPD/26348/2006]; Fundacao para a Ciencia e Tecnologia (FCT - Portugal); Fisheries Operational Programme (PROMAR); European Fisheries Fund [EFF 2007-2013]info:eu-repo/semantics/publishedVersio
Transmission Of Optical Coherent State Qubits
We discuss the long distance transmission of qubits encoded in optical
coherent states. Through absorption these qubits suffer from two main types of
errors, the reduction of the amplitude of the coherent states and accidental
application of the Pauli Z operator. We show how these errors can be fixed
using techniques of teleportation and error correcting codes.Comment: Added two pages of explanation/background/review material to increase
readability and clarity. Corrected minor typographical and linguistic error
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