9,272 research outputs found
t-tbar Pair production cross section measurement at the LHC
Measurement of pair production cross sections with an integrated
luminosity of around 1 fb at = 7 TeV obtained with the ATLAS
and CMS detectors are reported. The inclusive cross sections in dilepton (ee,
, and ), lepton+jets (e, ) and all hadronic decay
modes are measured. In addition to inclusive cross section measurement, the
study of jet multiplicity with additional jets are also presented, which is
important to constrain the initial state radiation. Measurement of the charge
asymmetry at the LHC is also presented. All measurements are compatible with
Standard Model predictions.Comment: Presented at the 2011 Hadron Collider Physics symposium (HCP-2011),
Paris, France, November 14-18 2011, 5 pages, 7 figure
Disruption of Molecular Clouds by Expansion of Dusty H II Regions
Dynamical expansion of H II regions around star clusters plays a key role in
dispersing the surrounding dense gas and therefore in limiting the efficiency
of star formation in molecular clouds. We use a semi-analytic method and
numerical simulations to explore expansion of spherical dusty H II regions and
surrounding neutral shells and the resulting cloud disruption. Our model for
shell expansion adopts the static solutions of Draine (2011) for dusty H II
regions and considers the contact outward forces on the shell due to radiation
and thermal pressures as well as the inward gravity from the central star and
the shell itself. We show that the internal structure we adopt and the shell
evolution from the semi-analytic approach are in good agreement with the
results of numerical simulations. Strong radiation pressure in the interior
controls the shell expansion indirectly by enhancing the density and pressure
at the ionization front. We calculate the minimum star formation efficiency
required for cloud disruption as a function of the cloud's
total mass and mean surface density. Within the adopted spherical geometry, we
find that typical giant molecular clouds in normal disk galaxies have
%, with comparable gas and radiation pressure
effects on shell expansion. Massive cluster-forming clumps require a
significantly higher efficiency of % for disruption,
produced mainly by radiation-driven expansion. The disruption time is typically
of the order of a free-fall timescale, suggesting that the cloud disruption
occurs rapidly once a sufficiently luminous H II region is formed. We also
discuss limitations of the spherical idealization.Comment: 23 pages, 14 figures; Accepted for publication in Ap
Modeling UV Radiation Feedback from Massive Stars: I. Implementation of Adaptive Ray Tracing Method and Tests
We present an implementation of an adaptive ray tracing (ART) module in the
Athena hydrodynamics code that accurately and efficiently handles the radiative
transfer involving multiple point sources on a three-dimensional Cartesian
grid. We adopt a recently proposed parallel algorithm that uses non-blocking,
asynchronous MPI communications to accelerate transport of rays across the
computational domain. We validate our implementation through several standard
test problems including the propagation of radiation in vacuum and the
expansions of various types of HII regions. Additionally, scaling tests show
that the cost of a full ray trace per source remains comparable to that of the
hydrodynamics update on up to processors. To demonstrate
application of our ART implementation, we perform a simulation of star cluster
formation in a marginally bound, turbulent cloud, finding that its star
formation efficiency is when both radiation pressure forces and
photoionization by UV radiation are treated. We directly compare the radiation
forces computed from the ART scheme with that from the M1 closure relation.
Although the ART and M1 schemes yield similar results on large scales, the
latter is unable to resolve the radiation field accurately near individual
point sources.Comment: 20 pages, 14 figures; accepted for publication in Ap
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