9,653 research outputs found
The role of low-mass star clusters in massive star formation. The Orion Case
To distinguish between the different theories proposed to explain massive
star formation, it is crucial to establish the distribution, the extinction,
and the density of low-mass stars in massive star-forming regions. We analyze
deep X-ray observations of the Orion massive star-forming region using the
Chandra Orion Ultradeep Project (COUP) catalog. We studied the stellar
distribution as a function of extinction, with cells of 0.03 pc x 0.03 pc, the
typical size of protostellar cores. We derived stellar density maps and
calculated cluster stellar densities. We found that low-mass stars cluster
toward the three massive star-forming regions: the Trapezium Cluster (TC), the
Orion Hot Core (OHC), and OMC1-S. We derived low-mass stellar densities of
10^{5} stars pc^{-3} in the TC and OMC1-S, and of 10^{6} stars pc^{-3} in the
OHC. The close association between the low-mass star clusters with massive star
cradles supports the role of these clusters in the formation of massive stars.
The X-ray observations show for the first time in the TC that low-mass stars
with intermediate extinction are clustered toward the position of the most
massive star, which is surrounded by a ring of non-extincted low-mass stars.
This 'envelope-core' structure is also supported by infrared and optical
observations. Our analysis suggests that at least two basic ingredients are
needed in massive star formation: the presence of dense gas and a cluster of
low-mass stars. The scenario that better explains our findings assumes high
fragmentation in the parental core, accretion at subcore scales that forms a
low-mass stellar cluster, and subsequent competitive accretion. Finally,
although coalescence does not seem a common mechanism for building up massive
stars, we show that a single stellar merger may have occurred in the evolution
of the OHC cluster, favored by the presence of disks, binaries, and gas
accretion.Comment: 17 pages, 11 figures, 3 Tables. Accepted for publication in A&
Friction mechanisms Wear mechanisms Abrasive wear Adhesive
Abstract Friction and wear tests were performed with a number of greases based on biogenic esters and thickened with two metal soaps and a highly dispersed silica acid gel. The series of experiments was performed on a Nonotribometer in material combination of sapphire ball on steel disks with a range of normal loads from 1 up to 500 mN. Results directly show influences of the bulk grease components on frictional and wear behavior. Comparison of frictional and wear results makes manifest that, while in most combinations of base oil and thickener, the highest influence is found in the thickening agent, some combinations are mainly influenced by the base oil. All frictional results along with wear widths and depths as well as micrographs of the prevailing wear mechanisms are presented and discussed
NGC7538 IRS1 - an ionized jet powered by accretion
Analysis of high spatial resolution VLA images shows that the free-free
emission from NGC7538 IRS1 is dominated by a collimated ionized wind. We have
re-analyzed high angular resolution VLA archive data from 6 cm to 7 mm, and
measured separately the flux density from the compact bipolar core and the
extended (1.5" - 3") lobes. We find that the flux density of the core is
proportional to the frequency to the power of alpha, with alpha being about
0.7. The frequency dependence of the total flux density is slightly steeper
with alpha = 0.8. A massive optically thick hypercompact core with a steep
density gradient can explain this frequency dependence, but it cannot explain
the extremely broad recombination line velocities observed in this source.
Neither can it explain why the core is bipolar rather than spherical, nor the
observed decrease of 4% in the flux density in less than 10 years. An ionized
wind modulated by accretion is expected to vary, because the accretion flow
from the surrounding cloud will vary over time. BIMA and CARMA continuum
observations at 3 mm show that the free-free emission still dominates at 3 mm.
HCO+ J = 1 - 0 observations combined with FCRAO single dish data show a clear
inverse P Cygni profile towards IRS1. These observations confirm that IRS1 is
heavily accreting with an accretion rate of about 2 times 10(-4) solar masses
per year.Comment: Accepted for Astrophysical Journal Letter
Magnetic field morphology in nearby molecular clouds as revealed by starlight and submillimetre polarization
Within four nearby (d < 160 pc) molecular clouds, we statistically evaluate
the structure of the interstellar magnetic field, projected on the plane of the
sky and integrated along the line of sight, as inferred from the polarized
thermal emission of Galactic dust observed by Planck at 353 GHz and from the
optical and NIR polarization of background starlight. We compare the dispersion
of the field orientation directly in vicinities with an area equivalent to that
subtended by the Planck effective beam at 353 GHz (10') and using the
second-order structure functions of the field orientation angles. We find that
the average dispersion of the starlight-inferred field orientations within
10'-diameter vicinities is less than 20 deg, and that at these scales the mean
field orientation is on average within 5 deg of that inferred from the
submillimetre polarization observations in the considered regions. We also find
that the dispersion of starlight polarization orientations and the polarization
fractions within these vicinities are well reproduced by a Gaussian model of
the turbulent structure of the magnetic field, in agreement with the findings
reported by the Planck collaboration at scales greater than 10' and for
comparable column densities. At scales greater than 10', we find differences of
up to 14.7 deg between the second-order structure functions obtained from
starlight and submillimetre polarization observations in the same positions in
the plane of the sky, but comparison with a Gaussian model of the turbulent
structure of the magnetic field indicates that these differences are small and
are consistent with the difference in angular resolution between both
techniques.Comment: 15 pages, 10 figures, submitted to A&
From Sasaki-Einstein spaces to quivers via BPS geodesics: Lpqr
The AdS/CFT correspondence between Sasaki-Einstein spaces and quiver gauge
theories is studied from the perspective of massless BPS geodesics. The
recently constructed toric Lpqr geometries are considered: we determine the
dual superconformal quivers and the spectrum of BPS mesons. The conformal
anomaly is compared with the volumes of the manifolds. The U(1)^2_F x U(1)_R
global symmetry quantum numbers of the mesonic operators are successfully
matched with the conserved momenta of the geodesics, providing a test of
AdS/CFT duality. The correspondence between BPS mesons and geodesics allows to
find new precise relations between the two sides of the duality. In particular
the parameters that characterize the geometry are mapped directly to the
parameters used for a-maximization in the field theory. The analysis simplifies
for the special case of the Lpqq models, which are shown to correspond to the
known "generalized conifolds". These geometries can break conformal invariance
through toric deformations of the complex structure.Comment: 30 pages, 8 figures, LaTeX. v2: One more figure. References added,
typos correcte
A microscopic theory of gauge mediation
We construct models of indirect gauge mediation where the dynamics
responsible for breaking supersymmetry simultaneously generates a weakly
coupled subsector of messengers. This provides a microscopic realization of
messenger gauge mediation where the messenger and hidden sector fields are
unified into a single sector. The UV theory is SQCD with massless and massive
quarks plus singlets, and at low energies it flows to a weakly coupled quiver
gauge theory. One node provides the primary source of supersymmetry breaking,
which is then transmitted to the node giving rise to the messenger fields.
These models break R-symmetry spontaneously, produce realistic gaugino and
sfermion masses, and give a heavy gravitino.Comment: 24 pages, 2 figures, accepted to JHEP for publicatio
A serological survey of bovine herpesvirus-1 infection in selected dairy herds in northern and central Italy.
Solving satisfiability problems by fluctuations: The dynamics of stochastic local search algorithms
Stochastic local search algorithms are frequently used to numerically solve
hard combinatorial optimization or decision problems. We give numerical and
approximate analytical descriptions of the dynamics of such algorithms applied
to random satisfiability problems. We find two different dynamical regimes,
depending on the number of constraints per variable: For low constraintness,
the problems are solved efficiently, i.e. in linear time. For higher
constraintness, the solution times become exponential. We observe that the
dynamical behavior is characterized by a fast equilibration and fluctuations
around this equilibrium. If the algorithm runs long enough, an exponentially
rare fluctuation towards a solution appears.Comment: 21 pages, 18 figures, revised version, to app. in PRE (2003
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