1,540 research outputs found
A discontinuity in the low-mass initial mass function
The origin of brown dwarfs (BDs) is still an unsolved mystery. While the
standard model describes the formation of BDs and stars in a similar way recent
data on the multiplicity properties of stars and BDs show them to have
different binary distribution functions. Here we show that proper treatment of
these uncovers a discontinuity of the multiplicity-corrected mass distribution
in the very-low-mass star (VLMS) and BD mass regime. A continuous IMF can be
discarded with extremely high confidence. This suggests that VLMSs and BDs on
the one hand, and stars on the other, are two correlated but disjoint
populations with different dynamical histories. The analysis presented here
suggests that about one BD forms per five stars and that the BD-star binary
fraction is about 2%-3% among stellar systems.Comment: 14 pages, 11 figures, uses emulateapj.cls. Minor corrections and 1
reference added after being accepted by the Ap
Phase transition in the 3-D massive Gross-Neveu model
We consider the 3-dimensional massive Gross-Neveu model at finite temperature
as an effective theory for strong interactions. Using the Matsubara imaginary
time formalism, we derive a closed form for the renormalized -dependent
four-point function. This gives a singularity, suggesting a phase transition.
Considering the free energy we obtain the -dependent mass, which goes to
zero for some temperature. These results lead us to the conclusion that there
is a second-order phase transition.Comment: 06 pages, 02 figures, LATE
Functional Determinants in Quantum Field Theory
Functional determinants of differential operators play a prominent role in
theoretical and mathematical physics, and in particular in quantum field
theory. They are, however, difficult to compute in non-trivial cases. For one
dimensional problems, a classical result of Gel'fand and Yaglom dramatically
simplifies the problem so that the functional determinant can be computed
without computing the spectrum of eigenvalues. Here I report recent progress in
extending this approach to higher dimensions (i.e., functional determinants of
partial differential operators), with applications in quantum field theory.Comment: Plenary talk at QTS5 (Quantum Theory and Symmetries); 16 pp, 2 fig
Self-diffusion coefficients of charged particles: Prediction of Nonlinear volume fraction dependence
We report on calculations of the translational and rotational short-time
self-diffusion coefficients and for suspensions of
charge-stabilized colloidal spheres. These diffusion coefficients are affected
by electrostatic forces and many-body hydrodynamic interactions (HI). Our
computations account for both two-body and three-body HI. For strongly charged
particles, we predict interesting nonlinear scaling relations and depending on volume fraction
, with essentially charge-independent parameters and . These
scaling relations are strikingly different from the corresponding results for
hard spheres. Our numerical results can be explained using a model of effective
hard spheres. Moreover, we perceptibly improve the known result for of
hard sphere suspensions.Comment: 8 pages, LaTeX, 3 Postscript figures included using eps
Complex and flexible catabolism in Aromatoleum aromaticum pCyN1
Large quantities of organic matter are continuously deposited, and (a)biotic gradients intersect in the soil-rhizosphere, where biodegradation contributes to the global cycles of elements. The betaproteobacterial genus Aromatoleum comprises cosmopolitan, facultative denitrifying degradation specialists. Aromatoleum aromaticum. pCyN1 stands out for anaerobically decomposing plant-derived monoterpenes in addition to monoaromatic hydrocarbons, polar aromatics and aliphatics. The catabolic network's structure and flexibility in A. aromaticum pCyN1 were studied across 34 growth conditions by superimposing proteome profiles onto the manually annotated 4.37 Mbp genome. Strain pCyN1 employs three fundamentally different enzymes for C-H-bond cleavage at the methyl groups of p-cymene/4-ethyltoluene, toluene and p-cresol respectively. Regulation of degradation modules displayed substrate specificities ranging from narrow (toluene and cyclohexane carboxylate) via medium-wide (one module shared by p-cymene, 4-ethyltoluene, alpha-phellandrene, alpha-terpinene, gamma-terpinene and limonene) to broad (central benzoyl-CoA pathway serving 16 aromatic substrates). Remarkably, three variants of ATP-dependent (class I) benzoyl-CoA reductase and four different beta-oxidation routes establish a degradation hub that accommodates the substrate diversity. The respiratory system displayed several conspicuous profiles, e.g. the presence of nitrous oxide reductase under oxic and of low-affinity oxidase under anoxic conditions. Overall, nutritional versatility in conjunction with network regulation endow A. aromaticum pCyN1 with broad adaptability
Photoproduction of scalar mesons on protons and nuclei
We study the photoproduction of scalar mesons close to the threshold of
f_0(980) and a_0(980) using a unitary chiral model. Peaks for both resonances
show up in the invariant mass distributions of pairs of pseudoscalar mesons. A
discussion is made on the photoproduction of these resonances in nuclei, which
can shed light on their nature, a subject of continuous debate.Comment: 25 pages, 9 figures, accepted for publication in Phys Rev
Quantum Mechanics of the Vacuum State in Two-Dimensional QCD with Adjoint Fermions
A study of two-dimensional QCD on a spatial circle with Majorana fermions in
the adjoint representation of the gauge groups SU(2) and SU(3) has been
performed. The main emphasis is put on the symmetry properties related to the
homotopically non-trivial gauge transformations and the discrete axial symmetry
of this model. Within a gauge fixed canonical framework, the delicate interplay
of topology on the one hand and Jacobians and boundary conditions arising in
the course of resolving Gauss's law on the other hand is exhibited. As a
result, a consistent description of the residual gauge symmetry (for
SU(N)) and the ``axial anomaly" emerges. For illustrative purposes, the vacuum
of the model is determined analytically in the limit of a small circle. There,
the Born-Oppenheimer approximation is justified and reduces the vacuum problem
to simple quantum mechanics. The issue of fermion condensates is addressed and
residual discrepancies with other approaches are pointed out.Comment: 44 pages; for hardcopies of figures, contact
[email protected]
Enhanced structural correlations accelerate diffusion in charge-stabilized colloidal suspensions
Theoretical calculations for colloidal charge-stabilized and hard sphere
suspensions show that hydrodynamic interactions yield a qualitatively different
particle concentration dependence of the short-time self-diffusion coefficient.
The effect, however, is numerically small and hardly accessible by conventional
light scattering experiments. Applying multiple-scattering decorrelation
equipment and a careful data analysis we show that the theoretical prediction
for charged particles is in agreement with our experimental results from
aqueous polystyrene latex suspensions.Comment: 1 ps-file (MS-Word), 14 page
Low-dose mistletoe lectin-I reduces melanoma growth and spread in a scid mouse xenograft model
This study investigates the effects of mistletoe lectin-I (ML-I) on melanoma growth and spread in vivo. The human melanoma cell line MV3 was xenografted into severe combined immunodeficient mice and vehicle solution or purified ML-I was administered at 30, 150 and 500 ng per kg body weight (20 mice per group) daily. After 19 days, mice were killed, primary tumours (PTs) and lungs were dissected out, and tumour weights, number of lung metastases (LMs), number of tumour-infiltrating dendritic cells (DCs), and apoptosis rates in the melanoma cells and in the DCs were assessed. A 35% reduction of PT weight (P=0.03) and a 55% decrease in number of LMs (P=0.016) were evident for low-dose ML-I (30 ng kg−1) treatment but not for higher doses. Mistletoe lectin-I increased apoptosis rates in the melanoma cells of PTs at all doses, while no induction of apoptosis was noted in the LMs. Low-dose ML-I significantly increased the number of DCs infiltrating the PTs (P<0.0001) and protected DCs against apoptosis, while higher doses induced apoptosis in the DCs (P<0.01). Our results demonstrate that low-dose ML-I reduced melanoma growth and number of metastases in vivo, primarily due to immunomodulatory effects
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