5,056 research outputs found
Flows, Fragmentation, and Star Formation. I. Low-mass Stars in Taurus
The remarkably filamentary spatial distribution of young stars in the Taurus
molecular cloud has significant implications for understanding low-mass star
formation in relatively quiescent conditions. The large scale and regular
spacing of the filaments suggests that small-scale turbulence is of limited
importance, which could be consistent with driving on large scales by flows
which produced the cloud. The small spatial dispersion of stars from gaseous
filaments indicates that the low-mass stars are generally born with small
velocity dispersions relative to their natal gas, of order the sound speed or
less. The spatial distribution of the stars exhibits a mean separation of about
0.25 pc, comparable to the estimated Jeans length in the densest gaseous
filaments, and is consistent with roughly uniform density along the filaments.
The efficiency of star formation in filaments is much higher than elsewhere,
with an associated higher frequency of protostars and accreting T Tauri stars.
The protostellar cores generally are aligned with the filaments, suggesting
that they are produced by gravitational fragmentation, resulting in initially
quasi-prolate cores. Given the absence of massive stars which could strongly
dominate cloud dynamics, Taurus provides important tests of theories of
dispersed low-mass star formation and numerical simulations of molecular cloud
structure and evolution.Comment: 32 pages, 9 figures: to appear in Ap
Predicting Hidden Links in Supply Networks
Manufacturing companies often lack visibility of the procurement interdependencies between the suppliers within their supply network. However, knowledge of these interdependencies is useful to plan for potential operational disruptions. In this paper, we develop the Supply Network Link Predictor (SNLP) method to infer supplier interdependencies using the manufacturer’s incomplete knowledge of the network.
SNLP uses topological data to extract relational features from the known network to train a classifier for predicting potential links. Using a test case from the automotive industry, four features are extracted: (i) number of existing supplier links, (ii) overlaps between supplier product portfolios, (iii) product outsourcing associations, and (iv) likelihood of buyers purchasing from two suppliers together. Naïve Bayes and Logistic Regression are then employed to predict whether these features can help predict interdependencies between two suppliers.
Our results show that these features can indeed be used to predict interdependencies in the network and that predictive accuracy is maximized by (i) and (iii). The findings give rise to the exciting possibility of using data analytics for improving supply chain visibility. We then proceed to discuss to what extent such approaches can be adopted and their limitations, highlighting next steps for future work in this area
Modeling two-language competition dynamics
During the last decade, much attention has been paid to language competition
in the complex systems community, that is, how the fractions of speakers of
several competing languages evolve in time. In this paper we review recent
advances in this direction and focus on three aspects. First we consider the
shift from two-state models to three state models that include the possibility
of bilingual individuals. The understanding of the role played by bilingualism
is essential in sociolinguistics. In particular, the question addressed is
whether bilingualism facilitates the coexistence of languages. Second, we will
analyze the effect of social interaction networks and physical barriers.
Finally, we will show how to analyze the issue of bilingualism from a game
theoretical perspective.Comment: 15 pages, 5 figures; published in the Special Issue of Advances in
Complex Systems "Language Dynamics
The Chromo-Dielectric Soliton Model: Quark Self Energy and Hadron Bags
The chromo-dielectric soliton model (CDM) is Lorentz- and chirally-invariant.
It has been demonstrated to exhibit dynamical chiral symmetry breaking and
spatial confinement in the locally uniform approximation. We here study the
full nonlocal quark self energy in a color-dielectric medium modeled by a two
parameter Fermi function. Here color confinement is manifest. The self energy
thus obtained is used to calculate quark wave functions in the medium which, in
turn, are used to calculate the nucleon and pion masses in the one gluon
exchange approximation. The nucleon mass is fixed to its empirical value using
scaling arguments; the pion mass (for massless current quarks) turns out to be
small but non-zero, depending on the model parameters.Comment: 24 pages, figures available from the author
On the interaction of a single-photon wave packet with an excited atom
The interaction of a single-photon wave packet with an initially excited
two-level atom in free space is studied in semiclassical and quantum
approaches. It is shown that the final state of the field does not contain
doubly occupied modes. The process of the atom's transition to the ground state
may be accelerated, decelerated or even reversed by the incoming photon,
depending on parameters. The spectrum of emitted radiation is close to the sum
of the spectrum of the incoming single-photon wave packet and the natural line
shape, with small and complicated deviations.Comment: 17 pages, 5 figure
New triple systems in the RasTyc sample of stellar X-ray sources
During the study of a large set of late-type stellar X-ray sources, we
discovered a large fraction of multiple systems. In this paper we investigate
the orbital elements and kinematic properties of three new spectroscopic triple
systems as well as spectral types and astrophysical parameters (T_eff, log g,
vsin i, log N(Li)) of their components. We conducted follow-up optical
observations, both photometric and spectroscopic at high resolution, of these
systems. We used a synthetic approach and the cross-correlation method to
derive most of the stellar parameters. We estimated reliable radial velocities
and deduced the orbital elements of the inner binaries. The comparison of the
observed spectra with synthetic composite ones, obtained as the weighted sum of
three spectra of non-active reference stars, allowed us to determine the
stellar parameters for each component of these systems. We found all are only
composed of main sequence stars. These three systems are certainly stable
hierarchical triples composed of short-period inner binaries plus a tertiary
component in a long-period orbit. From their kinematics and/or Lithium content,
these systems result to be fairly young.Comment: Accepted for publication in A&A (on July 22, 2008
``Superfast'' Reaction in Turbulent Flow with Potential Disorder
We explore the regime of ``superfast'' reactivity that has been predicted to
occur in turbulent flow in the presence of potential disorder. Computer
simulation studies confirm qualitative features of the previous renormalization
group predictions, which were based on a static model of turbulence. New
renormalization group calculations for a more realistic, dynamic model of
turbulence show that the superfast regime persists. This regime, with
concentration decay exponents greater than that for a well-mixed reaction,
appears to be a general result of the interplay among non-linear reaction
kinetics, turbulent transport, and local trapping by potential disorder.Comment: 14 pages. 4 figures. Uses IOP styles. To appear in J. Phys. A: Math.
Ge
A Case Study of Low-Mass Star Formation
This article synthesizes observational data from an extensive program aimed
toward a comprehensive understanding of star formation in a low-mass
star-forming molecular cloud. New observations and published data spanning from
the centimeter wave band to the near infrared reveal the high and low density
molecular gas, dust, and pre-main sequence stars in L1551.Comment: 24 pages, 21 figures, ApJS accepte
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